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Planet

Planet

Overview
A planet is a celestial body
Celestial Body
Celestial Body is a Croatian film directed by Lukas Nola. It was released in 2000....

 orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

ing a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...

s.
The term planet is ancient, with ties to history
History
History is the discovery, collection, organization, and presentation of information about past events. History can also mean the period of time after writing was invented. Scholars who write about history are called historians...

, science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

, mythology
Mythology
The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

, and religion
Religion
Religion is a collection of cultural systems, belief systems, and worldviews that establishes symbols that relate humanity to spirituality and, sometimes, to moral values. Many religions have narratives, symbols, traditions and sacred histories that are intended to give meaning to life or to...

. The planets were originally seen by many early cultures as divine, or as emissaries of the god
God
God is the English name given to a singular being in theistic and deistic religions who is either the sole deity in monotheism, or a single deity in polytheism....

s. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects.
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Timeline

1612   Galileo Galilei becomes the first astronomer to observe the planet Neptune, although he mistakenly catalogued it as a fixed star.

1915   Pluto is photographed for the first time but is not recognized as a planet.

1965   Venera program: The Soviet Union launches the Venera 3 space probe toward Venus, the first spacecraft to reach the surface of another planet.

1966   Venera 3 Soviet space probe crashes on Venus becoming the first spacecraft to land on another planet's surface.

1967   Venera program: Venera 4 is launched (it will become the first space probe to enter another planet's atmosphere and successfully return data).

1970   Venera Program: Venera 7 launched. It will later become the first spacecraft to successfully transmit data from the surface of another planet (Venus).

1972   The Pioneer 10 space probe is launched from Cape Canaveral, Florida with a mission to explore the outer planets.

1979   The American space probe ''Pioneer 11'' becomes the first spacecraft to visit Saturn when it passes the planet at a distance of 21,000 km.

1995   51 Pegasi is discovered to be the first major star apart from the Sun to have a planet (and extrasolar planet) orbiting around it.

 
Encyclopedia
A planet is a celestial body
Celestial Body
Celestial Body is a Croatian film directed by Lukas Nola. It was released in 2000....

 orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

ing a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...

s.
The term planet is ancient, with ties to history
History
History is the discovery, collection, organization, and presentation of information about past events. History can also mean the period of time after writing was invented. Scholars who write about history are called historians...

, science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

, mythology
Mythology
The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

, and religion
Religion
Religion is a collection of cultural systems, belief systems, and worldviews that establishes symbols that relate humanity to spirituality and, sometimes, to moral values. Many religions have narratives, symbols, traditions and sacred histories that are intended to give meaning to life or to...

. The planets were originally seen by many early cultures as divine, or as emissaries of the god
God
God is the English name given to a singular being in theistic and deistic religions who is either the sole deity in monotheism, or a single deity in polytheism....

s. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 officially adopted a resolution defining planets within the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were thought by Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 to orbit the Earth in deferent and epicycle
Deferent and epicycle
In the Ptolemaic system of astronomy, the epicycle was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets...

 motions. Though the idea that the planets orbited the Sun
Heliocentrism
Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a stationary Sun at the center of the universe. The word comes from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center...

 had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 astronomical observations, performed by Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

. By careful analysis of the observation data, Johannes Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

 found the planets' orbits to be not circular, but elliptical
Elliptic orbit
In astrodynamics or celestial mechanics an elliptic orbit is a Kepler orbit with the eccentricity less than 1; this includes the special case of a circular orbit, with eccentricity equal to zero. In a stricter sense, it is a Kepler orbit with the eccentricity greater than 0 and less than 1 . In a...

. As observational tools improved, astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

s saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice cap
Ice cap
An ice cap is an ice mass that covers less than 50 000 km² of land area . Masses of ice covering more than 50 000 km² are termed an ice sheet....

s and season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

s. Since the dawn of the Space Age
Space Age
The Space Age is a time period encompassing the activities related to the Space Race, space exploration, space technology, and the cultural developments influenced by these events. The Space Age is generally considered to have begun with Sputnik...

, close observation by probes
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

 has found that Earth and the other planets share characteristics such as volcanism
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

, hurricanes, tectonics
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...

, and even hydrology
Hydrology
Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...

.

Planets are generally divided into two main types: large, low-density gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

s, and smaller, rocky terrestrials
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, they are the four terrestrials, Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

, Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, then the four gas giants, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

, Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

, Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

, and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

. Six of the planets are orbited by one or more natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

s.
Additionally, the Solar System also contains at least five dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s and hundreds of thousands of small Solar System bodies
Small solar system body
A small Solar System body is an object in the Solar System that is neither a planet nor a dwarf planet, nor a satellite of a planet or dwarf planet:...

.

Since 1992, hundreds of planets around other stars ("extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

s" or "exoplanets") in the Milky Way Galaxy
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 have been discovered. As of , known extrasolar planets are listed in the Extrasolar Planets Encyclopaedia
Extrasolar Planets Encyclopaedia
The Extrasolar Planets Encyclopedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains a database of all the currently known and candidate extrasolar planets, with individual "note" pages for each planet and a full list...

, ranging from the size of terrestrial planets somewhat larger than Earth to gas giants larger than Jupiter.

History


The idea of planets has evolved over its history, from the divine wandering stars
Wandering Stars
Wandering Stars is an anthology of Jewish fantasy and science fiction, edited by Jack Dann, originally published by Harper & Row in 1974. It represented, according to the book cover, "the first time in science fiction that the Jew - and the richness of his themes and particular points of view --...

 of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.

The five classical planets, being visible to the naked eye, have been known since ancient times, and have had a significant impact on mythology
Mythology
The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

, religious cosmology
Religious cosmology
A Religious cosmology is a way of explaining the origin, the history and the evolution of the universe based on the religious mythology of a specific tradition...

, and ancient astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

. In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights ( "wandering stars") or simply "" ( "wanderers"), from which today's word "planet" was derived. In ancient Greece
Greece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....

, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

, Babylon
Babylon
Babylon was an Akkadian city-state of ancient Mesopotamia, the remains of which are found in present-day Al Hillah, Babil Province, Iraq, about 85 kilometers south of Baghdad...

 and indeed all pre-modern civilisations, it was almost universally believed that Earth was in the center of the Universe and that all the "planets" circled the Earth. The reasons for this perception were that stars and planets appeared to revolve around the Earth each day, and the apparently common-sense
Common sense
Common sense is defined by Merriam-Webster as, "sound and prudent judgment based on a simple perception of the situation or facts." Thus, "common sense" equates to the knowledge and experience which most people already have, or which the person using the term believes that they do or should have...

 perception that the Earth was solid and stable, and that it was not moving but at rest.

The name for planets in Chinese astronomy
Chinese astronomy
Astronomy in China has a very long history, with historians considering that "they [the Chinese] were the most persistent and accurate observers of celestial phenomena anywhere in the world before the Arabs."...

 had the same motive as the Greek name, 行星 "moving star". In Japanese during the Edo period
Edo period
The , or , is a division of Japanese history which was ruled by the shoguns of the Tokugawa family, running from 1603 to 1868. The political entity of this period was the Tokugawa shogunate....

 there were two competing terms, 惑星 "confused star" and 遊星 "wandering star". In modern Japan, terminology was unified in favour of 惑星, but in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 the alternative term 遊星 retains some currency.

Babylon



The first civilization known to possess a functional theory of the planets were the Babylonia
Babylonia
Babylonia was an ancient cultural region in central-southern Mesopotamia , with Babylon as its capital. Babylonia emerged as a major power when Hammurabi Babylonia was an ancient cultural region in central-southern Mesopotamia (present-day Iraq), with Babylon as its capital. Babylonia emerged as...

ns, who lived in Mesopotamia
Mesopotamia
Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...

 in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa
Venus tablet of Ammisaduqa
The Venus tablet of Ammisaduqa refers to the record of astronomical observations of Venus, as preserved in numerous cuneiform tablets dating from the first millennium BCE. It is believed that this astronomical record was first compiled during the reign of King Ammisaduqa , the fourth ruler after...

, a 7th century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC. The MUL.APIN
MUL.APIN
MUL.APIN is the conventional title given to a Babylonian compendium that deals with many diverse aspects of Babylonian astronomy and astrology....

 is a pair of cuneiform
Cuneiform
Cuneiform can refer to:*Cuneiform script, an ancient writing system originating in Mesopotamia in the 4th millennium BC*Cuneiform , three bones in the human foot*Cuneiform Records, a music record label...

 tablets dating from the 7th century BC that lays out the motions of the Sun, Moon and planets over the course of the year. The Babylonian astrologers
Babylonian astrology
In Babylon as well as in Assyria as a direct offshoot of Babylonian culture, astrology takes its place in theofficial cult as one of the two chief means at the disposal of the priests for ascertaining the will and intention of the gods, the other being through the inspection of the liver of the...

 also laid the foundations of what would eventually become Western astrology
Western astrology
Western astrology is the system of astrology most popular in Western countries. Western astrology is historically based on Ptolemy's Tetrabiblos , which in turn was a continuation of Hellenistic and ultimately Babylonian traditions....

. The Enuma anu enlil
Enuma anu enlil
Enuma Anu Enlil is a major series of 68 or 70 tablets dealing with Babylonian astrology...

, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omen
Omen
An omen is a phenomenon that is believed to foretell the future, often signifying the advent of change...

s and their relationships with various celestial phenomena including the motions of the planets.

The Sumer
Sumer
Sumer was a civilization and historical region in southern Mesopotamia, modern Iraq during the Chalcolithic and Early Bronze Age....

ians, predecessors of the Babylonians who are considered as one of the first civilizations
Cradle of Civilization
The cradle of civilization is a term referring to any of the possible locations for the emergence of civilization.It is usually applied to the Ancient Near Eastern Chalcolithic , especially in the Fertile Crescent , but also extended to sites in Armenia, and the Persian Plateau, besides other Asian...

 and are credited with the invention of writing
Writing
Writing is the representation of language in a textual medium through the use of a set of signs or symbols . It is distinguished from illustration, such as cave drawing and painting, and non-symbolic preservation of language via non-textual media, such as magnetic tape audio.Writing most likely...

, had identified at least Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 by 1500 BC. Shortly afterwards, the other inner planet Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

 and the outer planets Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 in early modern times.

Greco-Roman astronomy


Ptolemy's 7 planetary spheres
1
Moon
2
Mercury
3
Venus
4
Sun
5
Mars
6
Jupiter
7
Saturn


Because they were not as interested in divination as the Babylonians, the ancient Greeks initially did not attach as much significance to the planets. The Pythagoreans, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras
Pythagoras
Pythagoras of Samos was an Ionian Greek philosopher, mathematician, and founder of the religious movement called Pythagoreanism. Most of the information about Pythagoras was written down centuries after he lived, so very little reliable information is known about him...

 or Parmenides
Parmenides
Parmenides of Elea was an ancient Greek philosopher born in Elea, a Greek city on the southern coast of Italy. He was the founder of the Eleatic school of philosophy. The single known work of Parmenides is a poem, On Nature, which has survived only in fragmentary form. In this poem, Parmenides...

 are said to have first identified the evening star and morning star (Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

) as one and the same. In the 3rd century BC, Aristarchus of Samos
Aristarchus of Samos
Aristarchus, or more correctly Aristarchos , was a Greek astronomer and mathematician, born on the island of Samos, in Greece. He presented the first known heliocentric model of the solar system, placing the Sun, not the Earth, at the center of the known universe...

 proposed a heliocentric
Heliocentrism
Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a stationary Sun at the center of the universe. The word comes from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center...

 system, according to which the Earth and planets revolved around the sun. However, the geocentric system would remain dominant until the Scientific Revolution
Scientific revolution
The Scientific Revolution is an era associated primarily with the 16th and 17th centuries during which new ideas and knowledge in physics, astronomy, biology, medicine and chemistry transformed medieval and ancient views of nature and laid the foundations for modern science...

.

By the 1st century BC, during the Hellenistic period
Hellenistic period
The Hellenistic period or Hellenistic era describes the time which followed the conquests of Alexander the Great. It was so named by the historian J. G. Droysen. During this time, Greek cultural influence and power was at its zenith in Europe and Asia...

, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest
Almagest
The Almagest is a 2nd-century mathematical and astronomical treatise on the apparent motions of the stars and planetary paths. Written in Greek by Claudius Ptolemy, a Roman era scholar of Egypt,...

written by Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 in the 2nd century CE. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries. To the Greeks and Romans there were seven known planets, each presumed to be circling the Earth
Geocentric model
In astronomy, the geocentric model , is the superseded theory that the Earth is the center of the universe, and that all other objects orbit around it. This geocentric model served as the predominant cosmological system in many ancient civilizations such as ancient Greece...

 according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.

India


In 499 CE, the Indian astronomer Aryabhata
Aryabhata
Aryabhata was the first in the line of great mathematician-astronomers from the classical age of Indian mathematics and Indian astronomy...

 propounded a planetary model which explicitly incorporated the Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbit of planets are elliptical
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...

.
Aryabhata's followers were particularly strong in South India
South India
South India is the area encompassing India's states of Andhra Pradesh, Karnataka, Kerala and Tamil Nadu as well as the union territories of Lakshadweep and Pondicherry, occupying 19.31% of India's area...

, where his principles of the diurnal rotation of the earth, among others, were followed and a number of secondary works were based on them.

In 1500, Nilakantha Somayaji
Nilakantha Somayaji
Kelallur Nilakantha Somayaji was a major mathematician and astronomer of the Kerala school of astronomy and mathematics. One of his most influential works was the comprehensive astronomical treatise Tantrasamgraha completed in 1501...

 of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model. In his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits the Earth, similar to the Tychonic system
Tychonic system
The Tychonic system was a model of the solar system published by Tycho Brahe in the late 16th century which combined what he saw as the mathematical benefits of the Copernican system with the philosophical and "physical" benefits of the Ptolemaic system...

 later proposed by Tycho Brahe
Tycho Brahe
Tycho Brahe , born Tyge Ottesen Brahe, was a Danish nobleman known for his accurate and comprehensive astronomical and planetary observations...

 in the late 16th century. Most astronomers of the Kerala school who followed him accepted his planetary model.

Medieval Muslim astronomy


In the 11th century, the transit of Venus
Transit of Venus
A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, becoming visible against the solar disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun...

 was observed by Avicenna
Avicenna
Abū ʿAlī al-Ḥusayn ibn ʿAbd Allāh ibn Sīnā , commonly known as Ibn Sīnā or by his Latinized name Avicenna, was a Persian polymath, who wrote almost 450 treatises on a wide range of subjects, of which around 240 have survived...

, who established that Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 was, at least sometimes, below the Sun. In the 12th century, Ibn Bajjah
Ibn Bajjah
Abū-Bakr Muhammad ibn Yahya ibn al-Sāyigh , known as Ibn Bājjah , was an Andalusian polymath: an astronomer, logician, musician, philosopher, physician, physicist, psychologist, botanist, poet and scientist. He was known in the West by his Latinized name, Avempace...

 observed "two planets as black spots on the face of the Sun," which was later identified as a transit of Mercury
Transit of Mercury
A transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun....

 and Venus by the Maragha
Maragheh observatory
Maragheh observatory is an astronomical observatory which was established in 1259 CE by Nasir al-Din al-Tusi, an Iranian scientist and astronomer...

 astronomer Qotb al-Din Shirazi in the 13th century. However, Ibn Bajjah could not have observed a transit of Venus, as none occurred in his lifetime.

European Renaissance

Renaissance planets, ca. 1543 to 1781
1
Mercury
2
Venus
3
Earth
4
Mars
5
Jupiter
6
Saturn


With the advent of the Scientific Revolution
Scientific revolution
The Scientific Revolution is an era associated primarily with the 16th and 17th centuries during which new ideas and knowledge in physics, astronomy, biology, medicine and chemistry transformed medieval and ancient views of nature and laid the foundations for modern science...

, understanding of the term "planet" changed from something that moved across the sky (in relation to the star field); to a body that orbited the Earth (or that were believed to do so at the time); and in the 16th century to something that directly orbited the Sun when the heliocentric model of Copernicus
Nicolaus Copernicus
Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....

, Galileo
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

 and Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

 gained sway.

Thus the Earth became included in the list of planets, while the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century. Until the mid-19th century, the number of "planets" rose rapidly since any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community.

19th century

New planets, 1807–1845
1
Mercury
2
Venus
3
Earth
4
Mars
5
Vesta
6
Juno
7
Ceres
8
Pallas
9
Jupiter
10
Saturn
11
Uranus

In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas
2 Pallas
Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered , and one of the largest. It is estimated to constitute 7% of the mass of the asteroid belt, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta. It is however 20%...

, and Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

) were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the Asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

), and had a much smaller mass; as a result they were reclassified as "asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s." In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Since there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846, there was no apparent need to have a formal definition.

20th century

Planets 1854–1930, 2006–present
1
Mercury
2
Venus
3
Earth
4
Mars
5
Jupiter
6
Saturn
7
Uranus
8
Neptune

However, in the 20th century, Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 was discovered. After initial observations led to the belief it was larger than Earth, the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton
Raymond Lyttleton
Raymond Arthur Lyttleton FRS was a British mathematician and theoretical astronomer.He was born in the Oldbury, Worcestershire area and educated at King Edward VI Five Ways school in Birmingham, going from there to Clare College, Cambridge to read mathematics, graduating in 1933...

 suggested that Pluto may be an escaped satellite of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

, and Fred Whipple
Fred Lawrence Whipple
Fred Lawrence Whipple was an American astronomer, who worked at the Harvard College Observatory for over 70 years...

 suggested in 1964 that Pluto may be a comet. However, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status until 2006.
Planets 1930–2006
1
Mercury
2
Venus
3
Earth
4
Mars
5
Jupiter
6
Saturn
7
Uranus
8
Neptune
9
Pluto

In 1992, astronomers Aleksander Wolszczan
Aleksander Wolszczan
Aleksander Wolszczan is a Polish astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets.- Scientific career :...

 and Dale Frail
Dale Frail
Dale A. Frail is an astronomer working at the National Radio Astronomy Observatory in Socorro, New Mexico. He was born in Canada, spent much of his childhood in Europe, and his professional career has been based in the United States.-Career:...

 announced the discovery of planets around a pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

, PSR B1257+12
PSR B1257+12
PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located roughly 2000 light-years from the Sun. In 2007, it was confirmed that three extrasolar planets orbit the pulsar.- Pulsar :...

. This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor
Michel Mayor
Dr. Michel G. E. Mayor is a Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the Observatory of Geneva...

 and Didier Queloz
Didier Queloz
Didier Queloz is a Geneva-based astronomer with a prolific record in finding extrasolar planets. He is understudy to Michel Mayor.Didier Queloz was a Ph.D...

 of the University of Geneva
University of Geneva
The University of Geneva is a public research university located in Geneva, Switzerland.It was founded in 1559 by John Calvin, as a theological seminary and law school. It remained focused on theology until the 17th century, when it became a center for Enlightenment scholarship. In 1873, it...

 announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 star (51 Pegasi
51 Pegasi
51 Pegasi is a Sun-like star located 15.6 parsecs from Earth in the constellation Pegasus...

).

The discovery of extrasolar planets led to another ambiguity in defining a planet; the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as "brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

s". Brown dwarfs are generally considered stars due to their ability to fuse deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

, a heavier isotope of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

. While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.

21st century


With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

, or if it was large enough to generate energy by the thermonuclear fusion of deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

.

A growing number of astronomers argued for Pluto to be declassified as a planet, since many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands.

Some of them including Quaoar
50000 Quaoar
50000 Quaoar is a rocky trans-Neptunian object in the Kuiper belt with one known moon. Discovered on June 4, 2002 by astronomers Chad Trujillo and Michael Brown at the California Institute of Technology from images acquired at the Samuel Oschin Telescope at Palomar Observatory, it is thought by...

, Sedna
90377 Sedna
90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...

, and Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

 were heralded in the popular press as the tenth planet, failing however to receive widespread scientific recognition. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the necessity and public desire for an official definition of a planet.

Acknowledging the problem, the IAU set about creating the definition of planet
Definition of planet
The definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai , "wandering stars", for objects which apparently move over the sky...

, and produced one in August 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit
Clearing the neighbourhood
"Clearing the neighbourhood of its orbit" is a criterion for a celestial body to be considered a planet in the Solar System. This was one of the three criteria adopted by the International Astronomical Union in its 2006 definition of planet....

 (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s was created, initially containing three objects (Ceres, Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 and Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

).

Extrasolar planet definition


In 2003, The International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated the following working definition, mostly focused upon the boundary between planets and brown dwarves:
  1. Objects with true mass
    True mass
    The term true mass is synonymous with the term mass, but is used in astronomy to differentiate the measured mass of a planet from the lower limit of mass usually obtained from radial velocity techniques...

    es below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance
    Natural abundance
    In chemistry, natural abundance refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass of these isotopes is the atomic weight listed for the element in the periodic table...

     as the Sun) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
  2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarf
    Brown dwarf
    Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

    s", no matter how they formed or where they are located.
  3. Free-floating objects in young star cluster
    Star cluster
    Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...

    s with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).


This definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journal
Academic journal
An academic journal is a peer-reviewed periodical in which scholarship relating to a particular academic discipline is published. Academic journals serve as forums for the introduction and presentation for scrutiny of new research, and the critique of existing research...

s. Although temporary, it remains an effective working definition until a more permanent one is formally adopted. However, it does not address the dispute over the lower mass limit, and so it steered clear of the controversy regarding objects within the Solar System. This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2M1207b
2M1207b
2M1207b is a planetary-mass object orbiting the brown dwarf 2M1207, in the constellation Centaurus, approximately 170 light-years from Earth...

.

One definition of a sub-brown dwarf
Sub-brown dwarf
A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass for thermonuclear fusion of deuterium ....

 is a planet-mass object that formed through cloud-collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.
One reason for the dissent is that oftentimes it may not be possible to determine the formation process: for example an accretion-formed planet around a star may get ejected from the system to become free-floating, and likewise a cloud-collapse-formed sub-brown dwarf formed on its own in a star cluster may get captured into orbit around a star.
Dwarf planets 2006–present
Ceres Pluto Makemake Haumea Eris


The 13 Jupiter-mass cutoff is a rule of thumb rather than something of precise physical significance. The question arises: what is meant by deuterium burning
Deuterium burning
Deuterium burning is a nuclear fusion reaction that occurs in stars and some substellar objects, in which a deuterium nucleus and a proton combine to form a helium-3 nucleus...

? This question arises because large objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 MJ value is somewhere in between. The amount of deuterium burnt also depends not only on mass but on the composition of the planet, on the amount of helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 and deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 present.

Another criterion for separating planets and brown dwarfs, rather than deuterium burning, formation process or location is whether the core pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 is dominated by coulomb pressure or electron degeneracy.

2006 definition



The matter of the lower limit was addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, the assembly voted to pass a resolution that defined planets within the Solar System as:
Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto, Makemake and Eris) are classified as dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s, provided they are not also natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

s of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion. After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.

This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter
Steven Soter
Dr. Steven Soter, PhD, is an astrophysicist currently holding the positions of scientist-in-residence for New York University's Environmental Studies Program and of Research Associate for the Department of Astrophysics at the American Museum of Natural History...

:
In the aftermath of the IAU's 2006 vote, there has been controversy and debate about the definition, and many astronomers have stated that they will not use it. Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition.

Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status since its discovery in 1930. The discovery of Eris was widely reported in the media
Mass media
Mass media refers collectively to all media technologies which are intended to reach a large audience via mass communication. Broadcast media transmit their information electronically and comprise of television, film and radio, movies, CDs, DVDs and some other gadgets like cameras or video consoles...

 as the tenth planet and therefore the reclassification of all three objects as dwarf planets has attracted a lot of media and public attention as well.

Former classifications


The table below lists Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 bodies formerly considered to be planets:
Body (current classification) Notes
Star Dwarf planet Asteroid Moon
Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 
The Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

Classified as planets in antiquity
Ancient history
Ancient history is the study of the written past from the beginning of recorded human history to the Early Middle Ages. The span of recorded history is roughly 5,000 years, with Cuneiform script, the oldest discovered form of coherent writing, from the protoliterate period around the 30th century BC...

, in accordance with the definition then used.
Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

, Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

, Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...

, and Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...

The four largest moons of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

, known as the Galilean moons
Galilean moons
The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance...

 after their discoverer Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

. He referred to them as the "Medicean Planets" in honor of his patron
Patronage
Patronage is the support, encouragement, privilege, or financial aid that an organization or individual bestows to another. In the history of art, arts patronage refers to the support that kings or popes have provided to musicians, painters, and sculptors...

, the Medici family
Medici
The House of Medici or Famiglia de' Medici was a political dynasty, banking family and later royal house that first began to gather prominence under Cosimo de' Medici in the Republic of Florence during the late 14th century. The family originated in the Mugello region of the Tuscan countryside,...

.
Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

, Iapetus
Iapetus (moon)
Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671...

, Rhea
Rhea (moon)
Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.-Name:Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods"...

, Tethys
Tethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...

, and Dione
Dione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...

Five of Saturn's larger moons, discovered by Christiaan Huygens and Giovanni Domenico Cassini
Giovanni Domenico Cassini
This article is about the Italian-born astronomer. For his French-born great-grandson, see Jean-Dominique Cassini.Giovanni Domenico Cassini was an Italian/French mathematician, astronomer, engineer, and astrologer...

.
Ceres Pallas
2 Pallas
Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered , and one of the largest. It is estimated to constitute 7% of the mass of the asteroid belt, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta. It is however 20%...

, Juno
3 Juno
Juno , formal designation 3 Juno in the Minor Planet Center catalogue system, was the third asteroid to be discovered and is one of the larger main-belt asteroids, being one of the two largest stony asteroids, along with 15 Eunomia. Juno is estimated to contain 1% of the total mass of the asteroid...

, and Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

 
The first known asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s.
Ceres has subsequently been classified as a dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

 in 2006.
Astrea
5 Astraea
5 Astraea is a large main-belt asteroid. Its surface is highly reflective and its composition is probably a mixture of nickel-iron with magnesium- and iron-silicates....

, Hebe
6 Hebe
6 Hebe is a large main-belt asteroid, containing around half a percent of the mass of the belt. Its apparently high bulk density , however, means that by volume it does not rank among the top twenty asteroids...

, Iris
7 Iris
7 Iris is a large main-belt asteroid. Among the S-type asteroids, it ranks fifth in geometric mean diameter after Eunomia, Juno, Amphitrite and Herculina....

, Flora
8 Flora
8 Flora is a large, bright main-belt asteroid. It is the innermost large asteroid: no asteroid closer to the Sun has a diameter above 25 kilometres or two-elevenths that of Flora itself, and not until the tiny 149 Medusa was discovered was a single asteroid orbiting at a closer mean distance...

, Metis
9 Metis
9 Metis is one of the larger main-belt asteroids. It is composed of silicates and metallic nickel-iron, and may be the core remnant of a large asteroid that was destroyed by an ancient collision...

, Hygeia
10 Hygiea
10 Hygiea is an asteroid located in the asteroid belt. With somewhat oblong diameters of 350–500 km, and a mass estimated to be 2.9% of the total mass of the belt, it is the fourth largest asteroid by volume and mass...

, Parthenope
11 Parthenope
11 Parthenope is a large, bright main-belt asteroid.Parthenope was discovered by Annibale de Gasparis on May 11, 1850, the second of his nine asteroid discoveries. It was named after one of the Sirens in Greek mythology, said to have founded the city of Naples...

, Victoria
12 Victoria
12 Victoria is a large main-belt asteroid.It was discovered by J. R. Hind on September 13, 1850.Victoria is officially named after the Roman goddess of victory, but the name also honours Queen Victoria. The goddess Victoria was the daughter of Styx by the Titan Pallas...

, Egeria
13 Egeria
13 Egeria is a large main-belt G-type asteroid.It was discovered by A. de Gasparis on November 2, 1850, and was named by Urbain J. J. Le Verrier, whose computations led to the discovery of Neptune. Egeria was a goddess of Aricia, in Italy, and the wife of Numa Pompilius, second king of...

, Irene
14 Irene
14 Irene is a very large main-belt asteroid.14 Irene was discovered by J. R. Hind on May 19, 1851, and named after Eirene, a personification of peace in Greek mythology. She was one of the Horae, daughter of Zeus and Themis. The name was suggested by Sir John Herschel...

, Eunomia
15 Eunomia
15 Eunomia is a very large asteroid in the inner asteroid belt. It is the largest of the stony asteroids, and somewhere between the 8th-to-12th-largest main-belt asteroid overall...

 
More asteroids, discovered between 1845 and 1851. The rapidly expanding list of planets prompted their reclassification as asteroids by astronomers, and this was widely accepted by 1854.
Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

The first known Trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

 (i.e. minor planet with a semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

 beyond Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

). In 2006, Pluto was reclassified as a dwarf planet.
Eris (dwarf planet)
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

 (originally nicknamed Xena)
Discovered in 2003, this Trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

 (i.e. minor planet with a semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

 beyond Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

) was recognised in 2005, before, like Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

, in 2006 getting reclassified as a dwarf planet.

Mythology and naming



The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

, the two great luminaries the Sun and the Moon were called Helios
Helios
Helios was the personification of the Sun in Greek mythology. Homer often calls him simply Titan or Hyperion, while Hesiod and the Homeric Hymn separate him as a son of the Titans Hyperion and Theia or Euryphaessa and brother of the goddesses Selene, the moon, and Eos, the dawn...

and Selene
Selene
In Greek mythology, Selene was an archaic lunar deity and the daughter of the Titans Hyperion and Theia. In Roman mythology, the moon goddess is called Luna, Latin for "moon"....

; the farthest planet was called Phainon, the shiner; followed by Phaethon, "bright"; the red planet was known as Pyroeis, the "fiery"; the brightest was known as Phosphoros, the light bringer; and the fleeting final planet was called Stilbon, the gleamer. The Greeks also made each planet sacred to one among their pantheon of gods, the Olympians
Twelve Olympians
The Twelve Olympians, also known as the Dodekatheon , in Greek mythology, were the principal deities of the Greek pantheon, residing atop Mount Olympus. Zeus, Hera, Poseidon, Demeter, Hestia, and Hades were siblings. Ares, Hermes, Hephaestus, Athena, Apollo, and Artemis were children of Zeus...

: Helios and Selene were the names of both planets and gods; Phainon was sacred to Cronus
Cronus
In Greek mythology, Cronus or Kronos was the leader and the youngest of the first generation of Titans, divine descendants of Gaia, the earth, and Uranus, the sky...

, the Titan
Titan (mythology)
In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age....

 who fathered the Olympians; Phaethon was sacred to Zeus
Zeus
In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...

, Cronus's son who deposed him as king; Pyroeis was given to Ares
Ares
Ares is the Greek god of war. He is one of the Twelve Olympians, and the son of Zeus and Hera. In Greek literature, he often represents the physical or violent aspect of war, in contrast to the armored Athena, whose functions as a goddess of intelligence include military strategy and...

, son of Zeus and god of war; Phosphoros was ruled by Aphrodite
Aphrodite
Aphrodite is the Greek goddess of love, beauty, pleasure, and procreation.Her Roman equivalent is the goddess .Historically, her cult in Greece was imported from, or influenced by, the cult of Astarte in Phoenicia....

, the goddess of love; and Hermes
Hermes
Hermes is the great messenger of the gods in Greek mythology and a guide to the Underworld. Hermes was born on Mount Kyllini in Arcadia. An Olympian god, he is also the patron of boundaries and of the travelers who cross them, of shepherds and cowherds, of the cunning of thieves, of orators and...

, messenger of the gods and god of learning and wit, ruled over Stilbon.

The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros after their goddess of love, Ishtar
Ishtar
Ishtar is the Assyrian and Babylonian goddess of fertility, love, war, and sex. She is the counterpart to the Sumerian Inanna and to the cognate north-west Semitic goddess Astarte.-Characteristics:...

; Pyroeis after their god of war, Nergal
Nergal
The name Nergal, Nirgal, or Nirgali refers to a deity in Babylon with the main seat of his cult at Cuthah represented by the mound of Tell-Ibrahim. Nergal is mentioned in the Hebrew bible as the deity of the city of Cuth : "And the men of Babylon made Succoth-benoth, and the men of Cuth made Nergal"...

, Stilbon after their god of wisdom Nabu
Nabu
Nabu is the Assyrian and Babylonian god of wisdom and writing, worshipped by Babylonians as the son of Marduk and his consort, Sarpanitum, and as the grandson of Ea. Nabu's consort was Tashmetum....

, and Phaethon after their chief god, Marduk
Marduk
Marduk was the Babylonian name of a late-generation god from ancient Mesopotamia and patron deity of the city of Babylon, who, when Babylon became the political center of the Euphrates valley in the time of Hammurabi , started to...

. There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately. The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire
Roman Empire
The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

 and, later, the Catholic Church, use the Roman (or Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon
Roman mythology
Roman mythology is the body of traditional stories pertaining to ancient Rome's legendary origins and religious system, as represented in the literature and visual arts of the Romans...

 under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

. During the later period of the Roman Republic
Roman Republic
The Roman Republic was the period of the ancient Roman civilization where the government operated as a republic. It began with the overthrow of the Roman monarchy, traditionally dated around 508 BC, and its replacement by a government headed by two consuls, elected annually by the citizens and...

, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable. When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius
Mercury (mythology)
Mercury was a messenger who wore winged sandals, and a god of trade, the son of Maia Maiestas and Jupiter in Roman mythology. His name is related to the Latin word merx , mercari , and merces...

(for Hermes), Venus
Venus (mythology)
Venus is a Roman goddess principally associated with love, beauty, sex,sexual seduction and fertility, who played a key role in many Roman religious festivals and myths...

(Aphrodite), Mars
Mars (mythology)
Mars was the Roman god of war and also an agricultural guardian, a combination characteristic of early Rome. He was second in importance only to Jupiter, and he was the most prominent of the military gods worshipped by the Roman legions...

(Ares), Iuppiter
Jupiter (mythology)
In ancient Roman religion and myth, Jupiter or Jove is the king of the gods, and the god of the sky and thunder. He is the equivalent of Zeus in the Greek pantheon....

(Zeus) and Saturnus
Saturn (mythology)
In ancient Roman religion and myth, Saturn was a major god presiding over agriculture and the harvest time. His reign was depicted as a Golden Age of abundance and peace by many Roman authors. In medieval times he was known as the Roman god of agriculture, justice and strength. He held a sickle in...

(Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus
Neptune (mythology)
Neptune was the god of water and the sea in Roman mythology and religion. He is analogous with, but not identical to, the Greek god Poseidon. In the Greek-influenced tradition, Neptune was the brother of Jupiter and Pluto, each of them presiding over one of the three realms of the universe,...

(Poseidon
Poseidon
Poseidon was the god of the sea, and, as "Earth-Shaker," of the earthquakes in Greek mythology. The name of the sea-god Nethuns in Etruscan was adopted in Latin for Neptune in Roman mythology: both were sea gods analogous to Poseidon...

). Uranus is unique in that it is named by a Greek deity
Uranus (mythology)
Uranus , was the primal Greek god personifying the sky. His equivalent in Roman mythology was Caelus. In Ancient Greek literature, according to Hesiod in his Theogony, Uranus or Father Sky was the son and husband of Gaia, Mother Earth...

 rather than his Roman counterpart
Caelus
Caelus or Coelus was a primal god of the sky in Roman myth and theology, iconography, and literature...

.

Some Romans
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

, following a belief possibly originating in Mesopotamia
Mesopotamia
Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...

 but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet). Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the days of the week
Week-day names
The names of the days of the week from the Roman period have been both named after the seven planets of classical astronomy and numbered, beginning with Monday. In Slavic languages, a numbering system was adopted, but beginning with Monday. There was an even older tradition of names in Ancient...

 in the Roman calendar
Roman calendar
The Roman calendar changed its form several times in the time between the founding of Rome and the fall of the Roman Empire. This article generally discusses the early Roman or pre-Julian calendars...

 after the Nundinal cycle was rejected – and still preserved in many modern languages. Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden
Woden
Woden or Wodan is a major deity of Anglo-Saxon and Continental Germanic polytheism. Together with his Norse counterpart Odin, Woden represents a development of the Proto-Germanic god *Wōdanaz....

(Wednesday), Thunor
Thor
In Norse mythology, Thor is a hammer-wielding god associated with thunder, lightning, storms, oak trees, strength, the protection of mankind, and also hallowing, healing, and fertility...

(Thursday), and Fríge
Frige
*Frijjō is the reconstructed name or epithet of a hypothesized Common Germanic love goddess giving rise to both Frigg and Freyja....

(Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.

Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Since it was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god (the same is true, in English at least, of the Sun and the Moon, though they are no longer considered planets). The name originates from the 8th century Anglo-Saxon
Old English language
Old English or Anglo-Saxon is an early form of the English language that was spoken and written by the Anglo-Saxons and their descendants in parts of what are now England and southeastern Scotland between at least the mid-5th century and the mid-12th century...

 word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300. As with its equivalents in the other Germanic languages
Germanic languages
The Germanic languages constitute a sub-branch of the Indo-European language family. The common ancestor of all of the languages in this branch is called Proto-Germanic , which was spoken in approximately the mid-1st millennium BC in Iron Age northern Europe...

, it derives ultimately from the Proto-Germanic word ertho, "ground," as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jord. Many of the Romance languages
Romance languages
The Romance languages are a branch of the Indo-European language family, more precisely of the Italic languages subfamily, comprising all the languages that descend from Vulgar Latin, the language of ancient Rome...

 retain the old Roman word terra
Terra (mythology)
Terra or Tellus was a goddess personifying the Earth in Roman mythology. The names Terra Mater and Tellus Mater both mean "Mother Earth" in Latin; Mater is an honorific title also bestowed on other goddesses...

(or some variation of it) that was used with the meaning of "dry land" (as opposed to "sea"). However, the non-Romance languages use their own respective native words. The Greeks retain their original name, Γή
Gaia (mythology)
Gaia was the primordial Earth-goddess in ancient Greek religion. Gaia was the great mother of all: the heavenly gods and Titans were descended from her union with Uranus , the sea-gods from her union with Pontus , the Giants from her mating with Tartarus and mortal creatures were sprung or born...

(Ge or Yi).

Non-European cultures use other planetary naming systems. India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

 uses a naming system based on the Navagraha
Navagraha
Graha is a 'cosmic influencer' on the living beings of mother Bhumidevi . In Hindu astrology, the Navagraha are some of these major influencers.All the navagraha have relative movement with respect to the background of fixed stars in the zodiac...

, which incorporates the seven traditional planets (Surya
Surya
Surya Suraya or Phra Athit is the chief solar deity in Hinduism, one of the Adityas, son of Kasyapa and one of his wives, Aditi; of Indra; or of Dyaus Pitar . The term Surya also refers to the Sun, in general. Surya has hair and arms of gold...

 for the Sun, Chandra
Chandra
In Hinduism, Chandra is a lunar deity and a Graha. Chandra is also identified with the Vedic Lunar deity Soma . The Soma name refers particularly to the juice of sap in the plants and thus makes the Moon the lord of plants and vegetation. He is described as young, beautiful, fair; two-armed and...

 for the Moon, and Budha
Budha
In Hindu mythology, Budha is the name for the planet Mercury, a son of Chandra with Tara or Rohini. He is also the god of merchandise and protector of Merchants....

, Shukra
Shukra
Shukra , the Sanskrit for "clear, pure" or "brightness, clearness", is the name of the son of Bhrigu, and preceptor of the Daityas, and the guru of the Asuras, identified with the planet Venus, one of the Navagrahas...

, Mangala
Mangala
In Jyotish astrology, Mangala is the name for Mars, the red planet. Mars is also called Angaraka In Jyotish astrology, Mangala (Devanagari: मंगल) is the name for Mars, the red planet. Mars is also called Angaraka In Jyotish astrology, Mangala (Devanagari: मंगल) is the name for Mars, the red...

,
{{About|the astronomical object}}
{{pp-move-indef}}
{{Multiple image|direction=vertical|align=right|image1=1e7m comparison Uranus Neptune Sirius B Earth Venus.png|image2=1e6m comparison Mars Mercury Moon Pluto Haumea - no transparency.png|width=300|caption2=Planetary-sized objects to scale:
Top row: Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

;
second row: Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...

 star Sirius B, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

;
bottom row (reproduced and enlarged in lower image) – above: Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 and Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

;
below: the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, dwarf planets Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 and Haumea}}

A planet (from Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 {{lang|grc|πλανήτης αστήρ}} planētēs astēr "wandering star") is a celestial body
Celestial Body
Celestial Body is a Croatian film directed by Lukas Nola. It was released in 2000....

 orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

ing a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...

s.{{Ref label|A|a|none}}
The term planet is ancient, with ties to history
History
History is the discovery, collection, organization, and presentation of information about past events. History can also mean the period of time after writing was invented. Scholars who write about history are called historians...

, science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

, mythology
Mythology
The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

, and religion
Religion
Religion is a collection of cultural systems, belief systems, and worldviews that establishes symbols that relate humanity to spirituality and, sometimes, to moral values. Many religions have narratives, symbols, traditions and sacred histories that are intended to give meaning to life or to...

. The planets were originally seen by many early cultures as divine, or as emissaries of the god
God
God is the English name given to a singular being in theistic and deistic religions who is either the sole deity in monotheism, or a single deity in polytheism....

s. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 officially adopted a resolution defining planets within the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were thought by Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 to orbit the Earth in deferent and epicycle
Deferent and epicycle
In the Ptolemaic system of astronomy, the epicycle was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets...

 motions. Though the idea that the planets orbited the Sun
Heliocentrism
Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a stationary Sun at the center of the universe. The word comes from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center...

 had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 astronomical observations, performed by Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

. By careful analysis of the observation data, Johannes Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

 found the planets' orbits to be not circular, but elliptical
Elliptic orbit
In astrodynamics or celestial mechanics an elliptic orbit is a Kepler orbit with the eccentricity less than 1; this includes the special case of a circular orbit, with eccentricity equal to zero. In a stricter sense, it is a Kepler orbit with the eccentricity greater than 0 and less than 1 . In a...

. As observational tools improved, astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

s saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice cap
Ice cap
An ice cap is an ice mass that covers less than 50 000 km² of land area . Masses of ice covering more than 50 000 km² are termed an ice sheet....

s and season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

s. Since the dawn of the Space Age
Space Age
The Space Age is a time period encompassing the activities related to the Space Race, space exploration, space technology, and the cultural developments influenced by these events. The Space Age is generally considered to have begun with Sputnik...

, close observation by probes
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

 has found that Earth and the other planets share characteristics such as volcanism
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

, hurricanes, tectonics
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...

, and even hydrology
Hydrology
Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...

.

Planets are generally divided into two main types: large, low-density gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

s, and smaller, rocky terrestrials
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, they are the four terrestrials, Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

, Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, then the four gas giants, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

, Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

, Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

, and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

. Six of the planets are orbited by one or more natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

s.
Additionally, the Solar System also contains at least five dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s and hundreds of thousands of small Solar System bodies
Small solar system body
A small Solar System body is an object in the Solar System that is neither a planet nor a dwarf planet, nor a satellite of a planet or dwarf planet:...

.

Since 1992, hundreds of planets around other stars ("extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

s" or "exoplanets") in the Milky Way Galaxy
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 have been discovered. As of {{Extrasolar planet counts|asof}}, {{Extrasolar planet counts|planet_count}} known extrasolar planets are listed in the Extrasolar Planets Encyclopaedia
Extrasolar Planets Encyclopaedia
The Extrasolar Planets Encyclopedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains a database of all the currently known and candidate extrasolar planets, with individual "note" pages for each planet and a full list...

, ranging from the size of terrestrial planets somewhat larger than Earth to gas giants larger than Jupiter.

History


{{See|History of astronomy|Definition of planet}}
{{See also|Timeline of solar system astronomy}}
The idea of planets has evolved over its history, from the divine wandering stars
Wandering Stars
Wandering Stars is an anthology of Jewish fantasy and science fiction, edited by Jack Dann, originally published by Harper & Row in 1974. It represented, according to the book cover, "the first time in science fiction that the Jew - and the richness of his themes and particular points of view --...

 of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.

The five classical planets, being visible to the naked eye, have been known since ancient times, and have had a significant impact on mythology
Mythology
The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

, religious cosmology
Religious cosmology
A Religious cosmology is a way of explaining the origin, the history and the evolution of the universe based on the religious mythology of a specific tradition...

, and ancient astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

. In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights {{lang|grc|πλάνητες ἀστέρες}} ({{Transl|grc|planetes asteres}} "wandering stars") or simply "{{lang|grc|πλανήτοι}}" ({{Transl|grc|planētoi}} "wanderers"), from which today's word "planet" was derived. In ancient Greece
Greece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....

, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

, Babylon
Babylon
Babylon was an Akkadian city-state of ancient Mesopotamia, the remains of which are found in present-day Al Hillah, Babil Province, Iraq, about 85 kilometers south of Baghdad...

 and indeed all pre-modern civilisations, it was almost universally believed that Earth was in the center of the Universe and that all the "planets" circled the Earth. The reasons for this perception were that stars and planets appeared to revolve around the Earth each day, and the apparently common-sense
Common sense
Common sense is defined by Merriam-Webster as, "sound and prudent judgment based on a simple perception of the situation or facts." Thus, "common sense" equates to the knowledge and experience which most people already have, or which the person using the term believes that they do or should have...

 perception that the Earth was solid and stable, and that it was not moving but at rest.

The name for planets in Chinese astronomy
Chinese astronomy
Astronomy in China has a very long history, with historians considering that "they [the Chinese] were the most persistent and accurate observers of celestial phenomena anywhere in the world before the Arabs."...

 had the same motive as the Greek name, 行星 "moving star". In Japanese during the Edo period
Edo period
The , or , is a division of Japanese history which was ruled by the shoguns of the Tokugawa family, running from 1603 to 1868. The political entity of this period was the Tokugawa shogunate....

 there were two competing terms, 惑星 "confused star" and 遊星 "wandering star". In modern Japan, terminology was unified in favour of 惑星, but in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 the alternative term 遊星 retains some currency.{{citation needed|date=October 2011}}

Babylon


{{Main|Babylonian astronomy}}
The first civilization known to possess a functional theory of the planets were the Babylonia
Babylonia
Babylonia was an ancient cultural region in central-southern Mesopotamia , with Babylon as its capital. Babylonia emerged as a major power when Hammurabi Babylonia was an ancient cultural region in central-southern Mesopotamia (present-day Iraq), with Babylon as its capital. Babylonia emerged as...

ns, who lived in Mesopotamia
Mesopotamia
Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...

 in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa
Venus tablet of Ammisaduqa
The Venus tablet of Ammisaduqa refers to the record of astronomical observations of Venus, as preserved in numerous cuneiform tablets dating from the first millennium BCE. It is believed that this astronomical record was first compiled during the reign of King Ammisaduqa , the fourth ruler after...

, a 7th century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC. The MUL.APIN
MUL.APIN
MUL.APIN is the conventional title given to a Babylonian compendium that deals with many diverse aspects of Babylonian astronomy and astrology....

 is a pair of cuneiform
Cuneiform
Cuneiform can refer to:*Cuneiform script, an ancient writing system originating in Mesopotamia in the 4th millennium BC*Cuneiform , three bones in the human foot*Cuneiform Records, a music record label...

 tablets dating from the 7th century BC that lays out the motions of the Sun, Moon and planets over the course of the year. The Babylonian astrologers
Babylonian astrology
In Babylon as well as in Assyria as a direct offshoot of Babylonian culture, astrology takes its place in theofficial cult as one of the two chief means at the disposal of the priests for ascertaining the will and intention of the gods, the other being through the inspection of the liver of the...

 also laid the foundations of what would eventually become Western astrology
Western astrology
Western astrology is the system of astrology most popular in Western countries. Western astrology is historically based on Ptolemy's Tetrabiblos , which in turn was a continuation of Hellenistic and ultimately Babylonian traditions....

. The Enuma anu enlil
Enuma anu enlil
Enuma Anu Enlil is a major series of 68 or 70 tablets dealing with Babylonian astrology...

, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omen
Omen
An omen is a phenomenon that is believed to foretell the future, often signifying the advent of change...

s and their relationships with various celestial phenomena including the motions of the planets.

The Sumer
Sumer
Sumer was a civilization and historical region in southern Mesopotamia, modern Iraq during the Chalcolithic and Early Bronze Age....

ians, predecessors of the Babylonians who are considered as one of the first civilizations
Cradle of Civilization
The cradle of civilization is a term referring to any of the possible locations for the emergence of civilization.It is usually applied to the Ancient Near Eastern Chalcolithic , especially in the Fertile Crescent , but also extended to sites in Armenia, and the Persian Plateau, besides other Asian...

 and are credited with the invention of writing
Writing
Writing is the representation of language in a textual medium through the use of a set of signs or symbols . It is distinguished from illustration, such as cave drawing and painting, and non-symbolic preservation of language via non-textual media, such as magnetic tape audio.Writing most likely...

, had identified at least Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 by 1500 BC. Shortly afterwards, the other inner planet Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

 and the outer planets Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 in early modern times.

Greco-Roman astronomy


{{See also|Greek astronomy}}
Ptolemy's 7 planetary spheres
1
Moon
2
Mercury
3
Venus
4
Sun
5
Mars
6
Jupiter
7
Saturn


Because they were not as interested in divination as the Babylonians, the ancient Greeks initially did not attach as much significance to the planets. The Pythagoreans, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras
Pythagoras
Pythagoras of Samos was an Ionian Greek philosopher, mathematician, and founder of the religious movement called Pythagoreanism. Most of the information about Pythagoras was written down centuries after he lived, so very little reliable information is known about him...

 or Parmenides
Parmenides
Parmenides of Elea was an ancient Greek philosopher born in Elea, a Greek city on the southern coast of Italy. He was the founder of the Eleatic school of philosophy. The single known work of Parmenides is a poem, On Nature, which has survived only in fragmentary form. In this poem, Parmenides...

 are said to have first identified the evening star and morning star (Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

) as one and the same. In the 3rd century BC, Aristarchus of Samos
Aristarchus of Samos
Aristarchus, or more correctly Aristarchos , was a Greek astronomer and mathematician, born on the island of Samos, in Greece. He presented the first known heliocentric model of the solar system, placing the Sun, not the Earth, at the center of the known universe...

 proposed a heliocentric
Heliocentrism
Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a stationary Sun at the center of the universe. The word comes from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center...

 system, according to which the Earth and planets revolved around the sun. However, the geocentric system would remain dominant until the Scientific Revolution
Scientific revolution
The Scientific Revolution is an era associated primarily with the 16th and 17th centuries during which new ideas and knowledge in physics, astronomy, biology, medicine and chemistry transformed medieval and ancient views of nature and laid the foundations for modern science...

.

By the 1st century BC, during the Hellenistic period
Hellenistic period
The Hellenistic period or Hellenistic era describes the time which followed the conquests of Alexander the Great. It was so named by the historian J. G. Droysen. During this time, Greek cultural influence and power was at its zenith in Europe and Asia...

, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest
Almagest
The Almagest is a 2nd-century mathematical and astronomical treatise on the apparent motions of the stars and planetary paths. Written in Greek by Claudius Ptolemy, a Roman era scholar of Egypt,...

written by Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 in the 2nd century CE. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries. To the Greeks and Romans there were seven known planets, each presumed to be circling the Earth
Geocentric model
In astronomy, the geocentric model , is the superseded theory that the Earth is the center of the universe, and that all other objects orbit around it. This geocentric model served as the predominant cosmological system in many ancient civilizations such as ancient Greece...

 according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.

India


{{Main|Indian astronomy|Hindu cosmology}}

In 499 CE, the Indian astronomer Aryabhata
Aryabhata
Aryabhata was the first in the line of great mathematician-astronomers from the classical age of Indian mathematics and Indian astronomy...

 propounded a planetary model which explicitly incorporated the Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbit of planets are elliptical
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...

.{{Verify credibility|date=January 2011}}
Aryabhata's followers were particularly strong in South India
South India
South India is the area encompassing India's states of Andhra Pradesh, Karnataka, Kerala and Tamil Nadu as well as the union territories of Lakshadweep and Pondicherry, occupying 19.31% of India's area...

, where his principles of the diurnal rotation of the earth, among others, were followed and a number of secondary works were based on them. {{Page needed|date=October 2010}}

In 1500, Nilakantha Somayaji
Nilakantha Somayaji
Kelallur Nilakantha Somayaji was a major mathematician and astronomer of the Kerala school of astronomy and mathematics. One of his most influential works was the comprehensive astronomical treatise Tantrasamgraha completed in 1501...

 of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model. In his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits the Earth, similar to the Tychonic system
Tychonic system
The Tychonic system was a model of the solar system published by Tycho Brahe in the late 16th century which combined what he saw as the mathematical benefits of the Copernican system with the philosophical and "physical" benefits of the Ptolemaic system...

 later proposed by Tycho Brahe
Tycho Brahe
Tycho Brahe , born Tyge Ottesen Brahe, was a Danish nobleman known for his accurate and comprehensive astronomical and planetary observations...

 in the late 16th century. Most astronomers of the Kerala school who followed him accepted his planetary model.

Medieval Muslim astronomy


{{Main|Astronomy in medieval Islam|Islamic cosmology}}

In the 11th century, the transit of Venus
Transit of Venus
A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, becoming visible against the solar disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun...

 was observed by Avicenna
Avicenna
Abū ʿAlī al-Ḥusayn ibn ʿAbd Allāh ibn Sīnā , commonly known as Ibn Sīnā or by his Latinized name Avicenna, was a Persian polymath, who wrote almost 450 treatises on a wide range of subjects, of which around 240 have survived...

, who established that Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 was, at least sometimes, below the Sun. In the 12th century, Ibn Bajjah
Ibn Bajjah
Abū-Bakr Muhammad ibn Yahya ibn al-Sāyigh , known as Ibn Bājjah , was an Andalusian polymath: an astronomer, logician, musician, philosopher, physician, physicist, psychologist, botanist, poet and scientist. He was known in the West by his Latinized name, Avempace...

 observed "two planets as black spots on the face of the Sun," which was later identified as a transit of Mercury
Transit of Mercury
A transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun....

 and Venus by the Maragha
Maragheh observatory
Maragheh observatory is an astronomical observatory which was established in 1259 CE by Nasir al-Din al-Tusi, an Iranian scientist and astronomer...

 astronomer Qotb al-Din Shirazi in the 13th century. However, Ibn Bajjah could not have observed a transit of Venus, as none occurred in his lifetime.{{Citation needed|date=August 2011}}

European Renaissance

Renaissance planets, ca. 1543 to 1781
1
Mercury
2
Venus
3
Earth
4
Mars
5
Jupiter
6
Saturn

{{See also|Heliocentrism}}
With the advent of the Scientific Revolution
Scientific revolution
The Scientific Revolution is an era associated primarily with the 16th and 17th centuries during which new ideas and knowledge in physics, astronomy, biology, medicine and chemistry transformed medieval and ancient views of nature and laid the foundations for modern science...

, understanding of the term "planet" changed from something that moved across the sky (in relation to the star field); to a body that orbited the Earth (or that were believed to do so at the time); and in the 16th century to something that directly orbited the Sun when the heliocentric model of Copernicus
Nicolaus Copernicus
Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....

, Galileo
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

 and Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

 gained sway.

Thus the Earth became included in the list of planets, while the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century. Until the mid-19th century, the number of "planets" rose rapidly since any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community.

19th century

New planets, 1807–1845
1
Mercury
2
Venus
3
Earth
4
Mars
5
Vesta
6
Juno
7
Ceres
8
Pallas
9
Jupiter
10
Saturn
11
Uranus

In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas
2 Pallas
Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered , and one of the largest. It is estimated to constitute 7% of the mass of the asteroid belt, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta. It is however 20%...

, and Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

) were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the Asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

), and had a much smaller mass; as a result they were reclassified as "asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s." In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Since there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846, there was no apparent need to have a formal definition.

20th century

Planets 1854–1930, 2006–present
1
Mercury
2
Venus
3
Earth
4
Mars
5
Jupiter
6
Saturn
7
Uranus
8
Neptune

However, in the 20th century, Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 was discovered. After initial observations led to the belief it was larger than Earth, the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton
Raymond Lyttleton
Raymond Arthur Lyttleton FRS was a British mathematician and theoretical astronomer.He was born in the Oldbury, Worcestershire area and educated at King Edward VI Five Ways school in Birmingham, going from there to Clare College, Cambridge to read mathematics, graduating in 1933...

 suggested that Pluto may be an escaped satellite of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

, and Fred Whipple
Fred Lawrence Whipple
Fred Lawrence Whipple was an American astronomer, who worked at the Harvard College Observatory for over 70 years...

 suggested in 1964 that Pluto may be a comet. However, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status until 2006.
Planets 1930–2006
1
Mercury
2
Venus
3
Earth
4
Mars
5
Jupiter
6
Saturn
7
Uranus
8
Neptune
9
Pluto

In 1992, astronomers Aleksander Wolszczan
Aleksander Wolszczan
Aleksander Wolszczan is a Polish astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets.- Scientific career :...

 and Dale Frail
Dale Frail
Dale A. Frail is an astronomer working at the National Radio Astronomy Observatory in Socorro, New Mexico. He was born in Canada, spent much of his childhood in Europe, and his professional career has been based in the United States.-Career:...

 announced the discovery of planets around a pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

, PSR B1257+12
PSR B1257+12
PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located roughly 2000 light-years from the Sun. In 2007, it was confirmed that three extrasolar planets orbit the pulsar.- Pulsar :...

. This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor
Michel Mayor
Dr. Michel G. E. Mayor is a Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the Observatory of Geneva...

 and Didier Queloz
Didier Queloz
Didier Queloz is a Geneva-based astronomer with a prolific record in finding extrasolar planets. He is understudy to Michel Mayor.Didier Queloz was a Ph.D...

 of the University of Geneva
University of Geneva
The University of Geneva is a public research university located in Geneva, Switzerland.It was founded in 1559 by John Calvin, as a theological seminary and law school. It remained focused on theology until the 17th century, when it became a center for Enlightenment scholarship. In 1873, it...

 announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 star (51 Pegasi
51 Pegasi
51 Pegasi is a Sun-like star located 15.6 parsecs from Earth in the constellation Pegasus...

).

The discovery of extrasolar planets led to another ambiguity in defining a planet; the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as "brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

s". Brown dwarfs are generally considered stars due to their ability to fuse deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

, a heavier isotope of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

. While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.

21st century


With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

, or if it was large enough to generate energy by the thermonuclear fusion of deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

.

A growing number of astronomers argued for Pluto to be declassified as a planet, since many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands.

Some of them including Quaoar
50000 Quaoar
50000 Quaoar is a rocky trans-Neptunian object in the Kuiper belt with one known moon. Discovered on June 4, 2002 by astronomers Chad Trujillo and Michael Brown at the California Institute of Technology from images acquired at the Samuel Oschin Telescope at Palomar Observatory, it is thought by...

, Sedna
90377 Sedna
90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...

, and Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

 were heralded in the popular press as the tenth planet, failing however to receive widespread scientific recognition. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the necessity and public desire for an official definition of a planet.

Acknowledging the problem, the IAU set about creating the definition of planet
Definition of planet
The definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai , "wandering stars", for objects which apparently move over the sky...

, and produced one in August 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit
Clearing the neighbourhood
"Clearing the neighbourhood of its orbit" is a criterion for a celestial body to be considered a planet in the Solar System. This was one of the three criteria adopted by the International Astronomical Union in its 2006 definition of planet....

 (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s was created, initially containing three objects (Ceres, Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 and Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

).

Extrasolar planet definition


In 2003, The International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated the following working definition, mostly focused upon the boundary between planets and brown dwarves:
{{TNO imagemap}}
  1. Objects with true mass
    True mass
    The term true mass is synonymous with the term mass, but is used in astronomy to differentiate the measured mass of a planet from the lower limit of mass usually obtained from radial velocity techniques...

    es below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance
    Natural abundance
    In chemistry, natural abundance refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass of these isotopes is the atomic weight listed for the element in the periodic table...

     as the Sun) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
  2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarf
    Brown dwarf
    Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

    s", no matter how they formed or where they are located.
  3. Free-floating objects in young star cluster
    Star cluster
    Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...

    s with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).


This definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journal
Academic journal
An academic journal is a peer-reviewed periodical in which scholarship relating to a particular academic discipline is published. Academic journals serve as forums for the introduction and presentation for scrutiny of new research, and the critique of existing research...

s. Although temporary, it remains an effective working definition until a more permanent one is formally adopted. However, it does not address the dispute over the lower mass limit, and so it steered clear of the controversy regarding objects within the Solar System. This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2M1207b
2M1207b
2M1207b is a planetary-mass object orbiting the brown dwarf 2M1207, in the constellation Centaurus, approximately 170 light-years from Earth...

.

One definition of a sub-brown dwarf
Sub-brown dwarf
A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass for thermonuclear fusion of deuterium ....

 is a planet-mass object that formed through cloud-collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.
One reason for the dissent is that oftentimes it may not be possible to determine the formation process: for example an accretion-formed planet around a star may get ejected from the system to become free-floating, and likewise a cloud-collapse-formed sub-brown dwarf formed on its own in a star cluster may get captured into orbit around a star.
Dwarf planets 2006–present
Ceres Pluto Makemake Haumea Eris


The 13 Jupiter-mass cutoff is a rule of thumb rather than something of precise physical significance. The question arises: what is meant by deuterium burning
Deuterium burning
Deuterium burning is a nuclear fusion reaction that occurs in stars and some substellar objects, in which a deuterium nucleus and a proton combine to form a helium-3 nucleus...

? This question arises because large objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 MJ value is somewhere in between. The amount of deuterium burnt also depends not only on mass but on the composition of the planet, on the amount of helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 and deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 present.

Another criterion for separating planets and brown dwarfs, rather than deuterium burning, formation process or location is whether the core pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 is dominated by coulomb pressure or electron degeneracy.

2006 definition


{{Main|IAU definition of planet}}
The matter of the lower limit was addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, the assembly voted to pass a resolution that defined planets within the Solar System as:

{{quotation|A celestial body that is (a) in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...

 (nearly round) shape, and (c) has cleared the neighbourhood
Clearing the neighbourhood
"Clearing the neighbourhood of its orbit" is a criterion for a celestial body to be considered a planet in the Solar System. This was one of the three criteria adopted by the International Astronomical Union in its 2006 definition of planet....

 around its orbit.}}

Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto, Makemake and Eris) are classified as dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s, provided they are not also natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

s of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion. After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.

This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter
Steven Soter
Dr. Steven Soter, PhD, is an astrophysicist currently holding the positions of scientist-in-residence for New York University's Environmental Studies Program and of Research Associate for the Department of Astrophysics at the American Museum of Natural History...

:

{{quotation|The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Asteroids and comets, including KBOs [Kuiper belt objects], differ from planets in that they can collide with each other and with planets.}}

In the aftermath of the IAU's 2006 vote, there has been controversy and debate about the definition, and many astronomers have stated that they will not use it. Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition.

Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status since its discovery in 1930. The discovery of Eris was widely reported in the media
Mass media
Mass media refers collectively to all media technologies which are intended to reach a large audience via mass communication. Broadcast media transmit their information electronically and comprise of television, film and radio, movies, CDs, DVDs and some other gadgets like cameras or video consoles...

 as the tenth planet and therefore the reclassification of all three objects as dwarf planets has attracted a lot of media and public attention as well.

Former classifications


The table below lists Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 bodies formerly considered to be planets:
Body (current classification) Notes
Star Dwarf planet Asteroid Moon
Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 
The Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

Classified as planets in antiquity
Ancient history
Ancient history is the study of the written past from the beginning of recorded human history to the Early Middle Ages. The span of recorded history is roughly 5,000 years, with Cuneiform script, the oldest discovered form of coherent writing, from the protoliterate period around the 30th century BC...

, in accordance with the definition then used.
Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

, Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

, Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...

, and Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...

The four largest moons of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

, known as the Galilean moons
Galilean moons
The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance...

 after their discoverer Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

. He referred to them as the "Medicean Planets" in honor of his patron
Patronage
Patronage is the support, encouragement, privilege, or financial aid that an organization or individual bestows to another. In the history of art, arts patronage refers to the support that kings or popes have provided to musicians, painters, and sculptors...

, the Medici family
Medici
The House of Medici or Famiglia de' Medici was a political dynasty, banking family and later royal house that first began to gather prominence under Cosimo de' Medici in the Republic of Florence during the late 14th century. The family originated in the Mugello region of the Tuscan countryside,...

.
Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

,{{Ref label|B|b|none}} Iapetus
Iapetus (moon)
Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671...

,{{Ref label|C|c|none}} Rhea
Rhea (moon)
Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.-Name:Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods"...

,{{Ref label|C|c|none}} Tethys
Tethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...

,{{Ref label|D|d|none}} and Dione
Dione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...

{{Ref label|D|d|none}}
Five of Saturn's larger moons, discovered by Christiaan Huygens and Giovanni Domenico Cassini
Giovanni Domenico Cassini
This article is about the Italian-born astronomer. For his French-born great-grandson, see Jean-Dominique Cassini.Giovanni Domenico Cassini was an Italian/French mathematician, astronomer, engineer, and astrologer...

.
Ceres{{Ref label|E|e|none}} Pallas
2 Pallas
Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered , and one of the largest. It is estimated to constitute 7% of the mass of the asteroid belt, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta. It is however 20%...

, Juno
3 Juno
Juno , formal designation 3 Juno in the Minor Planet Center catalogue system, was the third asteroid to be discovered and is one of the larger main-belt asteroids, being one of the two largest stony asteroids, along with 15 Eunomia. Juno is estimated to contain 1% of the total mass of the asteroid...

, and Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

 
The first known asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s.
Ceres has subsequently been classified as a dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

 in 2006.
Astrea
5 Astraea
5 Astraea is a large main-belt asteroid. Its surface is highly reflective and its composition is probably a mixture of nickel-iron with magnesium- and iron-silicates....

, Hebe
6 Hebe
6 Hebe is a large main-belt asteroid, containing around half a percent of the mass of the belt. Its apparently high bulk density , however, means that by volume it does not rank among the top twenty asteroids...

, Iris
7 Iris
7 Iris is a large main-belt asteroid. Among the S-type asteroids, it ranks fifth in geometric mean diameter after Eunomia, Juno, Amphitrite and Herculina....

, Flora
8 Flora
8 Flora is a large, bright main-belt asteroid. It is the innermost large asteroid: no asteroid closer to the Sun has a diameter above 25 kilometres or two-elevenths that of Flora itself, and not until the tiny 149 Medusa was discovered was a single asteroid orbiting at a closer mean distance...

, Metis
9 Metis
9 Metis is one of the larger main-belt asteroids. It is composed of silicates and metallic nickel-iron, and may be the core remnant of a large asteroid that was destroyed by an ancient collision...

, Hygeia
10 Hygiea
10 Hygiea is an asteroid located in the asteroid belt. With somewhat oblong diameters of 350–500 km, and a mass estimated to be 2.9% of the total mass of the belt, it is the fourth largest asteroid by volume and mass...

, Parthenope
11 Parthenope
11 Parthenope is a large, bright main-belt asteroid.Parthenope was discovered by Annibale de Gasparis on May 11, 1850, the second of his nine asteroid discoveries. It was named after one of the Sirens in Greek mythology, said to have founded the city of Naples...

, Victoria
12 Victoria
12 Victoria is a large main-belt asteroid.It was discovered by J. R. Hind on September 13, 1850.Victoria is officially named after the Roman goddess of victory, but the name also honours Queen Victoria. The goddess Victoria was the daughter of Styx by the Titan Pallas...

, Egeria
13 Egeria
13 Egeria is a large main-belt G-type asteroid.It was discovered by A. de Gasparis on November 2, 1850, and was named by Urbain J. J. Le Verrier, whose computations led to the discovery of Neptune. Egeria was a goddess of Aricia, in Italy, and the wife of Numa Pompilius, second king of...

, Irene
14 Irene
14 Irene is a very large main-belt asteroid.14 Irene was discovered by J. R. Hind on May 19, 1851, and named after Eirene, a personification of peace in Greek mythology. She was one of the Horae, daughter of Zeus and Themis. The name was suggested by Sir John Herschel...

, Eunomia
15 Eunomia
15 Eunomia is a very large asteroid in the inner asteroid belt. It is the largest of the stony asteroids, and somewhere between the 8th-to-12th-largest main-belt asteroid overall...

 
More asteroids, discovered between 1845 and 1851. The rapidly expanding list of planets prompted their reclassification as asteroids by astronomers, and this was widely accepted by 1854.
Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

{{Ref label|F|f|none}}
The first known Trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

 (i.e. minor planet with a semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

 beyond Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

). In 2006, Pluto was reclassified as a dwarf planet.
Eris (dwarf planet)
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

 (originally nicknamed Xena)
Discovered in 2003, this Trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

 (i.e. minor planet with a semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

 beyond Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

) was recognised in 2005, before, like Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

, in 2006 getting reclassified as a dwarf planet.

Mythology and naming


{{See also|Weekday names|Naked-eye planet}}
The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

, the two great luminaries the Sun and the Moon were called Helios
Helios
Helios was the personification of the Sun in Greek mythology. Homer often calls him simply Titan or Hyperion, while Hesiod and the Homeric Hymn separate him as a son of the Titans Hyperion and Theia or Euryphaessa and brother of the goddesses Selene, the moon, and Eos, the dawn...

and Selene
Selene
In Greek mythology, Selene was an archaic lunar deity and the daughter of the Titans Hyperion and Theia. In Roman mythology, the moon goddess is called Luna, Latin for "moon"....

; the farthest planet was called Phainon, the shiner; followed by Phaethon, "bright"; the red planet was known as Pyroeis, the "fiery"; the brightest was known as Phosphoros, the light bringer; and the fleeting final planet was called Stilbon, the gleamer. The Greeks also made each planet sacred to one among their pantheon of gods, the Olympians
Twelve Olympians
The Twelve Olympians, also known as the Dodekatheon , in Greek mythology, were the principal deities of the Greek pantheon, residing atop Mount Olympus. Zeus, Hera, Poseidon, Demeter, Hestia, and Hades were siblings. Ares, Hermes, Hephaestus, Athena, Apollo, and Artemis were children of Zeus...

: Helios and Selene were the names of both planets and gods; Phainon was sacred to Cronus
Cronus
In Greek mythology, Cronus or Kronos was the leader and the youngest of the first generation of Titans, divine descendants of Gaia, the earth, and Uranus, the sky...

, the Titan
Titan (mythology)
In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age....

 who fathered the Olympians; Phaethon was sacred to Zeus
Zeus
In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...

, Cronus's son who deposed him as king; Pyroeis was given to Ares
Ares
Ares is the Greek god of war. He is one of the Twelve Olympians, and the son of Zeus and Hera. In Greek literature, he often represents the physical or violent aspect of war, in contrast to the armored Athena, whose functions as a goddess of intelligence include military strategy and...

, son of Zeus and god of war; Phosphoros was ruled by Aphrodite
Aphrodite
Aphrodite is the Greek goddess of love, beauty, pleasure, and procreation.Her Roman equivalent is the goddess .Historically, her cult in Greece was imported from, or influenced by, the cult of Astarte in Phoenicia....

, the goddess of love; and Hermes
Hermes
Hermes is the great messenger of the gods in Greek mythology and a guide to the Underworld. Hermes was born on Mount Kyllini in Arcadia. An Olympian god, he is also the patron of boundaries and of the travelers who cross them, of shepherds and cowherds, of the cunning of thieves, of orators and...

, messenger of the gods and god of learning and wit, ruled over Stilbon.

The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros after their goddess of love, Ishtar
Ishtar
Ishtar is the Assyrian and Babylonian goddess of fertility, love, war, and sex. She is the counterpart to the Sumerian Inanna and to the cognate north-west Semitic goddess Astarte.-Characteristics:...

; Pyroeis after their god of war, Nergal
Nergal
The name Nergal, Nirgal, or Nirgali refers to a deity in Babylon with the main seat of his cult at Cuthah represented by the mound of Tell-Ibrahim. Nergal is mentioned in the Hebrew bible as the deity of the city of Cuth : "And the men of Babylon made Succoth-benoth, and the men of Cuth made Nergal"...

, Stilbon after their god of wisdom Nabu
Nabu
Nabu is the Assyrian and Babylonian god of wisdom and writing, worshipped by Babylonians as the son of Marduk and his consort, Sarpanitum, and as the grandson of Ea. Nabu's consort was Tashmetum....

, and Phaethon after their chief god, Marduk
Marduk
Marduk was the Babylonian name of a late-generation god from ancient Mesopotamia and patron deity of the city of Babylon, who, when Babylon became the political center of the Euphrates valley in the time of Hammurabi , started to...

. There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately. The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire
Roman Empire
The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

 and, later, the Catholic Church, use the Roman (or Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon
Roman mythology
Roman mythology is the body of traditional stories pertaining to ancient Rome's legendary origins and religious system, as represented in the literature and visual arts of the Romans...

 under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

. During the later period of the Roman Republic
Roman Republic
The Roman Republic was the period of the ancient Roman civilization where the government operated as a republic. It began with the overthrow of the Roman monarchy, traditionally dated around 508 BC, and its replacement by a government headed by two consuls, elected annually by the citizens and...

, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable. When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius
Mercury (mythology)
Mercury was a messenger who wore winged sandals, and a god of trade, the son of Maia Maiestas and Jupiter in Roman mythology. His name is related to the Latin word merx , mercari , and merces...

(for Hermes), Venus
Venus (mythology)
Venus is a Roman goddess principally associated with love, beauty, sex,sexual seduction and fertility, who played a key role in many Roman religious festivals and myths...

(Aphrodite), Mars
Mars (mythology)
Mars was the Roman god of war and also an agricultural guardian, a combination characteristic of early Rome. He was second in importance only to Jupiter, and he was the most prominent of the military gods worshipped by the Roman legions...

(Ares), Iuppiter
Jupiter (mythology)
In ancient Roman religion and myth, Jupiter or Jove is the king of the gods, and the god of the sky and thunder. He is the equivalent of Zeus in the Greek pantheon....

(Zeus) and Saturnus
Saturn (mythology)
In ancient Roman religion and myth, Saturn was a major god presiding over agriculture and the harvest time. His reign was depicted as a Golden Age of abundance and peace by many Roman authors. In medieval times he was known as the Roman god of agriculture, justice and strength. He held a sickle in...

(Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus
Neptune (mythology)
Neptune was the god of water and the sea in Roman mythology and religion. He is analogous with, but not identical to, the Greek god Poseidon. In the Greek-influenced tradition, Neptune was the brother of Jupiter and Pluto, each of them presiding over one of the three realms of the universe,...

(Poseidon
Poseidon
Poseidon was the god of the sea, and, as "Earth-Shaker," of the earthquakes in Greek mythology. The name of the sea-god Nethuns in Etruscan was adopted in Latin for Neptune in Roman mythology: both were sea gods analogous to Poseidon...

). Uranus is unique in that it is named by a Greek deity
Uranus (mythology)
Uranus , was the primal Greek god personifying the sky. His equivalent in Roman mythology was Caelus. In Ancient Greek literature, according to Hesiod in his Theogony, Uranus or Father Sky was the son and husband of Gaia, Mother Earth...

 rather than his Roman counterpart
Caelus
Caelus or Coelus was a primal god of the sky in Roman myth and theology, iconography, and literature...

.

Some Romans
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

, following a belief possibly originating in Mesopotamia
Mesopotamia
Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...

 but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet). Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the days of the week
Week-day names
The names of the days of the week from the Roman period have been both named after the seven planets of classical astronomy and numbered, beginning with Monday. In Slavic languages, a numbering system was adopted, but beginning with Monday. There was an even older tradition of names in Ancient...

 in the Roman calendar
Roman calendar
The Roman calendar changed its form several times in the time between the founding of Rome and the fall of the Roman Empire. This article generally discusses the early Roman or pre-Julian calendars...

 after the Nundinal cycle was rejected – and still preserved in many modern languages. Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden
Woden
Woden or Wodan is a major deity of Anglo-Saxon and Continental Germanic polytheism. Together with his Norse counterpart Odin, Woden represents a development of the Proto-Germanic god *Wōdanaz....

(Wednesday), Thunor
Thor
In Norse mythology, Thor is a hammer-wielding god associated with thunder, lightning, storms, oak trees, strength, the protection of mankind, and also hallowing, healing, and fertility...

(Thursday), and Fríge
Frige
*Frijjō is the reconstructed name or epithet of a hypothesized Common Germanic love goddess giving rise to both Frigg and Freyja....

(Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.

Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Since it was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god (the same is true, in English at least, of the Sun and the Moon, though they are no longer considered planets). The name originates from the 8th century Anglo-Saxon
Old English language
Old English or Anglo-Saxon is an early form of the English language that was spoken and written by the Anglo-Saxons and their descendants in parts of what are now England and southeastern Scotland between at least the mid-5th century and the mid-12th century...

 word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300. As with its equivalents in the other Germanic languages
Germanic languages
The Germanic languages constitute a sub-branch of the Indo-European language family. The common ancestor of all of the languages in this branch is called Proto-Germanic , which was spoken in approximately the mid-1st millennium BC in Iron Age northern Europe...

, it derives ultimately from the Proto-Germanic word ertho, "ground," as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jord. Many of the Romance languages
Romance languages
The Romance languages are a branch of the Indo-European language family, more precisely of the Italic languages subfamily, comprising all the languages that descend from Vulgar Latin, the language of ancient Rome...

 retain the old Roman word terra
Terra (mythology)
Terra or Tellus was a goddess personifying the Earth in Roman mythology. The names Terra Mater and Tellus Mater both mean "Mother Earth" in Latin; Mater is an honorific title also bestowed on other goddesses...

(or some variation of it) that was used with the meaning of "dry land" (as opposed to "sea"). However, the non-Romance languages use their own respective native words. The Greeks retain their original name, Γή
Gaia (mythology)
Gaia was the primordial Earth-goddess in ancient Greek religion. Gaia was the great mother of all: the heavenly gods and Titans were descended from her union with Uranus , the sea-gods from her union with Pontus , the Giants from her mating with Tartarus and mortal creatures were sprung or born...

(Ge or Yi).

Non-European cultures use other planetary naming systems. India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

 uses a naming system based on the Navagraha
Navagraha
Graha is a 'cosmic influencer' on the living beings of mother Bhumidevi . In Hindu astrology, the Navagraha are some of these major influencers.All the navagraha have relative movement with respect to the background of fixed stars in the zodiac...

, which incorporates the seven traditional planets (Surya
Surya
Surya Suraya or Phra Athit is the chief solar deity in Hinduism, one of the Adityas, son of Kasyapa and one of his wives, Aditi; of Indra; or of Dyaus Pitar . The term Surya also refers to the Sun, in general. Surya has hair and arms of gold...

 for the Sun, Chandra
Chandra
In Hinduism, Chandra is a lunar deity and a Graha. Chandra is also identified with the Vedic Lunar deity Soma . The Soma name refers particularly to the juice of sap in the plants and thus makes the Moon the lord of plants and vegetation. He is described as young, beautiful, fair; two-armed and...

 for the Moon, and Budha
Budha
In Hindu mythology, Budha is the name for the planet Mercury, a son of Chandra with Tara or Rohini. He is also the god of merchandise and protector of Merchants....

, Shukra
Shukra
Shukra , the Sanskrit for "clear, pure" or "brightness, clearness", is the name of the son of Bhrigu, and preceptor of the Daityas, and the guru of the Asuras, identified with the planet Venus, one of the Navagrahas...

, Mangala
Mangala
In Jyotish astrology, Mangala is the name for Mars, the red planet. Mars is also called Angaraka In Jyotish astrology, Mangala (Devanagari: मंगल) is the name for Mars, the red planet. Mars is also called Angaraka In Jyotish astrology, Mangala (Devanagari: मंगल) is the name for Mars, the red...

,
{{About|the astronomical object}}
{{pp-move-indef}}
{{Multiple image|direction=vertical|align=right|image1=1e7m comparison Uranus Neptune Sirius B Earth Venus.png|image2=1e6m comparison Mars Mercury Moon Pluto Haumea - no transparency.png|width=300|caption2=Planetary-sized objects to scale:
Top row: Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

;
second row: Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...

 star Sirius B, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

;
bottom row (reproduced and enlarged in lower image) – above: Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 and Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

;
below: the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, dwarf planets Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 and Haumea}}

A planet (from Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 {{lang|grc|πλανήτης αστήρ}} planētēs astēr "wandering star") is a celestial body
Celestial Body
Celestial Body is a Croatian film directed by Lukas Nola. It was released in 2000....

 orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

ing a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...

s.{{Ref label|A|a|none}}
The term planet is ancient, with ties to history
History
History is the discovery, collection, organization, and presentation of information about past events. History can also mean the period of time after writing was invented. Scholars who write about history are called historians...

, science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

, mythology
Mythology
The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

, and religion
Religion
Religion is a collection of cultural systems, belief systems, and worldviews that establishes symbols that relate humanity to spirituality and, sometimes, to moral values. Many religions have narratives, symbols, traditions and sacred histories that are intended to give meaning to life or to...

. The planets were originally seen by many early cultures as divine, or as emissaries of the god
God
God is the English name given to a singular being in theistic and deistic religions who is either the sole deity in monotheism, or a single deity in polytheism....

s. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 officially adopted a resolution defining planets within the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were thought by Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 to orbit the Earth in deferent and epicycle
Deferent and epicycle
In the Ptolemaic system of astronomy, the epicycle was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets...

 motions. Though the idea that the planets orbited the Sun
Heliocentrism
Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a stationary Sun at the center of the universe. The word comes from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center...

 had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 astronomical observations, performed by Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

. By careful analysis of the observation data, Johannes Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

 found the planets' orbits to be not circular, but elliptical
Elliptic orbit
In astrodynamics or celestial mechanics an elliptic orbit is a Kepler orbit with the eccentricity less than 1; this includes the special case of a circular orbit, with eccentricity equal to zero. In a stricter sense, it is a Kepler orbit with the eccentricity greater than 0 and less than 1 . In a...

. As observational tools improved, astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

s saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice cap
Ice cap
An ice cap is an ice mass that covers less than 50 000 km² of land area . Masses of ice covering more than 50 000 km² are termed an ice sheet....

s and season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

s. Since the dawn of the Space Age
Space Age
The Space Age is a time period encompassing the activities related to the Space Race, space exploration, space technology, and the cultural developments influenced by these events. The Space Age is generally considered to have begun with Sputnik...

, close observation by probes
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

 has found that Earth and the other planets share characteristics such as volcanism
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

, hurricanes, tectonics
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...

, and even hydrology
Hydrology
Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...

.

Planets are generally divided into two main types: large, low-density gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

s, and smaller, rocky terrestrials
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, they are the four terrestrials, Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

, Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, then the four gas giants, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

, Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

, Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

, and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

. Six of the planets are orbited by one or more natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

s.
Additionally, the Solar System also contains at least five dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s and hundreds of thousands of small Solar System bodies
Small solar system body
A small Solar System body is an object in the Solar System that is neither a planet nor a dwarf planet, nor a satellite of a planet or dwarf planet:...

.

Since 1992, hundreds of planets around other stars ("extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

s" or "exoplanets") in the Milky Way Galaxy
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 have been discovered. As of {{Extrasolar planet counts|asof}}, {{Extrasolar planet counts|planet_count}} known extrasolar planets are listed in the Extrasolar Planets Encyclopaedia
Extrasolar Planets Encyclopaedia
The Extrasolar Planets Encyclopedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains a database of all the currently known and candidate extrasolar planets, with individual "note" pages for each planet and a full list...

, ranging from the size of terrestrial planets somewhat larger than Earth to gas giants larger than Jupiter.

History


{{See|History of astronomy|Definition of planet}}
{{See also|Timeline of solar system astronomy}}
The idea of planets has evolved over its history, from the divine wandering stars
Wandering Stars
Wandering Stars is an anthology of Jewish fantasy and science fiction, edited by Jack Dann, originally published by Harper & Row in 1974. It represented, according to the book cover, "the first time in science fiction that the Jew - and the richness of his themes and particular points of view --...

 of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.

The five classical planets, being visible to the naked eye, have been known since ancient times, and have had a significant impact on mythology
Mythology
The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

, religious cosmology
Religious cosmology
A Religious cosmology is a way of explaining the origin, the history and the evolution of the universe based on the religious mythology of a specific tradition...

, and ancient astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

. In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights {{lang|grc|πλάνητες ἀστέρες}} ({{Transl|grc|planetes asteres}} "wandering stars") or simply "{{lang|grc|πλανήτοι}}" ({{Transl|grc|planētoi}} "wanderers"), from which today's word "planet" was derived. In ancient Greece
Greece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....

, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

, Babylon
Babylon
Babylon was an Akkadian city-state of ancient Mesopotamia, the remains of which are found in present-day Al Hillah, Babil Province, Iraq, about 85 kilometers south of Baghdad...

 and indeed all pre-modern civilisations, it was almost universally believed that Earth was in the center of the Universe and that all the "planets" circled the Earth. The reasons for this perception were that stars and planets appeared to revolve around the Earth each day, and the apparently common-sense
Common sense
Common sense is defined by Merriam-Webster as, "sound and prudent judgment based on a simple perception of the situation or facts." Thus, "common sense" equates to the knowledge and experience which most people already have, or which the person using the term believes that they do or should have...

 perception that the Earth was solid and stable, and that it was not moving but at rest.

The name for planets in Chinese astronomy
Chinese astronomy
Astronomy in China has a very long history, with historians considering that "they [the Chinese] were the most persistent and accurate observers of celestial phenomena anywhere in the world before the Arabs."...

 had the same motive as the Greek name, 行星 "moving star". In Japanese during the Edo period
Edo period
The , or , is a division of Japanese history which was ruled by the shoguns of the Tokugawa family, running from 1603 to 1868. The political entity of this period was the Tokugawa shogunate....

 there were two competing terms, 惑星 "confused star" and 遊星 "wandering star". In modern Japan, terminology was unified in favour of 惑星, but in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 the alternative term 遊星 retains some currency.{{citation needed|date=October 2011}}

Babylon


{{Main|Babylonian astronomy}}
The first civilization known to possess a functional theory of the planets were the Babylonia
Babylonia
Babylonia was an ancient cultural region in central-southern Mesopotamia , with Babylon as its capital. Babylonia emerged as a major power when Hammurabi Babylonia was an ancient cultural region in central-southern Mesopotamia (present-day Iraq), with Babylon as its capital. Babylonia emerged as...

ns, who lived in Mesopotamia
Mesopotamia
Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...

 in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa
Venus tablet of Ammisaduqa
The Venus tablet of Ammisaduqa refers to the record of astronomical observations of Venus, as preserved in numerous cuneiform tablets dating from the first millennium BCE. It is believed that this astronomical record was first compiled during the reign of King Ammisaduqa , the fourth ruler after...

, a 7th century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC. The MUL.APIN
MUL.APIN
MUL.APIN is the conventional title given to a Babylonian compendium that deals with many diverse aspects of Babylonian astronomy and astrology....

 is a pair of cuneiform
Cuneiform
Cuneiform can refer to:*Cuneiform script, an ancient writing system originating in Mesopotamia in the 4th millennium BC*Cuneiform , three bones in the human foot*Cuneiform Records, a music record label...

 tablets dating from the 7th century BC that lays out the motions of the Sun, Moon and planets over the course of the year. The Babylonian astrologers
Babylonian astrology
In Babylon as well as in Assyria as a direct offshoot of Babylonian culture, astrology takes its place in theofficial cult as one of the two chief means at the disposal of the priests for ascertaining the will and intention of the gods, the other being through the inspection of the liver of the...

 also laid the foundations of what would eventually become Western astrology
Western astrology
Western astrology is the system of astrology most popular in Western countries. Western astrology is historically based on Ptolemy's Tetrabiblos , which in turn was a continuation of Hellenistic and ultimately Babylonian traditions....

. The Enuma anu enlil
Enuma anu enlil
Enuma Anu Enlil is a major series of 68 or 70 tablets dealing with Babylonian astrology...

, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omen
Omen
An omen is a phenomenon that is believed to foretell the future, often signifying the advent of change...

s and their relationships with various celestial phenomena including the motions of the planets.

The Sumer
Sumer
Sumer was a civilization and historical region in southern Mesopotamia, modern Iraq during the Chalcolithic and Early Bronze Age....

ians, predecessors of the Babylonians who are considered as one of the first civilizations
Cradle of Civilization
The cradle of civilization is a term referring to any of the possible locations for the emergence of civilization.It is usually applied to the Ancient Near Eastern Chalcolithic , especially in the Fertile Crescent , but also extended to sites in Armenia, and the Persian Plateau, besides other Asian...

 and are credited with the invention of writing
Writing
Writing is the representation of language in a textual medium through the use of a set of signs or symbols . It is distinguished from illustration, such as cave drawing and painting, and non-symbolic preservation of language via non-textual media, such as magnetic tape audio.Writing most likely...

, had identified at least Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 by 1500 BC. Shortly afterwards, the other inner planet Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

 and the outer planets Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 in early modern times.

Greco-Roman astronomy


{{See also|Greek astronomy}}
Ptolemy's 7 planetary spheres
1
Moon
2
Mercury
3
Venus
4
Sun
5
Mars
6
Jupiter
7
Saturn


Because they were not as interested in divination as the Babylonians, the ancient Greeks initially did not attach as much significance to the planets. The Pythagoreans, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras
Pythagoras
Pythagoras of Samos was an Ionian Greek philosopher, mathematician, and founder of the religious movement called Pythagoreanism. Most of the information about Pythagoras was written down centuries after he lived, so very little reliable information is known about him...

 or Parmenides
Parmenides
Parmenides of Elea was an ancient Greek philosopher born in Elea, a Greek city on the southern coast of Italy. He was the founder of the Eleatic school of philosophy. The single known work of Parmenides is a poem, On Nature, which has survived only in fragmentary form. In this poem, Parmenides...

 are said to have first identified the evening star and morning star (Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

) as one and the same. In the 3rd century BC, Aristarchus of Samos
Aristarchus of Samos
Aristarchus, or more correctly Aristarchos , was a Greek astronomer and mathematician, born on the island of Samos, in Greece. He presented the first known heliocentric model of the solar system, placing the Sun, not the Earth, at the center of the known universe...

 proposed a heliocentric
Heliocentrism
Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a stationary Sun at the center of the universe. The word comes from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center...

 system, according to which the Earth and planets revolved around the sun. However, the geocentric system would remain dominant until the Scientific Revolution
Scientific revolution
The Scientific Revolution is an era associated primarily with the 16th and 17th centuries during which new ideas and knowledge in physics, astronomy, biology, medicine and chemistry transformed medieval and ancient views of nature and laid the foundations for modern science...

.

By the 1st century BC, during the Hellenistic period
Hellenistic period
The Hellenistic period or Hellenistic era describes the time which followed the conquests of Alexander the Great. It was so named by the historian J. G. Droysen. During this time, Greek cultural influence and power was at its zenith in Europe and Asia...

, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest
Almagest
The Almagest is a 2nd-century mathematical and astronomical treatise on the apparent motions of the stars and planetary paths. Written in Greek by Claudius Ptolemy, a Roman era scholar of Egypt,...

written by Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 in the 2nd century CE. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries. To the Greeks and Romans there were seven known planets, each presumed to be circling the Earth
Geocentric model
In astronomy, the geocentric model , is the superseded theory that the Earth is the center of the universe, and that all other objects orbit around it. This geocentric model served as the predominant cosmological system in many ancient civilizations such as ancient Greece...

 according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.

India


{{Main|Indian astronomy|Hindu cosmology}}

In 499 CE, the Indian astronomer Aryabhata
Aryabhata
Aryabhata was the first in the line of great mathematician-astronomers from the classical age of Indian mathematics and Indian astronomy...

 propounded a planetary model which explicitly incorporated the Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbit of planets are elliptical
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...

.{{Verify credibility|date=January 2011}}
Aryabhata's followers were particularly strong in South India
South India
South India is the area encompassing India's states of Andhra Pradesh, Karnataka, Kerala and Tamil Nadu as well as the union territories of Lakshadweep and Pondicherry, occupying 19.31% of India's area...

, where his principles of the diurnal rotation of the earth, among others, were followed and a number of secondary works were based on them. {{Page needed|date=October 2010}}

In 1500, Nilakantha Somayaji
Nilakantha Somayaji
Kelallur Nilakantha Somayaji was a major mathematician and astronomer of the Kerala school of astronomy and mathematics. One of his most influential works was the comprehensive astronomical treatise Tantrasamgraha completed in 1501...

 of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model. In his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits the Earth, similar to the Tychonic system
Tychonic system
The Tychonic system was a model of the solar system published by Tycho Brahe in the late 16th century which combined what he saw as the mathematical benefits of the Copernican system with the philosophical and "physical" benefits of the Ptolemaic system...

 later proposed by Tycho Brahe
Tycho Brahe
Tycho Brahe , born Tyge Ottesen Brahe, was a Danish nobleman known for his accurate and comprehensive astronomical and planetary observations...

 in the late 16th century. Most astronomers of the Kerala school who followed him accepted his planetary model.

Medieval Muslim astronomy


{{Main|Astronomy in medieval Islam|Islamic cosmology}}

In the 11th century, the transit of Venus
Transit of Venus
A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, becoming visible against the solar disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun...

 was observed by Avicenna
Avicenna
Abū ʿAlī al-Ḥusayn ibn ʿAbd Allāh ibn Sīnā , commonly known as Ibn Sīnā or by his Latinized name Avicenna, was a Persian polymath, who wrote almost 450 treatises on a wide range of subjects, of which around 240 have survived...

, who established that Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 was, at least sometimes, below the Sun. In the 12th century, Ibn Bajjah
Ibn Bajjah
Abū-Bakr Muhammad ibn Yahya ibn al-Sāyigh , known as Ibn Bājjah , was an Andalusian polymath: an astronomer, logician, musician, philosopher, physician, physicist, psychologist, botanist, poet and scientist. He was known in the West by his Latinized name, Avempace...

 observed "two planets as black spots on the face of the Sun," which was later identified as a transit of Mercury
Transit of Mercury
A transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun....

 and Venus by the Maragha
Maragheh observatory
Maragheh observatory is an astronomical observatory which was established in 1259 CE by Nasir al-Din al-Tusi, an Iranian scientist and astronomer...

 astronomer Qotb al-Din Shirazi in the 13th century. However, Ibn Bajjah could not have observed a transit of Venus, as none occurred in his lifetime.{{Citation needed|date=August 2011}}

European Renaissance

Renaissance planets, ca. 1543 to 1781
1
Mercury
2
Venus
3
Earth
4
Mars
5
Jupiter
6
Saturn

{{See also|Heliocentrism}}
With the advent of the Scientific Revolution
Scientific revolution
The Scientific Revolution is an era associated primarily with the 16th and 17th centuries during which new ideas and knowledge in physics, astronomy, biology, medicine and chemistry transformed medieval and ancient views of nature and laid the foundations for modern science...

, understanding of the term "planet" changed from something that moved across the sky (in relation to the star field); to a body that orbited the Earth (or that were believed to do so at the time); and in the 16th century to something that directly orbited the Sun when the heliocentric model of Copernicus
Nicolaus Copernicus
Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....

, Galileo
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

 and Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

 gained sway.

Thus the Earth became included in the list of planets, while the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century. Until the mid-19th century, the number of "planets" rose rapidly since any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community.

19th century

New planets, 1807–1845
1
Mercury
2
Venus
3
Earth
4
Mars
5
Vesta
6
Juno
7
Ceres
8
Pallas
9
Jupiter
10
Saturn
11
Uranus

In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas
2 Pallas
Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered , and one of the largest. It is estimated to constitute 7% of the mass of the asteroid belt, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta. It is however 20%...

, and Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

) were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the Asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

), and had a much smaller mass; as a result they were reclassified as "asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s." In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Since there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846, there was no apparent need to have a formal definition.

20th century

Planets 1854–1930, 2006–present
1
Mercury
2
Venus
3
Earth
4
Mars
5
Jupiter
6
Saturn
7
Uranus
8
Neptune

However, in the 20th century, Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 was discovered. After initial observations led to the belief it was larger than Earth, the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton
Raymond Lyttleton
Raymond Arthur Lyttleton FRS was a British mathematician and theoretical astronomer.He was born in the Oldbury, Worcestershire area and educated at King Edward VI Five Ways school in Birmingham, going from there to Clare College, Cambridge to read mathematics, graduating in 1933...

 suggested that Pluto may be an escaped satellite of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

, and Fred Whipple
Fred Lawrence Whipple
Fred Lawrence Whipple was an American astronomer, who worked at the Harvard College Observatory for over 70 years...

 suggested in 1964 that Pluto may be a comet. However, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status until 2006.
Planets 1930–2006
1
Mercury
2
Venus
3
Earth
4
Mars
5
Jupiter
6
Saturn
7
Uranus
8
Neptune
9
Pluto

In 1992, astronomers Aleksander Wolszczan
Aleksander Wolszczan
Aleksander Wolszczan is a Polish astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets.- Scientific career :...

 and Dale Frail
Dale Frail
Dale A. Frail is an astronomer working at the National Radio Astronomy Observatory in Socorro, New Mexico. He was born in Canada, spent much of his childhood in Europe, and his professional career has been based in the United States.-Career:...

 announced the discovery of planets around a pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

, PSR B1257+12
PSR B1257+12
PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located roughly 2000 light-years from the Sun. In 2007, it was confirmed that three extrasolar planets orbit the pulsar.- Pulsar :...

. This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor
Michel Mayor
Dr. Michel G. E. Mayor is a Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the Observatory of Geneva...

 and Didier Queloz
Didier Queloz
Didier Queloz is a Geneva-based astronomer with a prolific record in finding extrasolar planets. He is understudy to Michel Mayor.Didier Queloz was a Ph.D...

 of the University of Geneva
University of Geneva
The University of Geneva is a public research university located in Geneva, Switzerland.It was founded in 1559 by John Calvin, as a theological seminary and law school. It remained focused on theology until the 17th century, when it became a center for Enlightenment scholarship. In 1873, it...

 announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 star (51 Pegasi
51 Pegasi
51 Pegasi is a Sun-like star located 15.6 parsecs from Earth in the constellation Pegasus...

).

The discovery of extrasolar planets led to another ambiguity in defining a planet; the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as "brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

s". Brown dwarfs are generally considered stars due to their ability to fuse deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

, a heavier isotope of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

. While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.

21st century


With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

, or if it was large enough to generate energy by the thermonuclear fusion of deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

.

A growing number of astronomers argued for Pluto to be declassified as a planet, since many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands.

Some of them including Quaoar
50000 Quaoar
50000 Quaoar is a rocky trans-Neptunian object in the Kuiper belt with one known moon. Discovered on June 4, 2002 by astronomers Chad Trujillo and Michael Brown at the California Institute of Technology from images acquired at the Samuel Oschin Telescope at Palomar Observatory, it is thought by...

, Sedna
90377 Sedna
90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...

, and Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

 were heralded in the popular press as the tenth planet, failing however to receive widespread scientific recognition. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the necessity and public desire for an official definition of a planet.

Acknowledging the problem, the IAU set about creating the definition of planet
Definition of planet
The definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai , "wandering stars", for objects which apparently move over the sky...

, and produced one in August 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit
Clearing the neighbourhood
"Clearing the neighbourhood of its orbit" is a criterion for a celestial body to be considered a planet in the Solar System. This was one of the three criteria adopted by the International Astronomical Union in its 2006 definition of planet....

 (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s was created, initially containing three objects (Ceres, Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 and Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

).

Extrasolar planet definition


In 2003, The International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated the following working definition, mostly focused upon the boundary between planets and brown dwarves:
{{TNO imagemap}}
  1. Objects with true mass
    True mass
    The term true mass is synonymous with the term mass, but is used in astronomy to differentiate the measured mass of a planet from the lower limit of mass usually obtained from radial velocity techniques...

    es below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance
    Natural abundance
    In chemistry, natural abundance refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass of these isotopes is the atomic weight listed for the element in the periodic table...

     as the Sun) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
  2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarf
    Brown dwarf
    Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

    s", no matter how they formed or where they are located.
  3. Free-floating objects in young star cluster
    Star cluster
    Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...

    s with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).


This definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journal
Academic journal
An academic journal is a peer-reviewed periodical in which scholarship relating to a particular academic discipline is published. Academic journals serve as forums for the introduction and presentation for scrutiny of new research, and the critique of existing research...

s. Although temporary, it remains an effective working definition until a more permanent one is formally adopted. However, it does not address the dispute over the lower mass limit, and so it steered clear of the controversy regarding objects within the Solar System. This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2M1207b
2M1207b
2M1207b is a planetary-mass object orbiting the brown dwarf 2M1207, in the constellation Centaurus, approximately 170 light-years from Earth...

.

One definition of a sub-brown dwarf
Sub-brown dwarf
A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass for thermonuclear fusion of deuterium ....

 is a planet-mass object that formed through cloud-collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.
One reason for the dissent is that oftentimes it may not be possible to determine the formation process: for example an accretion-formed planet around a star may get ejected from the system to become free-floating, and likewise a cloud-collapse-formed sub-brown dwarf formed on its own in a star cluster may get captured into orbit around a star.
Dwarf planets 2006–present
Ceres Pluto Makemake Haumea Eris


The 13 Jupiter-mass cutoff is a rule of thumb rather than something of precise physical significance. The question arises: what is meant by deuterium burning
Deuterium burning
Deuterium burning is a nuclear fusion reaction that occurs in stars and some substellar objects, in which a deuterium nucleus and a proton combine to form a helium-3 nucleus...

? This question arises because large objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 MJ value is somewhere in between. The amount of deuterium burnt also depends not only on mass but on the composition of the planet, on the amount of helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 and deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 present.

Another criterion for separating planets and brown dwarfs, rather than deuterium burning, formation process or location is whether the core pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 is dominated by coulomb pressure or electron degeneracy.

2006 definition


{{Main|IAU definition of planet}}
The matter of the lower limit was addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, the assembly voted to pass a resolution that defined planets within the Solar System as:

{{quotation|A celestial body that is (a) in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...

 (nearly round) shape, and (c) has cleared the neighbourhood
Clearing the neighbourhood
"Clearing the neighbourhood of its orbit" is a criterion for a celestial body to be considered a planet in the Solar System. This was one of the three criteria adopted by the International Astronomical Union in its 2006 definition of planet....

 around its orbit.}}

Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto, Makemake and Eris) are classified as dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s, provided they are not also natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

s of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion. After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.

This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter
Steven Soter
Dr. Steven Soter, PhD, is an astrophysicist currently holding the positions of scientist-in-residence for New York University's Environmental Studies Program and of Research Associate for the Department of Astrophysics at the American Museum of Natural History...

:

{{quotation|The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Asteroids and comets, including KBOs [Kuiper belt objects], differ from planets in that they can collide with each other and with planets.}}

In the aftermath of the IAU's 2006 vote, there has been controversy and debate about the definition, and many astronomers have stated that they will not use it. Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition.

Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status since its discovery in 1930. The discovery of Eris was widely reported in the media
Mass media
Mass media refers collectively to all media technologies which are intended to reach a large audience via mass communication. Broadcast media transmit their information electronically and comprise of television, film and radio, movies, CDs, DVDs and some other gadgets like cameras or video consoles...

 as the tenth planet and therefore the reclassification of all three objects as dwarf planets has attracted a lot of media and public attention as well.

Former classifications


The table below lists Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 bodies formerly considered to be planets:
Body (current classification) Notes
Star Dwarf planet Asteroid Moon
Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 
The Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

Classified as planets in antiquity
Ancient history
Ancient history is the study of the written past from the beginning of recorded human history to the Early Middle Ages. The span of recorded history is roughly 5,000 years, with Cuneiform script, the oldest discovered form of coherent writing, from the protoliterate period around the 30th century BC...

, in accordance with the definition then used.
Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

, Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

, Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...

, and Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...

The four largest moons of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

, known as the Galilean moons
Galilean moons
The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance...

 after their discoverer Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

. He referred to them as the "Medicean Planets" in honor of his patron
Patronage
Patronage is the support, encouragement, privilege, or financial aid that an organization or individual bestows to another. In the history of art, arts patronage refers to the support that kings or popes have provided to musicians, painters, and sculptors...

, the Medici family
Medici
The House of Medici or Famiglia de' Medici was a political dynasty, banking family and later royal house that first began to gather prominence under Cosimo de' Medici in the Republic of Florence during the late 14th century. The family originated in the Mugello region of the Tuscan countryside,...

.
Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

,{{Ref label|B|b|none}} Iapetus
Iapetus (moon)
Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671...

,{{Ref label|C|c|none}} Rhea
Rhea (moon)
Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.-Name:Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods"...

,{{Ref label|C|c|none}} Tethys
Tethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...

,{{Ref label|D|d|none}} and Dione
Dione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...

{{Ref label|D|d|none}}
Five of Saturn's larger moons, discovered by Christiaan Huygens and Giovanni Domenico Cassini
Giovanni Domenico Cassini
This article is about the Italian-born astronomer. For his French-born great-grandson, see Jean-Dominique Cassini.Giovanni Domenico Cassini was an Italian/French mathematician, astronomer, engineer, and astrologer...

.
Ceres{{Ref label|E|e|none}} Pallas
2 Pallas
Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered , and one of the largest. It is estimated to constitute 7% of the mass of the asteroid belt, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta. It is however 20%...

, Juno
3 Juno
Juno , formal designation 3 Juno in the Minor Planet Center catalogue system, was the third asteroid to be discovered and is one of the larger main-belt asteroids, being one of the two largest stony asteroids, along with 15 Eunomia. Juno is estimated to contain 1% of the total mass of the asteroid...

, and Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

 
The first known asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s.
Ceres has subsequently been classified as a dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

 in 2006.
Astrea
5 Astraea
5 Astraea is a large main-belt asteroid. Its surface is highly reflective and its composition is probably a mixture of nickel-iron with magnesium- and iron-silicates....

, Hebe
6 Hebe
6 Hebe is a large main-belt asteroid, containing around half a percent of the mass of the belt. Its apparently high bulk density , however, means that by volume it does not rank among the top twenty asteroids...

, Iris
7 Iris
7 Iris is a large main-belt asteroid. Among the S-type asteroids, it ranks fifth in geometric mean diameter after Eunomia, Juno, Amphitrite and Herculina....

, Flora
8 Flora
8 Flora is a large, bright main-belt asteroid. It is the innermost large asteroid: no asteroid closer to the Sun has a diameter above 25 kilometres or two-elevenths that of Flora itself, and not until the tiny 149 Medusa was discovered was a single asteroid orbiting at a closer mean distance...

, Metis
9 Metis
9 Metis is one of the larger main-belt asteroids. It is composed of silicates and metallic nickel-iron, and may be the core remnant of a large asteroid that was destroyed by an ancient collision...

, Hygeia
10 Hygiea
10 Hygiea is an asteroid located in the asteroid belt. With somewhat oblong diameters of 350–500 km, and a mass estimated to be 2.9% of the total mass of the belt, it is the fourth largest asteroid by volume and mass...

, Parthenope
11 Parthenope
11 Parthenope is a large, bright main-belt asteroid.Parthenope was discovered by Annibale de Gasparis on May 11, 1850, the second of his nine asteroid discoveries. It was named after one of the Sirens in Greek mythology, said to have founded the city of Naples...

, Victoria
12 Victoria
12 Victoria is a large main-belt asteroid.It was discovered by J. R. Hind on September 13, 1850.Victoria is officially named after the Roman goddess of victory, but the name also honours Queen Victoria. The goddess Victoria was the daughter of Styx by the Titan Pallas...

, Egeria
13 Egeria
13 Egeria is a large main-belt G-type asteroid.It was discovered by A. de Gasparis on November 2, 1850, and was named by Urbain J. J. Le Verrier, whose computations led to the discovery of Neptune. Egeria was a goddess of Aricia, in Italy, and the wife of Numa Pompilius, second king of...

, Irene
14 Irene
14 Irene is a very large main-belt asteroid.14 Irene was discovered by J. R. Hind on May 19, 1851, and named after Eirene, a personification of peace in Greek mythology. She was one of the Horae, daughter of Zeus and Themis. The name was suggested by Sir John Herschel...

, Eunomia
15 Eunomia
15 Eunomia is a very large asteroid in the inner asteroid belt. It is the largest of the stony asteroids, and somewhere between the 8th-to-12th-largest main-belt asteroid overall...

 
More asteroids, discovered between 1845 and 1851. The rapidly expanding list of planets prompted their reclassification as asteroids by astronomers, and this was widely accepted by 1854.
Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

{{Ref label|F|f|none}}
The first known Trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

 (i.e. minor planet with a semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

 beyond Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

). In 2006, Pluto was reclassified as a dwarf planet.
Eris (dwarf planet)
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

 (originally nicknamed Xena)
Discovered in 2003, this Trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

 (i.e. minor planet with a semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

 beyond Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

) was recognised in 2005, before, like Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

, in 2006 getting reclassified as a dwarf planet.

Mythology and naming


{{See also|Weekday names|Naked-eye planet}}
The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

, the two great luminaries the Sun and the Moon were called Helios
Helios
Helios was the personification of the Sun in Greek mythology. Homer often calls him simply Titan or Hyperion, while Hesiod and the Homeric Hymn separate him as a son of the Titans Hyperion and Theia or Euryphaessa and brother of the goddesses Selene, the moon, and Eos, the dawn...

and Selene
Selene
In Greek mythology, Selene was an archaic lunar deity and the daughter of the Titans Hyperion and Theia. In Roman mythology, the moon goddess is called Luna, Latin for "moon"....

; the farthest planet was called Phainon, the shiner; followed by Phaethon, "bright"; the red planet was known as Pyroeis, the "fiery"; the brightest was known as Phosphoros, the light bringer; and the fleeting final planet was called Stilbon, the gleamer. The Greeks also made each planet sacred to one among their pantheon of gods, the Olympians
Twelve Olympians
The Twelve Olympians, also known as the Dodekatheon , in Greek mythology, were the principal deities of the Greek pantheon, residing atop Mount Olympus. Zeus, Hera, Poseidon, Demeter, Hestia, and Hades were siblings. Ares, Hermes, Hephaestus, Athena, Apollo, and Artemis were children of Zeus...

: Helios and Selene were the names of both planets and gods; Phainon was sacred to Cronus
Cronus
In Greek mythology, Cronus or Kronos was the leader and the youngest of the first generation of Titans, divine descendants of Gaia, the earth, and Uranus, the sky...

, the Titan
Titan (mythology)
In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age....

 who fathered the Olympians; Phaethon was sacred to Zeus
Zeus
In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...

, Cronus's son who deposed him as king; Pyroeis was given to Ares
Ares
Ares is the Greek god of war. He is one of the Twelve Olympians, and the son of Zeus and Hera. In Greek literature, he often represents the physical or violent aspect of war, in contrast to the armored Athena, whose functions as a goddess of intelligence include military strategy and...

, son of Zeus and god of war; Phosphoros was ruled by Aphrodite
Aphrodite
Aphrodite is the Greek goddess of love, beauty, pleasure, and procreation.Her Roman equivalent is the goddess .Historically, her cult in Greece was imported from, or influenced by, the cult of Astarte in Phoenicia....

, the goddess of love; and Hermes
Hermes
Hermes is the great messenger of the gods in Greek mythology and a guide to the Underworld. Hermes was born on Mount Kyllini in Arcadia. An Olympian god, he is also the patron of boundaries and of the travelers who cross them, of shepherds and cowherds, of the cunning of thieves, of orators and...

, messenger of the gods and god of learning and wit, ruled over Stilbon.

The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros after their goddess of love, Ishtar
Ishtar
Ishtar is the Assyrian and Babylonian goddess of fertility, love, war, and sex. She is the counterpart to the Sumerian Inanna and to the cognate north-west Semitic goddess Astarte.-Characteristics:...

; Pyroeis after their god of war, Nergal
Nergal
The name Nergal, Nirgal, or Nirgali refers to a deity in Babylon with the main seat of his cult at Cuthah represented by the mound of Tell-Ibrahim. Nergal is mentioned in the Hebrew bible as the deity of the city of Cuth : "And the men of Babylon made Succoth-benoth, and the men of Cuth made Nergal"...

, Stilbon after their god of wisdom Nabu
Nabu
Nabu is the Assyrian and Babylonian god of wisdom and writing, worshipped by Babylonians as the son of Marduk and his consort, Sarpanitum, and as the grandson of Ea. Nabu's consort was Tashmetum....

, and Phaethon after their chief god, Marduk
Marduk
Marduk was the Babylonian name of a late-generation god from ancient Mesopotamia and patron deity of the city of Babylon, who, when Babylon became the political center of the Euphrates valley in the time of Hammurabi , started to...

. There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately. The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire
Roman Empire
The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

 and, later, the Catholic Church, use the Roman (or Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon
Roman mythology
Roman mythology is the body of traditional stories pertaining to ancient Rome's legendary origins and religious system, as represented in the literature and visual arts of the Romans...

 under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

. During the later period of the Roman Republic
Roman Republic
The Roman Republic was the period of the ancient Roman civilization where the government operated as a republic. It began with the overthrow of the Roman monarchy, traditionally dated around 508 BC, and its replacement by a government headed by two consuls, elected annually by the citizens and...

, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable. When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius
Mercury (mythology)
Mercury was a messenger who wore winged sandals, and a god of trade, the son of Maia Maiestas and Jupiter in Roman mythology. His name is related to the Latin word merx , mercari , and merces...

(for Hermes), Venus
Venus (mythology)
Venus is a Roman goddess principally associated with love, beauty, sex,sexual seduction and fertility, who played a key role in many Roman religious festivals and myths...

(Aphrodite), Mars
Mars (mythology)
Mars was the Roman god of war and also an agricultural guardian, a combination characteristic of early Rome. He was second in importance only to Jupiter, and he was the most prominent of the military gods worshipped by the Roman legions...

(Ares), Iuppiter
Jupiter (mythology)
In ancient Roman religion and myth, Jupiter or Jove is the king of the gods, and the god of the sky and thunder. He is the equivalent of Zeus in the Greek pantheon....

(Zeus) and Saturnus
Saturn (mythology)
In ancient Roman religion and myth, Saturn was a major god presiding over agriculture and the harvest time. His reign was depicted as a Golden Age of abundance and peace by many Roman authors. In medieval times he was known as the Roman god of agriculture, justice and strength. He held a sickle in...

(Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus
Neptune (mythology)
Neptune was the god of water and the sea in Roman mythology and religion. He is analogous with, but not identical to, the Greek god Poseidon. In the Greek-influenced tradition, Neptune was the brother of Jupiter and Pluto, each of them presiding over one of the three realms of the universe,...

(Poseidon
Poseidon
Poseidon was the god of the sea, and, as "Earth-Shaker," of the earthquakes in Greek mythology. The name of the sea-god Nethuns in Etruscan was adopted in Latin for Neptune in Roman mythology: both were sea gods analogous to Poseidon...

). Uranus is unique in that it is named by a Greek deity
Uranus (mythology)
Uranus , was the primal Greek god personifying the sky. His equivalent in Roman mythology was Caelus. In Ancient Greek literature, according to Hesiod in his Theogony, Uranus or Father Sky was the son and husband of Gaia, Mother Earth...

 rather than his Roman counterpart
Caelus
Caelus or Coelus was a primal god of the sky in Roman myth and theology, iconography, and literature...

.

Some Romans
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

, following a belief possibly originating in Mesopotamia
Mesopotamia
Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...

 but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet). Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the days of the week
Week-day names
The names of the days of the week from the Roman period have been both named after the seven planets of classical astronomy and numbered, beginning with Monday. In Slavic languages, a numbering system was adopted, but beginning with Monday. There was an even older tradition of names in Ancient...

 in the Roman calendar
Roman calendar
The Roman calendar changed its form several times in the time between the founding of Rome and the fall of the Roman Empire. This article generally discusses the early Roman or pre-Julian calendars...

 after the Nundinal cycle was rejected – and still preserved in many modern languages. Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden
Woden
Woden or Wodan is a major deity of Anglo-Saxon and Continental Germanic polytheism. Together with his Norse counterpart Odin, Woden represents a development of the Proto-Germanic god *Wōdanaz....

(Wednesday), Thunor
Thor
In Norse mythology, Thor is a hammer-wielding god associated with thunder, lightning, storms, oak trees, strength, the protection of mankind, and also hallowing, healing, and fertility...

(Thursday), and Fríge
Frige
*Frijjō is the reconstructed name or epithet of a hypothesized Common Germanic love goddess giving rise to both Frigg and Freyja....

(Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.

Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Since it was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god (the same is true, in English at least, of the Sun and the Moon, though they are no longer considered planets). The name originates from the 8th century Anglo-Saxon
Old English language
Old English or Anglo-Saxon is an early form of the English language that was spoken and written by the Anglo-Saxons and their descendants in parts of what are now England and southeastern Scotland between at least the mid-5th century and the mid-12th century...

 word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300. As with its equivalents in the other Germanic languages
Germanic languages
The Germanic languages constitute a sub-branch of the Indo-European language family. The common ancestor of all of the languages in this branch is called Proto-Germanic , which was spoken in approximately the mid-1st millennium BC in Iron Age northern Europe...

, it derives ultimately from the Proto-Germanic word ertho, "ground," as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jord. Many of the Romance languages
Romance languages
The Romance languages are a branch of the Indo-European language family, more precisely of the Italic languages subfamily, comprising all the languages that descend from Vulgar Latin, the language of ancient Rome...

 retain the old Roman word terra
Terra (mythology)
Terra or Tellus was a goddess personifying the Earth in Roman mythology. The names Terra Mater and Tellus Mater both mean "Mother Earth" in Latin; Mater is an honorific title also bestowed on other goddesses...

(or some variation of it) that was used with the meaning of "dry land" (as opposed to "sea"). However, the non-Romance languages use their own respective native words. The Greeks retain their original name, Γή
Gaia (mythology)
Gaia was the primordial Earth-goddess in ancient Greek religion. Gaia was the great mother of all: the heavenly gods and Titans were descended from her union with Uranus , the sea-gods from her union with Pontus , the Giants from her mating with Tartarus and mortal creatures were sprung or born...

(Ge or Yi).

Non-European cultures use other planetary naming systems. India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

 uses a naming system based on the Navagraha
Navagraha
Graha is a 'cosmic influencer' on the living beings of mother Bhumidevi . In Hindu astrology, the Navagraha are some of these major influencers.All the navagraha have relative movement with respect to the background of fixed stars in the zodiac...

, which incorporates the seven traditional planets (Surya
Surya
Surya Suraya or Phra Athit is the chief solar deity in Hinduism, one of the Adityas, son of Kasyapa and one of his wives, Aditi; of Indra; or of Dyaus Pitar . The term Surya also refers to the Sun, in general. Surya has hair and arms of gold...

 for the Sun, Chandra
Chandra
In Hinduism, Chandra is a lunar deity and a Graha. Chandra is also identified with the Vedic Lunar deity Soma . The Soma name refers particularly to the juice of sap in the plants and thus makes the Moon the lord of plants and vegetation. He is described as young, beautiful, fair; two-armed and...

 for the Moon, and Budha
Budha
In Hindu mythology, Budha is the name for the planet Mercury, a son of Chandra with Tara or Rohini. He is also the god of merchandise and protector of Merchants....

, Shukra
Shukra
Shukra , the Sanskrit for "clear, pure" or "brightness, clearness", is the name of the son of Bhrigu, and preceptor of the Daityas, and the guru of the Asuras, identified with the planet Venus, one of the Navagrahas...

, Mangala
Mangala
In Jyotish astrology, Mangala is the name for Mars, the red planet. Mars is also called Angaraka In Jyotish astrology, Mangala (Devanagari: मंगल) is the name for Mars, the red planet. Mars is also called Angaraka In Jyotish astrology, Mangala (Devanagari: मंगल) is the name for Mars, the red...

, {{IAST and Shani
Shani
Sanskrit Śhani शनि, Kannada Śhani ಶನಿ ದೇವರು,Shani/Sani , is one of the Navagraha or Jyotiṣa . Shani is embodied in the planet Saturn and is the Lord of Saturday....

 for the traditional planets Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending lunar node
Lunar node
The lunar nodes are the orbital nodes of the Moon, that is, the points where the orbit of the Moon crosses the ecliptic . The ascending node is where the moon crosses to the north of the ecliptic...

s Rahu
Rahu
In Hindu mythology, Rahu is a cut-off head of an asura, that swallows the sun or the moon causing eclipses. He is depicted in art as a serpent with no body riding a chariot drawn by eight black horses. Rahu is one of the navagrahas in Vedic astrology...

 and Ketu
Ketu (mythology)
Ketu is the descending lunar node. 'Ketu' is said to be the body of Rahu, after the head of the asura was cut off by God Vishnu. In Hindu mythology, Ketu is generally referred to as a "shadow" planet. It is believed to have a tremendous impact on human lives and also the whole creation...

. China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

 and the countries of eastern Asia historically subject to Chinese cultural influence
Sinosphere
In areal linguistics, Sinosphere refers to a grouping of countries and regions that are currently inhabited with a majority of Chinese population or were historically under Chinese cultural influence...

 (such as Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

, Korea
Korea
Korea ) is an East Asian geographic region that is currently divided into two separate sovereign states — North Korea and South Korea. Located on the Korean Peninsula, Korea is bordered by the People's Republic of China to the northwest, Russia to the northeast, and is separated from Japan to the...

 and Vietnam
Vietnam
Vietnam – sometimes spelled Viet Nam , officially the Socialist Republic of Vietnam – is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea –...

) use a naming system based on the five Chinese elements: water
Water (classical element)
Water is one of the elements in ancient Greek philosophy, in the Asian Indian system Panchamahabhuta, and in the Chinese cosmological and physiological system Wu Xing...

(Mercury), metal
Metal (classical element)
Metal , is the decline of the matter, or the matter's decline stage. Metal is the fourth phase of Wu Xing. Metal is yin in character, its motion is inwards and its energy is contracting. It is associated with the Autumn, the west, old age, the planet Venus, the color white, dry weather, and the...

(Venus), fire
Fire (classical element)
Fire has been an important part of all cultures and religions from pre-history to modern day and was vital to the development of civilization. It has been regarded in many different contexts throughout history, but especially as a metaphysical constant of the world.-Greek and Roman tradition:Fire...

(Mars), wood
Wood (classical element)
Tree , traditionally translated as Wood, is the growing of the matter, or the matter's growing stage. Tree is the first phase of Wu Xing. Tree is yang in character...

(Jupiter) and earth
Earth (classical element)
Earth, home and origin of humanity, has often been worshipped in its own right with its own unique spiritual tradition.-European tradition:Earth is one of the four classical elements in ancient Greek philosophy and science. It was commonly associated with qualities of heaviness, matter and the...

(Saturn).

Formation


{{Main|Nebular hypothesis}}
It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

 into a thin disk of gas and dust. A protostar
Protostar
A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...

 forms at the core, surrounded by a rotating protoplanetary disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...

. Through accretion
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...

 (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...

s form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanet
Protoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...

s. After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

.
When the protostar has grown such that it ignites to form a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

, the surviving disk is removed from the inside outward by photoevaporation, the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

, Poynting-Robertson drag
Poynting-Robertson effect
The Poynting–Robertson effect, also known as Poynting–Robertson drag, named after John Henry Poynting and Howard Percy Robertson, is a process by which solar radiation causes a dust grain in the Solar System to slowly spiral into the sun...

 and other effects. Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb. Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

s of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies
Small solar system body
A small Solar System body is an object in the Solar System that is neither a planet nor a dwarf planet, nor a satellite of a planet or dwarf planet:...

.

The energetic impacts of the smaller planetesimals (as well as radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core. Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s. (Smaller planets will lose any atmosphere they gain through various escape mechanisms
Atmospheric escape
Atmospheric escape is the loss of planetary atmospheric gases to outer space.- Thermal escape mechanisms :One classical thermal escape mechanism is Jeans escape. In a quantity of gas, the average velocity of a molecule is determined by temperature, but the velocity of individual molecules varies...

.)

With the discovery and observation of planetary system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...

s around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...

 – an astronomical term describing the abundance of chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

s with an atomic number
Atomic number
In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...

 greater than 2 (helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

) – is now believed to determine the likelihood that a star will have planets. Hence, it is thought that a metal-rich population I star will likely possess a more substantial planetary system than a metal-poor, population II star.

Solar System





{{Main|Solar System}}
{{See also|List of gravitationally rounded objects of the Solar System}}

According to the IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

's current definitions, there are eight planets and five dwarf planets in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. In increasing distance from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, the planets are:
  1. Mercury
    Mercury (planet)
    Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

  2. Venus
    Venus
    Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

  3. Earth
    Earth
    Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

  4. Mars
    Mars
    Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

  5. Jupiter
    Jupiter
    Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

  6. Saturn
    Saturn
    Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

  7. Uranus
    Uranus
    Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

  8. Neptune
    Neptune
    Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...



Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.

The planets of the Solar System can be divided into categories based on their composition:
  • Terrestrials
    Terrestrial planet
    A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

    : Planets that are similar to Earth, with bodies largely composed of rock
    Rock (geology)
    In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

    : Mercury, Venus, Earth and Mars. At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System, while Earth is the largest terrestrial planet.
  • Gas giant
    Gas giant
    A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

    s (Jovians)
    : Planets largely composed of gas
    Gas
    Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

    eous material and significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Jupiter, at 318 Earth masses, is the largest planet in the Solar System, while Saturn is one third as big, at 95 Earth masses.
    • Ice giant
      Ice giant
      Ice Giant or Ice giants may refer to:* A type of gas giant composed largely of materials less volatile than hydrogen and helium* The frost giants of Norse mythology, see also Hrimthurs...

      s
      , comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass (only 14 and 17 Earth masses), and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
  • Dwarf planet
    Dwarf planet
    A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

    s
    : Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently five dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Haumea, Makemake and Eris. Several other objects in both the Asteroid belt
    Asteroid belt
    The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

     and the Kuiper belt
    Kuiper belt
    The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

     are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain – namely that they are not dominant in their orbits
    Clearing the neighbourhood
    "Clearing the neighbourhood of its orbit" is a criterion for a celestial body to be considered a planet in the Solar System. This was one of the three criteria adopted by the International Astronomical Union in its 2006 definition of planet....

    . By definition, all dwarf planets are members of larger population
    Population
    A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

    s. Ceres is the largest body in the asteroid belt
    Asteroid belt
    The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

    , while Pluto, Haumea, and Makemake are members of the Kuiper belt and Eris is a member of the scattered disc
    Scattered disc
    The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...

    . Scientists such as Mike Brown
    Michael E. Brown
    Michael E. Brown has been a professor of planetary astronomy at the California Institute of Technology since 2003....

     believe that there are probably over one hundred trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.

Planetary attributes

Name Equatorial
diameter{{Ref label|a|a|a}}
Mass{{Ref label|a|a|a}} Orbital
radius (AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

)
Orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...


(years){{Ref label|a|a|a}}
Inclination
to Sun's equator
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...

 (°)
Orbital
eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

Rotation period
Rotation period
The rotation period of an astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars...


(days)
Confirmed
moons
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

{{Ref label|c|c|c}}
Rings Atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

Terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

0.382 0.06 0.39 0.24 3.38 0.206 58.64 0 no minimal
Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

0.949 0.82 0.72 0.62 3.86 0.007 −243.02 0 no CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, N2
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

{{Ref label|b|b|b}}
1.00 1.00 1.00 1.00 7.25 0.017 1.00 1
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

no N2, O2
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

0.532 0.11 1.52 1.88 5.65 0.093 1.03 2 no CO2, N2
Gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

11.209 317.8 5.20 11.86 6.09 0.048 0.41 64 yes
Rings of Jupiter
The planet Jupiter has a system of rings, known as the rings of Jupiter or the Jovian ring system. It was the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. It was first observed in 1979 by the Voyager 1 space probe and thoroughly investigated in the 1990s...

H2
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

, He
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

9.449 95.2 9.54 29.46 5.51 0.054 0.43 62 yes
Rings of Saturn
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...

H2, He
Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

4.007 14.6 19.22 84.01 6.48 0.047 −0.72 27 yes
Rings of Uranus
The planet Uranus has a system of rings intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10, 1977, by James L. Elliot, Edward W. Dunham, and Douglas J. Mink...

H2, He
Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

3.883 17.2 30.06 164.8 6.43 0.009 0.67 13 yes
Rings of Neptune
The rings of Neptune consist primarily of five principal rings predicted in 1984 by André Brahic and imaged in 1989 by the Voyager 2 spacecraft...

H2, He
Dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

Ceres 0.08 0.000 2 2.5–3.0 4.60 10.59 0.080 0.38 0 no none
Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

0.18 0.002 2 29.7–49.3 248.09 17.14 0.249 −6.39 4 no temporary
Haumea 0.15×0.12 0.000 7 35.2–51.5 282.76 28.19 0.189 0.16 2
Moons of Haumea
The outer Solar System dwarf planet Haumea has two known moons, Hiiaka and Namaka, named after Hawaiian goddesses. These small moons were discovered in 2005, from observations of Haumea made at the large telescopes of the W. M. Keck Observatory in Hawaii....

? ?
Makemake ~0.12 0.000 7 38.5–53.1 309.88 28.96 0.159 ? 0 ? ? {{Ref label|d|d|d}}
Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

0.19 0.002 5 37.8–97.6 ~557 44.19 0.442 ~0.3 1
Dysnomia (moon)
- References :...

? ? {{Ref label|d|d|d}}


Extrasolar planets


{{Main|Extrasolar planet}}

The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor
Michel Mayor
Dr. Michel G. E. Mayor is a Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the Observatory of Geneva...

 and Didier Queloz
Didier Queloz
Didier Queloz is a Geneva-based astronomer with a prolific record in finding extrasolar planets. He is understudy to Michel Mayor.Didier Queloz was a Ph.D...

 of the University of Geneva
University of Geneva
The University of Geneva is a public research university located in Geneva, Switzerland.It was founded in 1559 by John Calvin, as a theological seminary and law school. It remained focused on theology until the 17th century, when it became a center for Enlightenment scholarship. In 1873, it...

 announced the detection of an exoplanet around 51 Pegasi
51 Pegasi
51 Pegasi is a Sun-like star located 15.6 parsecs from Earth in the constellation Pegasus...

. Of the {{Extrasolar planet counts|planet_count}} extrasolar planets discovered by {{Extrasolar planet counts|asof}}, most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass have been observed. The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

s, such as PSR B1257+12
PSR B1257+12
PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located roughly 2000 light-years from the Sun. In 2007, it was confirmed that three extrasolar planets orbit the pulsar.- Pulsar :...

.

There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses, such as those orbiting the stars Mu Arae
Mu Arae
Mu Arae , often referred to by its designation in the Henry Draper catalogue HD 160691, is a main sequence G-type star around 50 light-years away from Earth in the constellation of Ara. The star has a planetary system with four known planets, three of them with masses comparable to that of Jupiter...

, 55 Cancri
55 Cancri
55 Cancri , also cataloged Rho1 Cancri or abbreviated 55 Cnc, is a binary star approximately 41 light-years away from Earth in the constellation of Cancer...

 and GJ 436. These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths).

Another new category are the so-called "super-Earth
Super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability...

s", possibly terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

s larger than Earth but smaller than Neptune or Uranus. To date, about twenty possible super-Earths (depending on mass limits) have been found, including OGLE-2005-BLG-390Lb
OGLE-2005-BLG-390Lb
OGLE-2005-BLG-390Lb is a 'super-Earth' extrasolar planet orbiting the star OGLE-2005-BLG-390L, which is situated 21,500 ± 3,300 light years away from Earth, near the center of the Milky Way galaxy...

 and MOA-2007-BLG-192Lb
MOA-2007-BLG-192Lb
MOA-2007-BLG-192Lb, occasionally shortened to MOA-192 b, is an extrasolar planet approximately 3,000 light-years away in the constellation of Sagittarius. The planet was discovered orbiting the brown dwarf or low-mass star MOA-2007-BLG-192L. At a mass of approximately 3.3 times Earth, it is one of...

, frigid icy worlds discovered through gravitational microlensing
Gravitational microlensing
Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects ranging from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit lots of light ...

, Kepler 10b, a planet with a diameter roughly 1.4 times that of Earth, (making it the smallest super-Earth yet measured) and five of the six planets orbiting the nearby red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

 Gliese 581
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...

. Gliese 581 d
Gliese 581 d
Gliese 581 d or Gl 581 d is an extrasolar planet orbiting the star Gliese 581 approximately 20 light-years away in the constellation of Libra. It is the third planet discovered in the system and the fifth in order from the star....

 is roughly 7.7 times Earth's mass, while Gliese 581 c
Gliese 581 c
Gliese 581 c or Gl 581 c is a planet orbiting the red dwarf star Gliese 581. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth, it is classified as a super-Earth...

 is five times Earth's mass and was initially thought to be the first terrestrial planet found within a star's habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

. However, more detailed studies revealed that it was slightly too close to its star to be habitable, and that the farther planet in the system, Gliese 581 d, though it is much colder than Earth, could potentially be habitable if its atmosphere contained sufficient greenhouse gases.

It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planet
Carbon planet
A carbon planet, also referred to as a diamond planet or carbide planet, is a theoretical type of planet proposed by Marc Kuchner that could form if protoplanetary discs are carbon-rich and oxygen-poor. According to planetary science, it would develop differently from Earth, Mars and Venus, planets...

s. In particular, some of the newly discovered planets, known as hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...

s, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There may also exist a class of hot Jupiters, called Chthonian planet
Chthonian planet
A chthonian planet is a hypothetical class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a star...

s, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered.

More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the COROT
Corot
Corot may refer to:* Jean-Baptiste-Camille Corot, French landscape painter * COROT, a space mission with the dual aims of finding extrasolar planets and performing asteroseismology* COROT-7, a dwarf star in the Monoceros constellation...

 and Kepler
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...

 spacecraft are searching for stellar luminosity variations due to transiting planets
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...

. Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's, Terrestrial Planet Finder
Terrestrial Planet Finder
The Terrestrial Planet Finder was a proposed project by NASA to construct a system of telescopes for detecting extrasolar terrestrial planets. TPF was postponed several times and finally cancelled...

, and Space Interferometry Mission
Space Interferometry Mission
The Space Interferometry Mission, or SIM, also known as SIM Lite , was a planned space telescope developed by the U.S. National Aeronautics and Space Administration , in conjunction with contractor Northrop Grumman...

 programs, and the CNES' PEGASE
PEGASE
PEGASE is a proposed space mission to build a double-aperture interferometer composed of three free-flying satellites. The goal of the mission is the study of Hot Jupiters , brown dwarfs and the interior of protoplanetary disks. The mission would be performed by the Centre National d'Études...

. The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...

. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 (HD 209458 b
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....

 and HD 189733 b
HD 189733 b
HD 189733 b is an extrasolar planet approximately 63 light-years away in the constellation of Vulpecula . The planet was discovered orbiting the star HD 189733 on October 5, 2005, when astronomers in France observed the planet transiting across the face of the star. The planet is classified as a...

). The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation
Drake equation
The Drake equation is an equation used to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy. It is used in the fields of exobiology and the Search for ExtraTerrestrial Intelligence...

 which estimates the number of intelligent, communicating civilizations
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

 that exist in our galaxy.

Planetary-mass objects


A planetary-mass object, PMO, or planemo is a celestial object with a mass that falls within the range of the definition of a planet – i.e. a mass greater than that of a minor object
Small solar system body
A small Solar System body is an object in the Solar System that is neither a planet nor a dwarf planet, nor a satellite of a planet or dwarf planet:...

, yet smaller than that of a nuclear-reactive brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

 or star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

. By definition, all planets are planetary-mass objects, but the purpose of the term is to describe objects which do not conform to typical expectations for a planet. Free-floating planets not orbiting a star may be rogue planets ejected from their system, or objects that have formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs).

Rogue planets


{{Main|Rogue planet}}
Several computer simulation
Computer simulation
A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...

s of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

. Some scientists have argued that such objects found roaming in deep space should be classed as "planets", although others have suggested that they could be low-mass stars.

Sub-brown dwarfs


{{Main|Sub-brown dwarf}}
Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as Cha 110913-773444
Cha 110913-773444
Cha 110913-773444 is an astronomical object surrounded by what appears to be a protoplanetary disk...

, or orbiting a larger object such as 2MASS J04414489+2301513
2MASS J04414489+2301513
2MASS J04414489+2301513 is a young brown dwarf approximately 450 light years away with an orbiting companion about 5-10 times the mass of Jupiter.The mass of the primary brown dwarf is roughly 20 times the mass of Jupiter and its age is roughly one million years.It is not clear whether this...

.

For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515
Oph 162225-240515
Oph 162225-240515, often abbreviated Oph1622, is a pair of brown dwarfs that have been reported as orbiting each other. The bodies are located in the constellation Ophiuchus and are about 400 light years away...

, which the discoverers described as "planemos", or "planetary-mass objects". However, recent analysis of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

s.

Former stars


In close binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

 systems one of the stars can lose mass to a heavier companion. See accretion-powered pulsars. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

 PSR J1719-1438.

Satellite planets and belt planets


Some large satellites are of similar size or larger than the planet Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

, e.g. Jupiter's Galilean moons
Galilean moons
The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance...

 and Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

. Alan Stern
Alan Stern
S. Alan Stern is an American planetary scientist. He is the principal investigator of the New Horizons mission to Pluto....

 has argued that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet, and proposes the term satellite planet for a planet-sized object orbiting another planet. Likewise, planet-sized objects in the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

 or Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

 should also be planets according to Stern.

Attributes


Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whilst others are also common to extrasolar planets.

Orbit


According to current definitions, all planets must revolve around stars; thus, any potential "rogue planet
Rogue Planet
- Literature :* "Rogue Planet" , a Dan Dare story that ran in the original Eagle comic from Volume 6, Issue 48 to Volume 8, Issue 7* Rogue Planet , a 2000 novel set in the Star Wars galaxy- Other :...

s" are excluded. In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates (counter-clockwise as seen from above the Sun's north pole). At least one extrasolar planet, WASP-17b
WASP-17b
WASP-17b is an exoplanet in the constellation Scorpius that is orbiting the star WASP-17. Its discovery was announced on 11 August 2009. It is the first planet discovered to have a retrograde orbit, meaning it orbits in a direction counter to the rotation of its host star. This discovery changed...

, has been found to orbit in the opposite direction to its star's rotation. The period of one revolution of a planet's orbit is known as its sidereal period or year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....

. A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), while its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory.

Each planet's orbit is delineated by a set of elements
Orbital elements
Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are generally considered in classical two-body systems, where a Kepler orbit is used...

:
  • The eccentricity
    Orbital eccentricity
    The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

    of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with high eccentricities have more elliptical orbits. The planets in the Solar System have very low eccentricities, and thus nearly circular orbits. Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.
  • The semi-major axis
    Semi-major axis
    The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

    is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not the same as its apastron, as no planet's orbit has its star at its exact centre.
  • The inclination of a planet tells how far above or below an established reference plane its orbit lies. In the Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic
    Ecliptic
    The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...

    . For extrasolar planets, the plane, known as the sky plane or plane of the sky, is the plane of the observer's line of sight from Earth. The eight planets of the Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it. The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes. The longitude of the ascending node
    Longitude of the ascending node
    The longitude of the ascending node is one of the orbital elements used to specify the orbit of an object in space. It is the angle from a reference direction, called the origin of longitude, to the direction of the ascending node, measured in a reference plane...

     is the angle between the reference plane's 0 longitude and the planet's ascending node. The argument of periapsis
    Argument of periapsis
    The argument of periapsis , symbolized as ω, is one of the orbital elements of an orbiting body...

     (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.

Axial tilt


Planets also have varying degrees of axial tilt
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...

; they lie at an angle to the plane of their stars' equators
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...

. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

s; changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...

. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices. Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars.

Rotation


The planets rotate around invisible axes through their centres. A planet's rotation period
Rotation period
The rotation period of an astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars...

 is known as a stellar day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...

. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the sun's north pole, the exceptions being Venus and Uranus which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise. However, regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit.

The rotation of a planet can be induced by several factors during formation. A net angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

 can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the gas giants can also contribute to the angular momentum. Finally, during the last stages of planet building, a stochastic process
Stochastic process
In probability theory, a stochastic process , or sometimes random process, is the counterpart to a deterministic process...

 of protoplanetary accretion can randomly alter the spin axis of the planet. There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours. The rotational periods of extrasolar planets are not known; however their proximity to their stars means that hot Jupiters are tidally locked
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...

 (their orbits are in sync with their rotations). This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.

Orbital clearing


The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...

s in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

's official definition of a planet in August, 2006. This criterion excludes such planetary bodies as Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

, Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

 and Ceres from full-fledged planethood, making them instead dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s. Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.

Mass


A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...

. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.

Mass is also the prime attribute by which planets are distinguished from star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s. The upper mass limit for planethood is roughly 13 times Jupiter's mass for objects with solar-type isotopic abundance, beyond which it achieves conditions suitable for nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

. Other than the Sun, no objects of such mass exist in the Solar System; but there are exoplanets of this size. The 13MJ limit is not universally agreed upon and the Extrasolar Planets Encyclopaedia
Extrasolar Planets Encyclopaedia
The Extrasolar Planets Encyclopedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains a database of all the currently known and candidate extrasolar planets, with individual "note" pages for each planet and a full list...

 includes objects up to 20 Jupiter masses, and the Exoplanet Data Explorer
Exoplanet Data Explorer
The Exoplanet Data Explorer / Exoplanet Orbit Database lists extrasolar planets up to 24 Jupiter masses."We have retained the generous upper mass limit of 24 Jupiter masses in our definition of a “planet”, for the same reasons as in the Catalog: at the moment, any mass limit is arbitrary and will...

 up to 24 Jupiter masses.

The smallest known planet, excluding dwarf planets and satellites, is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

. Its mass is roughly half that of the planet Mercury.

Internal differentiation


Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...

 interior consisting of a dense planetary core
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...

 surrounded by a mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

 which either is or was a fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

. The terrestrial planets are sealed within hard crusts
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

, but in the gas giants the mantle simply dissolves into the upper cloud layers. The terrestrial planets possess cores of magnetic elements such as iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

, and mantles of silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

s. Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 are believed to possess cores of rock and metal surrounded by mantles of metallic hydrogen
Metallic hydrogen
Metallic hydrogen is a state of hydrogen which results when it is sufficiently compressed and undergoes a phase transition; it is an example of degenerate matter. Solid metallic hydrogen is predicted to consist of a crystal lattice of hydrogen nuclei , with a spacing which is significantly smaller...

. Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

, which are smaller, possess rocky cores surrounded by mantles of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

, methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 and other ices
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...

. The fluid action within these planets' cores creates a geodynamo that generates a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

.

Atmosphere


{{See also|Extraterrestrial atmospheres}}

All of the Solar System planets have atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

s as their large masses mean gravity is strong enough to keep gaseous particles close to the surface. The larger gas giants are massive enough to keep large amounts of the light gases hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 close by, while the smaller planets lose these gases into space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

. The composition of the Earth's atmosphere is different from the other planets because the various life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

 processes that have transpired on the planet have introduced free molecular oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

. The only solar planet without a substantial atmosphere is Mercury which had it mostly, although not entirely, blasted away by the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

.

Planetary atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storm
Dust storm
A dust / sand storm is a meteorological phenomenon common in arid and semi-arid regions. Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface. Particles are transported by saltation and suspension, causing soil to move from one place and deposition...

s (on Mars), an Earth-sized anticyclone
Anticyclonic storm
An anticyclonic storm is a weather storm where winds around the storm flow contrary to the direction dictated by the Coriolis effect about a region of low pressure. In the northern hemisphere, anticyclonic storms involve clockwise wind flow; in the southern hemisphere, they involve counterclockwise...

 on Jupiter (called the Great Red Spot), and holes in the atmosphere
Great Dark Spot
The Great Dark Spot is the name given to a series of dark spots on Neptune similar in appearance to Jupiter's Great Red Spot. The first one was observed in 1989 by NASA's Voyager 2 probe. Like Jupiter's spot, they are anticyclonic storms...

 (on Neptune). At least one extrasolar planet, HD 189733 b
HD 189733 b
HD 189733 b is an extrasolar planet approximately 63 light-years away in the constellation of Vulpecula . The planet was discovered orbiting the star HD 189733 on October 5, 2005, when astronomers in France observed the planet transiting across the face of the star. The planet is classified as a...

, has been claimed to possess such a weather system, similar to the Great Red Spot but twice as large.

Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. These planets may have vast differences in temperature between their day and night sides which produce supersonic winds, although the day and night sides of HD 189733 b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.

Magnetosphere


One important characteristic of the planets is their intrinsic magnetic moment
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...

s which in turn give rise to magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

s. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

 with the solar wind, which cannot effectively protect the planet.

Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field. In addition, the moon of Jupiter Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...

 also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System (so strong in fact that it poses a serious health risk to future manned missions to its moons). The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.

In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star HD 179949
HD 179949
HD 179949 is a 6th magnitude star in the constellation of Sagittarius. It is a yellow-white dwarf , a type of star hotter and more luminous than our Sun...

, which appeared to be creating a sunspot on the surface of its parent star. The team hypothesised that the planet's magnetosphere was transferring energy onto the star's surface, increasing its already high 7,760 °C temperature by an additional 400 °C.

Secondary characteristics


Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

 with each other or with smaller bodies (this is also common in satellite systems). All except Mercury and Venus have natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

s, often called "moons". Earth has one, Mars has two, and the gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

s have numerous moons in complex planetary-type systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

).

The four gas giants are also orbited by planetary ring
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...

s of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny 'moonlets' whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's Roche limit
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...

 and were torn apart by tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...

s.

No secondary characteristics have been observed around extrasolar planets. However the sub-brown dwarf
Sub-brown dwarf
A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass for thermonuclear fusion of deuterium ....

 Cha 110913-773444
Cha 110913-773444
Cha 110913-773444 is an astronomical object surrounded by what appears to be a protoplanetary disk...

, which has been described as a rogue planet
Rogue Planet
- Literature :* "Rogue Planet" , a Dan Dare story that ran in the original Eagle comic from Volume 6, Issue 48 to Volume 8, Issue 7* Rogue Planet , a 2000 novel set in the Star Wars galaxy- Other :...

, is believed to be orbited by a tiny protoplanetary disc.
{{Clear}}

External links


{{Commons category|Planets}}
{{Wiktionary|planet}}

{{Solar System}}
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