A
planet (from
GreekAncient Greek is the historical stage in the development of the Greek language spanning across the Archaic , Classical , and Hellenistic periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek...
, alternative form of "wanderer"), is a celestial body
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
ing a
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
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
planetesimalPlanetesimals 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 dust grains that collide and stick to form larger and larger bodies...
s.
The term
planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of the gods. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the
International Astronomical UnionThe International Astronomical Union 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 SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
. This definition has been both praised and criticized, and remains disputed by some scientists.
The planets were thought by
PtolemyClaudius Ptolemaeus , known in English as Ptolemy , was a Roman citizen of Greek ancestry. He was a mathematician, astronomer, geographer, astrologer and a poet of a single epigram in the Greek Anthology. He lived in Egypt under the Roman Empire, and is believed to have been born in the town of...
to orbit the Earth in
deferent and epicycleIn 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 SunIn astronomy, heliocentrism is the theory that the Sun is stationary and at the center of the universe. The word came from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. Discussions on the possibility of heliocentrism date to classical...
had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by
Galileo GalileiGalileo 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 KeplerJohannes Kepler was a German mathematician, astronomer and astrologer, and 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...
found the planets' orbits to be not circular, but elliptical. As observational tools improved,
astronomerAn 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 share such features as ice-caps and seasons. Since the dawn of the
Space AgeThe Space Age is a contemporary period encompassing the activities related to the Space Race, space exploration, space technology, and the cultural developments influenced by these events...
, close observation by
probesA 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
volcanism3. Conduit
4. Base
5. Sill
6. Dike
7. Layers of ash emitted by the volcano
8. Flank| 9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14. Crater
15...
, hurricanes,
tectonicsTectonics 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
hydrologyHydrology is the study of the movement, distribution, and quality of water throughout Earth, and thus addresses both the hydrologic cycle and water resources...
. Since 1992, through the discovery of hundreds of planets around other stars, called
extrasolar planetAn extrasolar planet, or exoplanet, is a planet beyond our solar system, orbiting a star other than our Sun. , 403 exoplanets are listed in the Extrasolar Planets Encyclopaedia. The vast majority have been detected through radial velocity observations and other indirect methods rather than actual...
s, scientists are beginning to understand that planets throughout the
Milky Way GalaxyThe Milky Way, or simply the Galaxy, is the galaxy in which the Solar System is located. It is a barred spiral galaxy that is part of the Local Group of galaxies...
share characteristics in common with our own. As of October 2009, there are 403 known extrasolar planets, ranging from the size of gas giants to that of terrestrial planets.
Planets are generally divided into two main types: large, low-density
gas giantA gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune...
s, and smaller, rocky
terrestrialsA terrestrial planet, telluric planet, rocky planet or inner planet is a planet that is primarily composed of silicate rocks. Within the solar system, the terrestrial planets are the closest planets to the Sun...
. Under IAU definitions, there are eight planets in the Solar System. In order from the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, they are the four terrestrials,
MercuryFor the liquid metallic element, see Mercury .Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 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...
,
VenusVenus is the second-closest planet to 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...
,
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, and
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
, then the four gas giants,
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
,
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
,
UranusUranus is the seventh planet from the Sun, and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus the father of Kronos and grandfather of Zeus...
, and
NeptuneNeptune is the eighth planet from the Sun in our 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 Earth masses and...
. The Solar System also contains at least five
dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
s: Ceres,
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
(originally classified as the Solar System's ninth planet), Makemake, Haumea and
ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
. With the exception of Mercury, Venus, Ceres and Makemake, all of these are orbited by one or more
natural satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
s.
History
The idea of planets has evolved over its history, from the divine
wandering starsWandering Stars is an anthology of Jewish fantasy and science fiction, edited by Jack Dann, originally published by Harper & Row in 1974.Table of Contents:*Introduction: "Why Me?" by Isaac Asimov*"On Venus, Have We Got a Rabbi" by William Tenn...
of antiquity to the earthly objects of the scientific age. The concept has also now 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.
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,
ChinaAstronomy in China has a very long history, and historians consider that, 'they were the most persistent and accurate observers of celestial phenomena anywhere in the world before the Arabs'....
,
BabylonBabylon was a 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 centre of the
UniverseThe Universe comprises everything that physically exists, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them...
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 senseCommon sense , based on a strict construction of the term, consists of what people in common would agree on: that which they "sense" as their common natural understanding...
perception that the Earth was solid and stable, and that it is not moving but at rest.
Babylon
The first Western civilisation known to possess a functional theory of the planets were the Babylonians, who lived in
MesopotamiaMesopotamia "land between the rivers" is a name for the Tigris–Euphrates region in the eastern Mediterranean, largely corresponding to Iraq, as well as northeastern Syria, some parts of southeastern Turkey, and some parts of the Khūzestān Province of southwestern...
in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian
Venus tablet of AmmisaduqaThe Venus tablet of Ammisaduqa recovered from the library at Nineveh, is a 7th century BC cuneiform tablet that bears much older records of the rise times of Venus and its first and last visibility on the horizon before or after sunrise and sunset in the form of lunar dates...
, 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 Babylonians also laid the foundations of what would eventually become
Western astrologyWestern astrology is the system of astrology most popular in Western countries. Western astrology was founded by Ptolemy's Tetrabiblos in the 2nd century AD, and forms a continuation of Hellenistic astrology and ultimately Babylonian astrology....
. The
Enuma anu enlilEnuma 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
omenAn omen is a phenomenon that is believed to foretell the future, often signifying the advent of change. Omens may be considered "good" or "bad", but the term is more often used in a foreboding sense, as with the word "ominous".-In ancient Rome:Ancient Roman religion employed two distinct types of...
s and their relationships with various celestial phenomena including the motions of the planets. The
SumerSumer was a civilization and historical region in southern Iraq . It is the earliest known civilization in the world and is known as the Cradle of Civilization...
ians, predecessors of the Babylonians who are considered as one of the
first civilizationsThe cradle of civilization is 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 Anatolia and the Persian Plateau,besides other Asian cultures situated...
and are credited with the invention of
writingWriting 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 the recording of language via a non-textual medium such as magnetic tape audio.In Eurasia writing began as a...
, had identified at least Venus by 1500 BC.
Ancient Greece to Medieval Europe
Ptolemy's "planetary spheres"
| Modern |
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
|
Mercury |
Venus |
the Sun The Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
|
Mars |
Jupiter |
Saturn |
| Medieval Europe |
☾ LVNA |
☿ MERCVRIVS |
♀VENVS |
☉ SOL |
♂ MARS |
♃ IVPITER |
♄ SATVRNVS |
The
ancient GreekAncient Greece is the civilisation belonging to the period of Greek history lasting from the Greek Dark Ages ca. 1100 BC and the Dorian invasion, to 146 BC and the Roman conquest of Greece after the Battle of Corinth. It is generally considered to be the seminal culture which provided the...
cosmological system was taken from that of the
BabyloniaBabylonia was a civilization in Lower Mesopotamia , with Babylon as its capital. Babylonia emerged when Hammurabi created an empire out of the territories of the former kingdoms of Sumer and Akkad...
ns, from whom they began to acquire astronomical learning from around 600 BC, including the
constellationIn modern astronomy, a constellation is an area of the celestial sphere, defined by exact boundaries.The term "constellation" can also be used loosely to refer to just the more prominent visible stars that seem to form a pattern in that area.-Definitions:...
s and the
zodiacIn astronomy, the zodiac is the ring of constellations that lines the ecliptic, which is the apparent path of the Sun across the sky over the course of the year. The Moon and planets also lie within the ecliptic, and so are also within the constellations of the zodiac. In astrology, the zodiac...
. In the 6th century BC, the Babylonians' astronomical knowledge at the time was far in advance of the Greeks. The earliest known Greek sources, such as the
IliadThe Iliad is an epic poem recounting significant events during a portion of the final year of the Trojan War — the Greek siege of the city of Ilion — hence the title...
and the
OdysseyThe Odyssey is one of two major ancient Greek epic poems attributed to Homer. It is, in part, a sequel to the Iliad, the other work traditionally ascribed to Homer. The poem is fundamental to the modern Western canon. Indeed it is the second—the Iliad being the first—extant work of...
, do not mention the planets.
By the first century BC, 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
AlmagestAlmagest is the Latin form of the Arabic name of a mathematical and astronomical treatise proposing the complex motions of the stars and planetary paths, originally written in Greek as by Ptolemy of Alexandria, Egypt, written in the 2nd century...
written by
PtolemyClaudius Ptolemaeus , known in English as Ptolemy , was a Roman citizen of Greek ancestry. He was a mathematician, astronomer, geographer, astrologer and a poet of a single epigram in the Greek Anthology. He lived in Egypt under the Roman Empire, and is believed to have been born in the town of...
in the 2nd century AD. 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 EarthIn astronomy, the geocentric model or the Ptolemaic worldview of the universe is the theory, now superseded, that the Earth is the center of the universe and other objects go around it. Belief in this system was common in ancient Greece...
according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
, Mercury, Venus, the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, Mars, Jupiter, and Saturn.
European Renaissance
Renaissance planets
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
The five
naked-eye planetIn antiquity the classical planets were the non-fixed objects visible in the sky, known to various ancient cultures. The classical planets were therefore the Sun and Moon and the five non-earth planets of our solar system closest to the sun ; all easily visible without a telescope. They are the...
s may have been known since ancient times, and have had a significant impact on
mythologyMythology is the study of myths and or of a body of myths. For example, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece. The term "myth" is often used colloquially to refer to a false story;...
,
religious cosmologyReligious cosmologies are ways of explaining the history and evolution of the universe based, at least in part, on the acceptance of principles that cannot be justified by accepted scientific arguments...
, and ancient
astronomyAstronomy is the scientific study of celestial objects and phenomena that originate outside the Earth's atmosphere...
. As scientific knowledge progressed, however, 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
CopernicusNicolaus Copernicus was the first astronomer to formulate a comprehensive heliocentric cosmology, which displaced the Earth from the center of the universe...
,
GalileoGalileo 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
KeplerJohannes Kepler was a German mathematician, astronomer and astrologer, and 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...
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
Planets in early 1800s
| Mercury |
Venus |
Earth |
Mars |
Vesta |
Juno |
Ceres |
Pallas |
Jupiter |
Saturn |
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,
Pallas2 Pallas is one of the largest asteroids and is located in the main asteroid belt. It was the second asteroid to be discovered, by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802...
, and
Vesta4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt...
) were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the
Asteroid beltThe 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 "
asteroidthumb|260px|right|[[253 Mathilde]], a [[C-type asteroid]] measuring about across. Photograph taken in 1997 by the [[NEAR Shoemaker]] probe.Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are...
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 from late 1800s to 1930
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
However, in the 20th century,
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest 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 LyttletonRaymond Arthur Lyttleton was a British astronomer.- External links :*...
suggested that Pluto may be an escaped satellite of
NeptuneNeptune is the eighth planet from the Sun in our 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 Earth masses and...
, and
Fred WhippleFred 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 |
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
Pluto |
In 1992, astronomers
Aleksander WolszczanAleksander Wolszczan is a Polish astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets.- Scientific career :...
and
Dale FrailDale A. Frail is a Canadian radio astronomer working for the National Radio Astronomy Observatory in Socorro, New Mexico. He received his Ph.D. from the University of Toronto in 1989...
announced the discovery of planets around a
pulsarPulsars are highly magnetized, rotating neutron stars that emit a beam of electromagnetic radiation. The observed periods of their pulses range from 1.4 milliseconds to 8.5 seconds. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the...
,
PSR B1257+12PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located 980 light-years from the Sun. As of 2007, it is 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 MayorMichel G. E. Mayor is a Swiss professor in the Department of Astronomy at the University of Geneva.Together with Didier Queloz in 1995 he discovered 51 Pegasi b, the first extrasolar planet orbiting a sun-like star, 51 Pegasi....
and
Didier QuelozDidier Queloz is a Geneva-based astronomer with a prolific record in finding extrasolar planets. He is understudy to Michel Mayor....
of the
University of GenevaThe University of Geneva is a university in Geneva, Switzerland.Founded by John Calvin in 1559 as a theological seminary that also taught law, it remained focused on theology until the 17th century, when it became a center for Enlightenment scholarship. In 1873 it dropped its religious...
announced the first definitive detection of an exoplanet orbiting an ordinary
main-sequenceThe main sequence is a continuous and distinctive band of stars that appear 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 Pegasi51 Pegasi is a Sun-like star located 15.4 parsecs from Earth in the constellation Pegasus. It was the first Sun-like star, other than the Sun, found to have a planet orbiting it, a discovery that was announced in 1995....
).
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 dwarfBrown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which 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
deuteriumDeuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in of hydrogen...
, a heavier isotope of
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
. 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
| Planets 2006- |
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
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
beltThe 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
deuteriumDeuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in of hydrogen...
.
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 beltThe 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 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 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
Quaoar50000 Quaoar is a binary trans-Neptunian object and dwarf planet candidate orbiting the Sun in the Kuiper belt. It was 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...
,
Sedna90377 Sedna is a trans-Neptunian object and a likely dwarf planet discovered by Michael Brown , Chad Trujillo and David Rabinowitz on November 14, 2003. It is currently 88 AU from the Sun, about three times as distant as Neptune...
, and
ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
were heralded in the popular press as the
tenth planet, failing however to receive widespread scientific recognition. The discovery of Eris, an object more massive than Pluto, brought things to a head.
Acknowledging the problem, the IAU set about creating the
definition of planetFrom its beginnings denoting the "wandering stars" of the classical world, the definition of "planet" has been fraught with ambiguity. In its long life, the word has meant many different things, often simultaneously...
, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of
dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
s was created, initially containing three objects (Ceres, Pluto and Eris).
Extrasolar planet definition
| Dwarf Planets 2006- |
| Ceres |
Pluto |
Makemake |
Haumea |
Eris |
In 2003, The
International Astronomical UnionThe International Astronomical Union 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:
Image:EightTNOs.png|thumb|275px|Comparison of ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
, PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
, Makemake, Haumea, Sedna90377 Sedna is a trans-Neptunian object and a likely dwarf planet discovered by Michael Brown , Chad Trujillo and David Rabinowitz on November 14, 2003. It is currently 88 AU from the Sun, about three times as distant as Neptune...
, Orcus90482 Orcus is a Kuiper Belt object and a likely dwarf planet that was discovered by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. The discovery images of this object were acquired on February 17, 2004...
, Quaoar50000 Quaoar is a binary trans-Neptunian object and dwarf planet candidate orbiting the Sun in the Kuiper belt. It was 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...
, Varuna20000 Varuna is a large classical Kuiper Belt object and a potential dwarf planet. It previously had the provisional designation ' and has been precovered in plates dating back to 1953.- Name :...
, and EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
(all to scale).
- Earth
rect 646 1714 2142 1994 The EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
- Eris and Dysnomia
circle 226 412 16 DysnomiaDysnomia, officially Eris I Dysnomia, is the only known moon of the dwarf planet Eris. It was discovered in 2005 by Mike Brown and the laser guide star adaptive optics team at the W. M...
circle 350 626 197 ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
- Pluto and Charon
circle 1252 684 86 CharonCharon, discovered in 1978 at the United States Naval Observatory Flagstaff Station, is the largest satellite of the dwarf planet Pluto. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
circle 1038 632 188 PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
- Makemake
circle 1786 614 142 Makemake
- Haumea
circle 2438 616 155 Haumea
- Sedna
circle 342 1305 137 Sedna90377 Sedna is a trans-Neptunian object and a likely dwarf planet discovered by Michael Brown , Chad Trujillo and David Rabinowitz on November 14, 2003. It is currently 88 AU from the Sun, about three times as distant as Neptune...
- Orcus
circle 1088 1305 114 Orcus90482 Orcus is a Kuiper Belt object and a likely dwarf planet that was discovered by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. The discovery images of this object were acquired on February 17, 2004...
- Quaoar
circle 1784 1305 97 Quaoar50000 Quaoar is a binary trans-Neptunian object and dwarf planet candidate orbiting the Sun in the Kuiper belt. It was 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...
- Varuna
circle 2420 1305 58 Varuna20000 Varuna is a large classical Kuiper Belt object and a potential dwarf planet. It previously had the provisional designation ' and has been precovered in plates dating back to 1953.- Name :...
- link to image (under all other links)
rect 0 0 2749 1994
desc bottom-right
- - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
- Notes:
- Details on the new coding for clickable images is here: mw:Extension:ImageMap
- While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
- Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.
- Objects with 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 abundanceIn chemistry, natural abundance refers to the abundance 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.
- Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarf
Brown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth...
s", no matter how they formed or where they are located.
- Free-floating objects in young 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, a more loosely clustered group 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 journalAn 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. Content typically takes the...
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.
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 planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
s, provided they are not also
natural satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
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 SoterDr. 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. The next IAU
conferenceAn academic conference is a conference for researchers to present and discuss their work. Together with academic or scientific journals, conferences provide an important channel for exchange of information between researchers....
is not until 2009, when modifications could be made to the IAU definition, also possibly including extrasolar planets.
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
mediaMass media denotes a section of the media specifically designed to reach a very large audience such as the population of a nation state. The term was coined in the 1920s with the advent of nationwide radio networks, mass-circulation newspapers and magazines. However, some forms of mass media such...
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 SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
bodies formerly considered to be planets:
| Bodies |
Notes |
| Sun The Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
|
Classified as planets in antiquityAncient history is the study of the written past from the beginning of recorded human history in the Old World until the Early Middle Ages in Europe and the Qin Dynasty in China....
, in accordance with the definition then used. |
IoIo is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometres, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus.With over 400 active volcanoes, Io is the most...
, EuropaEuropa is the sixth moon of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei , and named after a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete...
, GanymedeGanymede is a moon of Jupiter and the largest moon in the Solar System. Completing an orbit in roughly seven days, it is the seventh moon and third Galilean moon from Jupiter. Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. It is larger in diameter...
, and CallistoCallisto is a moon of the planet Jupiter, 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 planet Mercury but only about a third of its mass...
|
The four largest moons of JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
, known as the Galilean moonsThe Galilean moons are the four moons of Jupiter discovered by Galileo Galilei on January 7, 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 GalileiGalileo 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 patronPatronage 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 familyThe House of Medici was a political dynasty, banking family and later royal house who 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, gradually rising until...
. |
TitanTitan , or Saturn VI, is the largest moon of Saturn, the only moon 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....
, IapetusIapetus , occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the solar system, discovered by Giovanni Domenico Cassini in 1671...
, RheaRhea 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"...
, TethysTethys is a moon of Saturn that was discovered by Giovanni Domenico Cassini in 1684.-Name:Tethys is named after the titan Tethys of Greek mythology...
, and DioneDione 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 Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
's larger moonsThe moons of Saturn are numerous and diverse, ranging from tiny moonlets to the enormous Titan. Saturn has 61 moons with confirmed orbits, 52 of which have names, and most of which are quite small. There are also hundreds of known moonlets embedded within Saturn's rings...
, discovered by Christiaan HuygensChristiaan Huygens, FRS was a prominent Dutch mathematician, astronomer, physicist, horologist, and writer of early science fiction...
and Giovanni Domenico CassiniThis 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...
. |
Pallas2 Pallas is one of the largest asteroids and is located in the main asteroid belt. It was the second asteroid to be discovered, by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802...
, JunoJuno , 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 Vesta4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt...
|
The first known asteroids, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s.
Ceres has subsequently been classified as a dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
. |
Astrea5 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...
, Hebe6 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...
, Iris7 Iris is a large main belt asteroid. Among S-type asteroids it ranks fifth in geometric mean diameter after Eunomia, Juno, Amphitrite and Herculina....
, Flora8 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...
, Metis9 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...
, Hygeia10 Hygiea is an asteroid located in the main 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 object in the region by volume and mass...
, Parthenope11 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...
, Victoria12 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 of the United Kingdom. The goddess Victoria was the daughter of Styx by the Titan Pallas...
, Egeria13 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...
, Irene14 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...
, Eunomia15 Eunomia is a very large asteroid in the inner main 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. |
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
|
Trans-Neptunian objectA trans-Neptunian object is any object in the Solar System that orbits the Sun at a greater distance on average than Neptune. The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space....
with a semi-major axisIn geometry, the semi-major axis is used to describe the dimensions of ellipses and hyperbolae.- Ellipse :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 NeptuneNeptune is the eighth planet from the Sun in our 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 Earth masses and...
. In 2006, Pluto was reclassified as a dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
. |
Mythology
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 GreeceAncient Greece is the civilisation belonging to the period of Greek history lasting from the Greek Dark Ages ca. 1100 BC and the Dorian invasion, to 146 BC and the Roman conquest of Greece after the Battle of Corinth. It is generally considered to be the seminal culture which provided the...
, the two great luminaries the Sun and the Moon were called
HeliosIn Greek mythology the sun was personified as Helios 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
SeleneSelene is the Titan goddess of the moon.In Greek mythology, Seléne 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 of their pantheon of gods, the
OlympiansThe Twelve Olympians, also known as the Dodekatheon , in Greek mythology, were the principal gods of the Greek pantheon, residing atop Mount Olympus. The first ancient reference of religious ceremonies for them is found in the Homeric Hymn to Hermes...
: Helios and Selene were the names of both planets and gods; Phainon was sacred to
KronosCronus or Kronos was the leader and the youngest of the first generation of Titans, divine descendants of Gaia, the earth goddess, and Ouranos, the sky...
, the
TitanIn Greek mythology, the Titans , were a race of powerful deities that ruled during the legendary Golden Age...
who fathered the Olympians; Phaethon was sacred to
ZeúsIn Greek mythology, Zeus is the king of the gods, the ruler of Mount Olympus and the god of the sky and thunder. His symbols are the thunderbolt, eagle, bull, and oak. In addition to his Indo-European inheritance, the classical "cloud-gatherer" also derives certain iconographic traits from the...
, Kronos's son who deposed him as king; Pyroeis was given to
AresIn Greek mythology, Ares is the son of Zeus and Hera. Though often referred to as the Olympian god of warfare, he is more accurately the god of bloodlust, or slaughter personified: "Ares is apparently an ancient abstract noun meaning throng of battle, war."-Etymology:Ares is the god of war...
, son of Zeus and god of war; Phosphorus was ruled by
AphroditeAphrodite is the Greek goddess of love, beauty and raw sexuality. According to Greek poet Hesiod, she was born when Cronus cut off Ouranos's genitals and threw them into the sea, and from the aphros arose Aphrodite.Because of her beauty other gods feared that jealousy would interrupt the peace...
, the goddess of love; and
HermesHermes is the Messenger of the gods in Greek mythology as well as a guide to the Underworld. An Olympian god, he is also the patron of boundaries and of the travelers who cross them, of shepherds and cowherds, of thieves and road travelers, of orators and wit, of literature and poets, of...
, 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 Phosphorus after their goddess of love,
IshtarIshtar is the Assyrian and Babylonian counterpart to the Sumerian Inanna and to the cognate northwest Semitic goddess Astarte.-Characteristics:Ishtar is a goddess of fertility, love, war, and sex...
; Pyroeis after their god of war,
NergalThe name Nergal refers to a deity in Babylonia 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
NabuNabu is the 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,
MardukMarduk 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 slowly...
. 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 godsThe Twelve Olympians, also known as the Dodekatheon , in Greek mythology, were the principal gods of the Greek pantheon, residing atop Mount Olympus. The first ancient reference of religious ceremonies for them is found in the Homeric Hymn to Hermes...
. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the
Roman EmpireThe Roman Empire was the post-Republican phase of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean. The term is used to describe the Roman state during and after the time of the first emperor,...
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 pantheonRoman mythology, or Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its main city, Ancient Rome. It can be considered as having two parts; One part, largely later and literary, consists of borrowings from Greek mythology...
under different names but lacked the rich narrative traditions that Greek poetic culture had given
their godsGreek 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 RepublicThe Roman Republic was the phase of the ancient Roman civilization characterized by a republican form of government. It began with the overthrow of the Roman monarchy, c...
, 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:
MercuriusMercury was a messenger, and a god of trade, profit and commerce, the son of Maia Maiestas, also known as Ops, the Roman version of Rhea, and Jupiter. His name is related to the Latin word merx...
(for Hermes),
VenusVenus was a major Roman goddess principally associated with love, beauty and fertility, who played a key role in many Roman religious festivals and myths. From the third century BC, the increasing Hellenization of Roman upper classes identified her as the equivalent of the Greek goddess...
(Aphrodite),
MarsMars was the Roman god of war, the son of Juno and Jupiter, husband of Bellona, and the lover of Venus. He was the most prominent of the military gods that were worshipped by the Roman legions. The martial Romans considered him second in importance only to Jupiter...
(Ares),
IuppiterIn Roman mythology, Jupiter or Jove was the king of the gods, and the god of sky and thunder. He is the equivalent of Zeus in the Greek pantheon. He was called Iuppiter Optimus Maximus ; as the patron deity of the Roman state, he ruled over laws and social order...
(Zeus) and
SaturnusSaturn was a major Roman god of agriculture and harvest. In medieval times he was known as the Roman god of agriculture, justice and strength; he held a sickle in his left hand and a bundle of wheat in his right. His mother's name was Helen, or Hel...
(Kronos). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained:
Uranus (
OuranosUranus is the Latinized form of Ouranos , the Greek word for sky. In Greek mythology Ouranos or Father Sky, is personified as the son and husband of Gaia, Mother Earth...
) and
NeptūnusNeptune is the god of water and the sea in Roman mythology, a brother of Jupiter and Pluto. He is analogous with but not identical to the god Poseidon of Greek mythology. The Roman conception of Neptune owed a great deal to the Etruscan god Nethuns....
(
PoseidonIn Greek mythology, Poseidon was the god of the sea and, as "Earth-Shaker," of earthquakes. 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...
).
Some
RomansAncient Rome was a civilization that grew out of a small agricultural community founded on the Italian Peninsula as early as the 10th century BC. Located along the Mediterranean Sea, it became one of the largest empires in the ancient world....
, following a belief possibly originating in
MesopotamiaMesopotamia "land between the rivers" is a name for the Tigris–Euphrates region in the eastern Mediterranean, largely corresponding to Iraq, as well as northeastern Syria, some parts of southeastern Turkey, and some parts of the Khūzestān Province of southwestern...
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 in the
Roman calendarThe Roman calendar changed its form several times in the time between the foundation 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 many modern languages. Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after
TiwTyr is the god of single combat, victory and heroic glory in Norse mythology, portrayed as a one-handed man.Corresponding names in other Germanic languages are Gothic Teiws , Old English Tīw and Old High German Ziu, all from Proto-Germanic *Tîwaz.In the late Icelandic Eddas, Tyr is portrayed,...
, (Tuesday)
WódenWōden is a god in Anglo-Saxon paganism, together with Norse Odin representing a development of a Proto-Germanic god, *Wōdanaz. Other West Germanic forms of the name include Old High German Wuotan, Low German and Dutch Wodan....
(Wednesday),
ThunorThor is the red-haired and bearded god of thunder in Germanic mythology and Germanic paganism, and its subsets: Norse paganism, Anglo-Saxon paganism and Continental Germanic paganism....
(Thursday), and
Fríge*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.
Since Earth 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-SaxonOld English , also called Anglo-Saxon, is an early form of the English language that was spoken and written in parts of what are now England and south-eastern Scotland between at least the mid-5th century and the mid-12th century. What survives through writing represents primarily the literary...
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. It is the only planet whose name in English is not derived from
GrecoGreek 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...
-
Roman mythologyRoman mythology, or Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its main city, Ancient Rome. It can be considered as having two parts; One part, largely later and literary, consists of borrowings from Greek mythology...
. Many of the
Romance languagesThe Romance languages are a branch of the Indo-European language family comprising all the languages that descend from Latin, the language of ancient Rome...
retain the old Roman word
terraTerra Mater 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.-Form and function:...
(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,
ΓήGaia Gaia Gaia ( or ; "land" or "earth", from the Ancient Greek Γαῖα; also Gæa or Gea (Koine and Modern Greek Γῆ) is the primal Greek goddess personifying the Earth....
(
Ge or
Yi); the
Germanic languagesThe Germanic languages are a group of related languages that constitute a branch of the Indo-European language family. The common ancestor of all the languages in this branch is Proto-Germanic, spoken in approximately the mid-1st millennium BC in Iron Age northern Europe...
, including English, use a variation of an ancient Germanic word
ertho, "ground," as can be seen in the English
Earth, the German
Erde, the Dutch
Aarde, and the Scandinavian
Jorde.
Non-European cultures use other planetary naming systems.
IndiaIndia, 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, and the most populous democracy in the world. Bounded by the Indian Ocean on the south, the Arabian Sea on the west, and the Bay of Bengal...
uses a naming system based on the
NavagrahaGraha is a 'cosmic influencer' on the living beings of mother Bhumidevi . In Hindu Astrology, the Navagraha are some of these major influencers."The Sanskrit word captures the idea that these nine grahas are living energies which put out waves of energy...
, which incorporates the seven traditional planets (
SuryaIn Hinduism, Sūrya is the chief solar deity, 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,
ChandraIn 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
BudhaIn 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....
,
ShukraShukra , the Sanskrit for "clear, pure" or "brightness, clearness", is the name the son of Bhrgu and Urjaswathi, and preceptor of the Daityas, and the guru of the Asuras, identified with the planet Venus, one of the Navagrahas...
,
MangalaIn Jyotish astrology, Mangala is the name for Mars, the red planet. Mars is also called Angaraka or Bhauma in Sanskrit. He is the god of war and is celibate. He is considered the son of Prithvi or Bhumi, the Earth Goddess...
,
{{Otheruses4|the astronomical objects|other uses|planet (disambiguation)}}
A
planet (from
GreekAncient Greek is the historical stage in the development of the Greek language spanning across the Archaic , Classical , and Hellenistic periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek...
{{polytonic|πλανήτης}}, alternative form of {{polytonic|πλάνης}} "wanderer"), is a celestial body
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
ing a
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
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
planetesimalPlanetesimals 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 dust grains that collide and stick to form larger and larger bodies...
s.{{Ref_label|A|a|none}}
The term
planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of the gods. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the
International Astronomical UnionThe International Astronomical Union 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 SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
. This definition has been both praised and criticized, and remains disputed by some scientists.
The planets were thought by
PtolemyClaudius Ptolemaeus , known in English as Ptolemy , was a Roman citizen of Greek ancestry. He was a mathematician, astronomer, geographer, astrologer and a poet of a single epigram in the Greek Anthology. He lived in Egypt under the Roman Empire, and is believed to have been born in the town of...
to orbit the Earth in
deferent and epicycleIn 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 SunIn astronomy, heliocentrism is the theory that the Sun is stationary and at the center of the universe. The word came from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. Discussions on the possibility of heliocentrism date to classical...
had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by
Galileo GalileiGalileo 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 KeplerJohannes Kepler was a German mathematician, astronomer and astrologer, and 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...
found the planets' orbits to be not circular, but elliptical. As observational tools improved,
astronomerAn 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 share such features as ice-caps and seasons. Since the dawn of the
Space AgeThe Space Age is a contemporary period encompassing the activities related to the Space Race, space exploration, space technology, and the cultural developments influenced by these events...
, close observation by
probesA 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
volcanism3. Conduit
4. Base
5. Sill
6. Dike
7. Layers of ash emitted by the volcano
8. Flank| 9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14. Crater
15...
, hurricanes,
tectonicsTectonics 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
hydrologyHydrology is the study of the movement, distribution, and quality of water throughout Earth, and thus addresses both the hydrologic cycle and water resources...
. Since 1992, through the discovery of hundreds of planets around other stars, called
extrasolar planetAn extrasolar planet, or exoplanet, is a planet beyond our solar system, orbiting a star other than our Sun. , 403 exoplanets are listed in the Extrasolar Planets Encyclopaedia. The vast majority have been detected through radial velocity observations and other indirect methods rather than actual...
s, scientists are beginning to understand that planets throughout the
Milky Way GalaxyThe Milky Way, or simply the Galaxy, is the galaxy in which the Solar System is located. It is a barred spiral galaxy that is part of the Local Group of galaxies...
share characteristics in common with our own. As of October 2009, there are 403 known extrasolar planets, ranging from the size of gas giants to that of terrestrial planets.
Planets are generally divided into two main types: large, low-density
gas giantA gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune...
s, and smaller, rocky
terrestrialsA terrestrial planet, telluric planet, rocky planet or inner planet is a planet that is primarily composed of silicate rocks. Within the solar system, the terrestrial planets are the closest planets to the Sun...
. Under IAU definitions, there are eight planets in the Solar System. In order from the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, they are the four terrestrials,
MercuryFor the liquid metallic element, see Mercury .Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 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...
,
VenusVenus is the second-closest planet to 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...
,
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, and
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
, then the four gas giants,
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
,
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
,
UranusUranus is the seventh planet from the Sun, and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus the father of Kronos and grandfather of Zeus...
, and
NeptuneNeptune is the eighth planet from the Sun in our 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 Earth masses and...
. The Solar System also contains at least five
dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
s: Ceres,
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
(originally classified as the Solar System's ninth planet), Makemake, Haumea and
ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
. With the exception of Mercury, Venus, Ceres and Makemake, all of these are orbited by one or more
natural satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
s.
History
{{Main|History of astronomy|Definition of planet}}
{{See also|Geocentric model|Timeline of solar system astronomy}}
The idea of planets has evolved over its history, from the divine
wandering starsWandering Stars is an anthology of Jewish fantasy and science fiction, edited by Jack Dann, originally published by Harper & Row in 1974.Table of Contents:*Introduction: "Why Me?" by Isaac Asimov*"On Venus, Have We Got a Rabbi" by William Tenn...
of antiquity to the earthly objects of the scientific age. The concept has also now 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.
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,
ChinaAstronomy in China has a very long history, and historians consider that, 'they were the most persistent and accurate observers of celestial phenomena anywhere in the world before the Arabs'....
,
BabylonBabylon was a 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 centre of the
UniverseThe Universe comprises everything that physically exists, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them...
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 senseCommon sense , based on a strict construction of the term, consists of what people in common would agree on: that which they "sense" as their common natural understanding...
perception that the Earth was solid and stable, and that it is not moving but at rest.
Babylon
The first Western civilisation known to possess a functional theory of the planets were the Babylonians, who lived in
MesopotamiaMesopotamia "land between the rivers" is a name for the Tigris–Euphrates region in the eastern Mediterranean, largely corresponding to Iraq, as well as northeastern Syria, some parts of southeastern Turkey, and some parts of the Khūzestān Province of southwestern...
in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian
Venus tablet of AmmisaduqaThe Venus tablet of Ammisaduqa recovered from the library at Nineveh, is a 7th century BC cuneiform tablet that bears much older records of the rise times of Venus and its first and last visibility on the horizon before or after sunrise and sunset in the form of lunar dates...
, 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 Babylonians also laid the foundations of what would eventually become
Western astrologyWestern astrology is the system of astrology most popular in Western countries. Western astrology was founded by Ptolemy's Tetrabiblos in the 2nd century AD, and forms a continuation of Hellenistic astrology and ultimately Babylonian astrology....
. The
Enuma anu enlilEnuma 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
omenAn omen is a phenomenon that is believed to foretell the future, often signifying the advent of change. Omens may be considered "good" or "bad", but the term is more often used in a foreboding sense, as with the word "ominous".-In ancient Rome:Ancient Roman religion employed two distinct types of...
s and their relationships with various celestial phenomena including the motions of the planets. The
SumerSumer was a civilization and historical region in southern Iraq . It is the earliest known civilization in the world and is known as the Cradle of Civilization...
ians, predecessors of the Babylonians who are considered as one of the
first civilizationsThe cradle of civilization is 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 Anatolia and the Persian Plateau,besides other Asian cultures situated...
and are credited with the invention of
writingWriting 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 the recording of language via a non-textual medium such as magnetic tape audio.In Eurasia writing began as a...
, had identified at least Venus by 1500 BC.
Ancient Greece to Medieval Europe
Ptolemy's "planetary spheres"
| Modern |
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
|
Mercury |
Venus |
the Sun The Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
|
Mars |
Jupiter |
Saturn |
| Medieval Europe |
☾ LVNA |
☿ MERCVRIVS |
♀VENVS |
☉ SOL |
♂ MARS |
♃ IVPITER |
♄ SATVRNVS |
The
ancient GreekAncient Greece is the civilisation belonging to the period of Greek history lasting from the Greek Dark Ages ca. 1100 BC and the Dorian invasion, to 146 BC and the Roman conquest of Greece after the Battle of Corinth. It is generally considered to be the seminal culture which provided the...
cosmological system was taken from that of the
BabyloniaBabylonia was a civilization in Lower Mesopotamia , with Babylon as its capital. Babylonia emerged when Hammurabi created an empire out of the territories of the former kingdoms of Sumer and Akkad...
ns, from whom they began to acquire astronomical learning from around 600 BC, including the
constellationIn modern astronomy, a constellation is an area of the celestial sphere, defined by exact boundaries.The term "constellation" can also be used loosely to refer to just the more prominent visible stars that seem to form a pattern in that area.-Definitions:...
s and the
zodiacIn astronomy, the zodiac is the ring of constellations that lines the ecliptic, which is the apparent path of the Sun across the sky over the course of the year. The Moon and planets also lie within the ecliptic, and so are also within the constellations of the zodiac. In astrology, the zodiac...
. In the 6th century BC, the Babylonians' astronomical knowledge at the time was far in advance of the Greeks. The earliest known Greek sources, such as the
IliadThe Iliad is an epic poem recounting significant events during a portion of the final year of the Trojan War — the Greek siege of the city of Ilion — hence the title...
and the
OdysseyThe Odyssey is one of two major ancient Greek epic poems attributed to Homer. It is, in part, a sequel to the Iliad, the other work traditionally ascribed to Homer. The poem is fundamental to the modern Western canon. Indeed it is the second—the Iliad being the first—extant work of...
, do not mention the planets.
By the first century BC, 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
AlmagestAlmagest is the Latin form of the Arabic name of a mathematical and astronomical treatise proposing the complex motions of the stars and planetary paths, originally written in Greek as by Ptolemy of Alexandria, Egypt, written in the 2nd century...
written by
PtolemyClaudius Ptolemaeus , known in English as Ptolemy , was a Roman citizen of Greek ancestry. He was a mathematician, astronomer, geographer, astrologer and a poet of a single epigram in the Greek Anthology. He lived in Egypt under the Roman Empire, and is believed to have been born in the town of...
in the 2nd century AD. 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 EarthIn astronomy, the geocentric model or the Ptolemaic worldview of the universe is the theory, now superseded, that the Earth is the center of the universe and other objects go around it. Belief in this system was common in ancient Greece...
according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
, Mercury, Venus, the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, Mars, Jupiter, and Saturn.
European Renaissance
Renaissance planets
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
{{See also|Heliocentrism}}
The five
naked-eye planetIn antiquity the classical planets were the non-fixed objects visible in the sky, known to various ancient cultures. The classical planets were therefore the Sun and Moon and the five non-earth planets of our solar system closest to the sun ; all easily visible without a telescope. They are the...
s may have been known since ancient times, and have had a significant impact on
mythologyMythology is the study of myths and or of a body of myths. For example, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece. The term "myth" is often used colloquially to refer to a false story;...
,
religious cosmologyReligious cosmologies are ways of explaining the history and evolution of the universe based, at least in part, on the acceptance of principles that cannot be justified by accepted scientific arguments...
, and ancient
astronomyAstronomy is the scientific study of celestial objects and phenomena that originate outside the Earth's atmosphere...
. As scientific knowledge progressed, however, 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
CopernicusNicolaus Copernicus was the first astronomer to formulate a comprehensive heliocentric cosmology, which displaced the Earth from the center of the universe...
,
GalileoGalileo 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
KeplerJohannes Kepler was a German mathematician, astronomer and astrologer, and 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...
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
Planets in early 1800s
| Mercury |
Venus |
Earth |
Mars |
Vesta |
Juno |
Ceres |
Pallas |
Jupiter |
Saturn |
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,
Pallas2 Pallas is one of the largest asteroids and is located in the main asteroid belt. It was the second asteroid to be discovered, by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802...
, and
Vesta4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt...
) were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the
Asteroid beltThe 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 "
asteroidthumb|260px|right|[[253 Mathilde]], a [[C-type asteroid]] measuring about across. Photograph taken in 1997 by the [[NEAR Shoemaker]] probe.Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are...
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 from late 1800s to 1930
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
However, in the 20th century,
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest 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 LyttletonRaymond Arthur Lyttleton was a British astronomer.- External links :*...
suggested that Pluto may be an escaped satellite of
NeptuneNeptune is the eighth planet from the Sun in our 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 Earth masses and...
, and
Fred WhippleFred 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 |
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
Pluto |
In 1992, astronomers
Aleksander WolszczanAleksander Wolszczan is a Polish astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets.- Scientific career :...
and
Dale FrailDale A. Frail is a Canadian radio astronomer working for the National Radio Astronomy Observatory in Socorro, New Mexico. He received his Ph.D. from the University of Toronto in 1989...
announced the discovery of planets around a
pulsarPulsars are highly magnetized, rotating neutron stars that emit a beam of electromagnetic radiation. The observed periods of their pulses range from 1.4 milliseconds to 8.5 seconds. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the...
,
PSR B1257+12PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located 980 light-years from the Sun. As of 2007, it is 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 MayorMichel G. E. Mayor is a Swiss professor in the Department of Astronomy at the University of Geneva.Together with Didier Queloz in 1995 he discovered 51 Pegasi b, the first extrasolar planet orbiting a sun-like star, 51 Pegasi....
and
Didier QuelozDidier Queloz is a Geneva-based astronomer with a prolific record in finding extrasolar planets. He is understudy to Michel Mayor....
of the
University of GenevaThe University of Geneva is a university in Geneva, Switzerland.Founded by John Calvin in 1559 as a theological seminary that also taught law, it remained focused on theology until the 17th century, when it became a center for Enlightenment scholarship. In 1873 it dropped its religious...
announced the first definitive detection of an exoplanet orbiting an ordinary
main-sequenceThe main sequence is a continuous and distinctive band of stars that appear 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 Pegasi51 Pegasi is a Sun-like star located 15.4 parsecs from Earth in the constellation Pegasus. It was the first Sun-like star, other than the Sun, found to have a planet orbiting it, a discovery that was announced in 1995....
).
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 dwarfBrown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which 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
deuteriumDeuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in of hydrogen...
, a heavier isotope of
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
. 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
| Planets 2006- |
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
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
beltThe 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
deuteriumDeuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in of hydrogen...
.
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 beltThe 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 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 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
Quaoar50000 Quaoar is a binary trans-Neptunian object and dwarf planet candidate orbiting the Sun in the Kuiper belt. It was 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...
,
Sedna90377 Sedna is a trans-Neptunian object and a likely dwarf planet discovered by Michael Brown , Chad Trujillo and David Rabinowitz on November 14, 2003. It is currently 88 AU from the Sun, about three times as distant as Neptune...
, and
ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
were heralded in the popular press as the
tenth planet, failing however to receive widespread scientific recognition. The discovery of Eris, an object more massive than Pluto, brought things to a head.
Acknowledging the problem, the IAU set about creating the
definition of planetFrom its beginnings denoting the "wandering stars" of the classical world, the definition of "planet" has been fraught with ambiguity. In its long life, the word has meant many different things, often simultaneously...
, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of
dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
s was created, initially containing three objects (Ceres, Pluto and Eris).
Extrasolar planet definition
| Dwarf Planets 2006- |
| Ceres |
Pluto |
Makemake |
Haumea |
Eris |
In 2003, The
International Astronomical UnionThe International Astronomical Union 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:
Image:EightTNOs.png|thumb|275px|Comparison of ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
, PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
, Makemake, Haumea, Sedna90377 Sedna is a trans-Neptunian object and a likely dwarf planet discovered by Michael Brown , Chad Trujillo and David Rabinowitz on November 14, 2003. It is currently 88 AU from the Sun, about three times as distant as Neptune...
, Orcus90482 Orcus is a Kuiper Belt object and a likely dwarf planet that was discovered by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. The discovery images of this object were acquired on February 17, 2004...
, Quaoar50000 Quaoar is a binary trans-Neptunian object and dwarf planet candidate orbiting the Sun in the Kuiper belt. It was 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...
, Varuna20000 Varuna is a large classical Kuiper Belt object and a potential dwarf planet. It previously had the provisional designation ' and has been precovered in plates dating back to 1953.- Name :...
, and EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
(all to scale).
- Earth
rect 646 1714 2142 1994 The EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
- Eris and Dysnomia
circle 226 412 16 DysnomiaDysnomia, officially Eris I Dysnomia, is the only known moon of the dwarf planet Eris. It was discovered in 2005 by Mike Brown and the laser guide star adaptive optics team at the W. M...
circle 350 626 197 ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
- Pluto and Charon
circle 1252 684 86 CharonCharon, discovered in 1978 at the United States Naval Observatory Flagstaff Station, is the largest satellite of the dwarf planet Pluto. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
circle 1038 632 188 PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
- Makemake
circle 1786 614 142 Makemake
- Haumea
circle 2438 616 155 Haumea
- Sedna
circle 342 1305 137 Sedna90377 Sedna is a trans-Neptunian object and a likely dwarf planet discovered by Michael Brown , Chad Trujillo and David Rabinowitz on November 14, 2003. It is currently 88 AU from the Sun, about three times as distant as Neptune...
- Orcus
circle 1088 1305 114 Orcus90482 Orcus is a Kuiper Belt object and a likely dwarf planet that was discovered by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. The discovery images of this object were acquired on February 17, 2004...
- Quaoar
circle 1784 1305 97 Quaoar50000 Quaoar is a binary trans-Neptunian object and dwarf planet candidate orbiting the Sun in the Kuiper belt. It was 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...
- Varuna
circle 2420 1305 58 Varuna20000 Varuna is a large classical Kuiper Belt object and a potential dwarf planet. It previously had the provisional designation ' and has been precovered in plates dating back to 1953.- Name :...
- link to image (under all other links)
rect 0 0 2749 1994
desc bottom-right
- - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
- Notes:
- Details on the new coding for clickable images is here: mw:Extension:ImageMap
- While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
- Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.
- Objects with 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 abundanceIn chemistry, natural abundance refers to the abundance 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.
- Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarf
Brown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth...
s", no matter how they formed or where they are located.
- Free-floating objects in young 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, a more loosely clustered group 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 journalAn 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. Content typically takes the...
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.
2006 definition
{{Main|2006 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 equilibriumHydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient wihch creates a pressure gradient force in the opposite direction...
(nearly round) shape, and (c) has
cleared the neighbourhoodIn the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence...
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 planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
s, provided they are not also
natural satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
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 SoterDr. 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. The next IAU
conferenceAn academic conference is a conference for researchers to present and discuss their work. Together with academic or scientific journals, conferences provide an important channel for exchange of information between researchers....
is not until 2009, when modifications could be made to the IAU definition, also possibly including extrasolar planets.
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
mediaMass media denotes a section of the media specifically designed to reach a very large audience such as the population of a nation state. The term was coined in the 1920s with the advent of nationwide radio networks, mass-circulation newspapers and magazines. However, some forms of mass media such...
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 SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
bodies formerly considered to be planets:
| Bodies |
Notes |
| Sun The Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
|
Classified as planets in antiquityAncient history is the study of the written past from the beginning of recorded human history in the Old World until the Early Middle Ages in Europe and the Qin Dynasty in China....
, in accordance with the definition then used. |
IoIo is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometres, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus.With over 400 active volcanoes, Io is the most...
, EuropaEuropa is the sixth moon of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei , and named after a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete...
, GanymedeGanymede is a moon of Jupiter and the largest moon in the Solar System. Completing an orbit in roughly seven days, it is the seventh moon and third Galilean moon from Jupiter. Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. It is larger in diameter...
, and CallistoCallisto is a moon of the planet Jupiter, 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 planet Mercury but only about a third of its mass...
|
The four largest moons of JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
, known as the Galilean moonsThe Galilean moons are the four moons of Jupiter discovered by Galileo Galilei on January 7, 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 GalileiGalileo 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 patronPatronage 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 familyThe House of Medici was a political dynasty, banking family and later royal house who 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, gradually rising until...
. |
TitanTitan , or Saturn VI, is the largest moon of Saturn, the only moon 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}} IapetusIapetus , occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the solar system, discovered by Giovanni Domenico Cassini in 1671...
,{{Ref_label|C|c|none}} RheaRhea 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}} TethysTethys is a moon of Saturn that was discovered by Giovanni Domenico Cassini in 1684.-Name:Tethys is named after the titan Tethys of Greek mythology...
,{{Ref_label|D|d|none}} and DioneDione 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 Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
's larger moonsThe moons of Saturn are numerous and diverse, ranging from tiny moonlets to the enormous Titan. Saturn has 61 moons with confirmed orbits, 52 of which have names, and most of which are quite small. There are also hundreds of known moonlets embedded within Saturn's rings...
, discovered by Christiaan HuygensChristiaan Huygens, FRS was a prominent Dutch mathematician, astronomer, physicist, horologist, and writer of early science fiction...
and Giovanni Domenico CassiniThis 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...
. |
E|e|none}} Pallas2 Pallas is one of the largest asteroids and is located in the main asteroid belt. It was the second asteroid to be discovered, by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802...
, JunoJuno , 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 Vesta4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt...
|
The first known asteroids, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s.
Ceres has subsequently been classified as a dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
. |
Astrea5 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...
, Hebe6 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...
, Iris7 Iris is a large main belt asteroid. Among S-type asteroids it ranks fifth in geometric mean diameter after Eunomia, Juno, Amphitrite and Herculina....
, Flora8 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...
, Metis9 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...
, Hygeia10 Hygiea is an asteroid located in the main 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 object in the region by volume and mass...
, Parthenope11 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...
, Victoria12 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 of the United Kingdom. The goddess Victoria was the daughter of Styx by the Titan Pallas...
, Egeria13 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...
, Irene14 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...
, Eunomia15 Eunomia is a very large asteroid in the inner main 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. |
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
{{Ref_label>F|f|none}} |
Trans-Neptunian objectA trans-Neptunian object is any object in the Solar System that orbits the Sun at a greater distance on average than Neptune. The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space....
with a semi-major axisIn geometry, the semi-major axis is used to describe the dimensions of ellipses and hyperbolae.- Ellipse :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 NeptuneNeptune is the eighth planet from the Sun in our 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 Earth masses and...
. In 2006, Pluto was reclassified as a dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
. |
Mythology
{{See also|Week-day 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 GreeceAncient Greece is the civilisation belonging to the period of Greek history lasting from the Greek Dark Ages ca. 1100 BC and the Dorian invasion, to 146 BC and the Roman conquest of Greece after the Battle of Corinth. It is generally considered to be the seminal culture which provided the...
, the two great luminaries the Sun and the Moon were called
HeliosIn Greek mythology the sun was personified as Helios 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
SeleneSelene is the Titan goddess of the moon.In Greek mythology, Seléne 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 of their pantheon of gods, the
OlympiansThe Twelve Olympians, also known as the Dodekatheon , in Greek mythology, were the principal gods of the Greek pantheon, residing atop Mount Olympus. The first ancient reference of religious ceremonies for them is found in the Homeric Hymn to Hermes...
: Helios and Selene were the names of both planets and gods; Phainon was sacred to
KronosCronus or Kronos was the leader and the youngest of the first generation of Titans, divine descendants of Gaia, the earth goddess, and Ouranos, the sky...
, the
TitanIn Greek mythology, the Titans , were a race of powerful deities that ruled during the legendary Golden Age...
who fathered the Olympians; Phaethon was sacred to
ZeúsIn Greek mythology, Zeus is the king of the gods, the ruler of Mount Olympus and the god of the sky and thunder. His symbols are the thunderbolt, eagle, bull, and oak. In addition to his Indo-European inheritance, the classical "cloud-gatherer" also derives certain iconographic traits from the...
, Kronos's son who deposed him as king; Pyroeis was given to
AresIn Greek mythology, Ares is the son of Zeus and Hera. Though often referred to as the Olympian god of warfare, he is more accurately the god of bloodlust, or slaughter personified: "Ares is apparently an ancient abstract noun meaning throng of battle, war."-Etymology:Ares is the god of war...
, son of Zeus and god of war; Phosphorus was ruled by
AphroditeAphrodite is the Greek goddess of love, beauty and raw sexuality. According to Greek poet Hesiod, she was born when Cronus cut off Ouranos's genitals and threw them into the sea, and from the aphros arose Aphrodite.Because of her beauty other gods feared that jealousy would interrupt the peace...
, the goddess of love; and
HermesHermes is the Messenger of the gods in Greek mythology as well as a guide to the Underworld. An Olympian god, he is also the patron of boundaries and of the travelers who cross them, of shepherds and cowherds, of thieves and road travelers, of orators and wit, of literature and poets, of...
, 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 Phosphorus after their goddess of love,
IshtarIshtar is the Assyrian and Babylonian counterpart to the Sumerian Inanna and to the cognate northwest Semitic goddess Astarte.-Characteristics:Ishtar is a goddess of fertility, love, war, and sex...
; Pyroeis after their god of war,
NergalThe name Nergal refers to a deity in Babylonia 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
NabuNabu is the 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,
MardukMarduk 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 slowly...
. 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 godsThe Twelve Olympians, also known as the Dodekatheon , in Greek mythology, were the principal gods of the Greek pantheon, residing atop Mount Olympus. The first ancient reference of religious ceremonies for them is found in the Homeric Hymn to Hermes...
. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the
Roman EmpireThe Roman Empire was the post-Republican phase of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean. The term is used to describe the Roman state during and after the time of the first emperor,...
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 pantheonRoman mythology, or Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its main city, Ancient Rome. It can be considered as having two parts; One part, largely later and literary, consists of borrowings from Greek mythology...
under different names but lacked the rich narrative traditions that Greek poetic culture had given
their godsGreek 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 RepublicThe Roman Republic was the phase of the ancient Roman civilization characterized by a republican form of government. It began with the overthrow of the Roman monarchy, c...
, 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:
MercuriusMercury was a messenger, and a god of trade, profit and commerce, the son of Maia Maiestas, also known as Ops, the Roman version of Rhea, and Jupiter. His name is related to the Latin word merx...
(for Hermes),
VenusVenus was a major Roman goddess principally associated with love, beauty and fertility, who played a key role in many Roman religious festivals and myths. From the third century BC, the increasing Hellenization of Roman upper classes identified her as the equivalent of the Greek goddess...
(Aphrodite),
MarsMars was the Roman god of war, the son of Juno and Jupiter, husband of Bellona, and the lover of Venus. He was the most prominent of the military gods that were worshipped by the Roman legions. The martial Romans considered him second in importance only to Jupiter...
(Ares),
IuppiterIn Roman mythology, Jupiter or Jove was the king of the gods, and the god of sky and thunder. He is the equivalent of Zeus in the Greek pantheon. He was called Iuppiter Optimus Maximus ; as the patron deity of the Roman state, he ruled over laws and social order...
(Zeus) and
SaturnusSaturn was a major Roman god of agriculture and harvest. In medieval times he was known as the Roman god of agriculture, justice and strength; he held a sickle in his left hand and a bundle of wheat in his right. His mother's name was Helen, or Hel...
(Kronos). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained:
Uranus (
OuranosUranus is the Latinized form of Ouranos , the Greek word for sky. In Greek mythology Ouranos or Father Sky, is personified as the son and husband of Gaia, Mother Earth...
) and
NeptūnusNeptune is the god of water and the sea in Roman mythology, a brother of Jupiter and Pluto. He is analogous with but not identical to the god Poseidon of Greek mythology. The Roman conception of Neptune owed a great deal to the Etruscan god Nethuns....
(
PoseidonIn Greek mythology, Poseidon was the god of the sea and, as "Earth-Shaker," of earthquakes. 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...
).
Some
RomansAncient Rome was a civilization that grew out of a small agricultural community founded on the Italian Peninsula as early as the 10th century BC. Located along the Mediterranean Sea, it became one of the largest empires in the ancient world....
, following a belief possibly originating in
MesopotamiaMesopotamia "land between the rivers" is a name for the Tigris–Euphrates region in the eastern Mediterranean, largely corresponding to Iraq, as well as northeastern Syria, some parts of southeastern Turkey, and some parts of the Khūzestān Province of southwestern...
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 in the
Roman calendarThe Roman calendar changed its form several times in the time between the foundation 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 many modern languages. Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after
TiwTyr is the god of single combat, victory and heroic glory in Norse mythology, portrayed as a one-handed man.Corresponding names in other Germanic languages are Gothic Teiws , Old English Tīw and Old High German Ziu, all from Proto-Germanic *Tîwaz.In the late Icelandic Eddas, Tyr is portrayed,...
, (Tuesday)
WódenWōden is a god in Anglo-Saxon paganism, together with Norse Odin representing a development of a Proto-Germanic god, *Wōdanaz. Other West Germanic forms of the name include Old High German Wuotan, Low German and Dutch Wodan....
(Wednesday),
ThunorThor is the red-haired and bearded god of thunder in Germanic mythology and Germanic paganism, and its subsets: Norse paganism, Anglo-Saxon paganism and Continental Germanic paganism....
(Thursday), and
Fríge*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.
Since Earth 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-SaxonOld English , also called Anglo-Saxon, is an early form of the English language that was spoken and written in parts of what are now England and south-eastern Scotland between at least the mid-5th century and the mid-12th century. What survives through writing represents primarily the literary...
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. It is the only planet whose name in English is not derived from
GrecoGreek 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...
-
Roman mythologyRoman mythology, or Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its main city, Ancient Rome. It can be considered as having two parts; One part, largely later and literary, consists of borrowings from Greek mythology...
. Many of the
Romance languagesThe Romance languages are a branch of the Indo-European language family comprising all the languages that descend from Latin, the language of ancient Rome...
retain the old Roman word
terraTerra Mater 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.-Form and function:...
(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,
ΓήGaia Gaia Gaia ( or ; "land" or "earth", from the Ancient Greek Γαῖα; also Gæa or Gea (Koine and Modern Greek Γῆ) is the primal Greek goddess personifying the Earth....
(
Ge or
Yi); the
Germanic languagesThe Germanic languages are a group of related languages that constitute a branch of the Indo-European language family. The common ancestor of all the languages in this branch is Proto-Germanic, spoken in approximately the mid-1st millennium BC in Iron Age northern Europe...
, including English, use a variation of an ancient Germanic word
ertho, "ground," as can be seen in the English
Earth, the German
Erde, the Dutch
Aarde, and the Scandinavian
Jorde.
Non-European cultures use other planetary naming systems.
IndiaIndia, 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, and the most populous democracy in the world. Bounded by the Indian Ocean on the south, the Arabian Sea on the west, and the Bay of Bengal...
uses a naming system based on the
NavagrahaGraha is a 'cosmic influencer' on the living beings of mother Bhumidevi . In Hindu Astrology, the Navagraha are some of these major influencers."The Sanskrit word captures the idea that these nine grahas are living energies which put out waves of energy...
, which incorporates the seven traditional planets (
SuryaIn Hinduism, Sūrya is the chief solar deity, 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,
ChandraIn 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
BudhaIn 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....
,
ShukraShukra , the Sanskrit for "clear, pure" or "brightness, clearness", is the name the son of Bhrgu and Urjaswathi, and preceptor of the Daityas, and the guru of the Asuras, identified with the planet Venus, one of the Navagrahas...
,
MangalaIn Jyotish astrology, Mangala is the name for Mars, the red planet. Mars is also called Angaraka or Bhauma in Sanskrit. He is the god of war and is celibate. He is considered the son of Prithvi or Bhumi, the Earth Goddess...
,
{{Otheruses4|the astronomical objects|other uses|planet (disambiguation)}}
A
planet (from
GreekAncient Greek is the historical stage in the development of the Greek language spanning across the Archaic , Classical , and Hellenistic periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek...
{{polytonic|πλανήτης}}, alternative form of {{polytonic|πλάνης}} "wanderer"), is a celestial body
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
ing a
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
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
planetesimalPlanetesimals 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 dust grains that collide and stick to form larger and larger bodies...
s.{{Ref_label|A|a|none}}
The term
planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of the gods. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the
International Astronomical UnionThe International Astronomical Union 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 SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
. This definition has been both praised and criticized, and remains disputed by some scientists.
The planets were thought by
PtolemyClaudius Ptolemaeus , known in English as Ptolemy , was a Roman citizen of Greek ancestry. He was a mathematician, astronomer, geographer, astrologer and a poet of a single epigram in the Greek Anthology. He lived in Egypt under the Roman Empire, and is believed to have been born in the town of...
to orbit the Earth in
deferent and epicycleIn 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 SunIn astronomy, heliocentrism is the theory that the Sun is stationary and at the center of the universe. The word came from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. Discussions on the possibility of heliocentrism date to classical...
had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by
Galileo GalileiGalileo 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 KeplerJohannes Kepler was a German mathematician, astronomer and astrologer, and 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...
found the planets' orbits to be not circular, but elliptical. As observational tools improved,
astronomerAn 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 share such features as ice-caps and seasons. Since the dawn of the
Space AgeThe Space Age is a contemporary period encompassing the activities related to the Space Race, space exploration, space technology, and the cultural developments influenced by these events...
, close observation by
probesA 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
volcanism3. Conduit
4. Base
5. Sill
6. Dike
7. Layers of ash emitted by the volcano
8. Flank| 9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14. Crater
15...
, hurricanes,
tectonicsTectonics 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
hydrologyHydrology is the study of the movement, distribution, and quality of water throughout Earth, and thus addresses both the hydrologic cycle and water resources...
. Since 1992, through the discovery of hundreds of planets around other stars, called
extrasolar planetAn extrasolar planet, or exoplanet, is a planet beyond our solar system, orbiting a star other than our Sun. , 403 exoplanets are listed in the Extrasolar Planets Encyclopaedia. The vast majority have been detected through radial velocity observations and other indirect methods rather than actual...
s, scientists are beginning to understand that planets throughout the
Milky Way GalaxyThe Milky Way, or simply the Galaxy, is the galaxy in which the Solar System is located. It is a barred spiral galaxy that is part of the Local Group of galaxies...
share characteristics in common with our own. As of October 2009, there are 403 known extrasolar planets, ranging from the size of gas giants to that of terrestrial planets.
Planets are generally divided into two main types: large, low-density
gas giantA gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune...
s, and smaller, rocky
terrestrialsA terrestrial planet, telluric planet, rocky planet or inner planet is a planet that is primarily composed of silicate rocks. Within the solar system, the terrestrial planets are the closest planets to the Sun...
. Under IAU definitions, there are eight planets in the Solar System. In order from the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, they are the four terrestrials,
MercuryFor the liquid metallic element, see Mercury .Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 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...
,
VenusVenus is the second-closest planet to 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...
,
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, and
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
, then the four gas giants,
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
,
SaturnSaturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
,
UranusUranus is the seventh planet from the Sun, and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus the father of Kronos and grandfather of Zeus...
, and
NeptuneNeptune is the eighth planet from the Sun in our 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 Earth masses and...
. The Solar System also contains at least five
dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
s: Ceres,
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
(originally classified as the Solar System's ninth planet), Makemake, Haumea and
ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
. With the exception of Mercury, Venus, Ceres and Makemake, all of these are orbited by one or more
natural satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
s.
History
{{Main|History of astronomy|Definition of planet}}
{{See also|Geocentric model|Timeline of solar system astronomy}}
The idea of planets has evolved over its history, from the divine
wandering starsWandering Stars is an anthology of Jewish fantasy and science fiction, edited by Jack Dann, originally published by Harper & Row in 1974.Table of Contents:*Introduction: "Why Me?" by Isaac Asimov*"On Venus, Have We Got a Rabbi" by William Tenn...
of antiquity to the earthly objects of the scientific age. The concept has also now 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.
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,
ChinaAstronomy in China has a very long history, and historians consider that, 'they were the most persistent and accurate observers of celestial phenomena anywhere in the world before the Arabs'....
,
BabylonBabylon was a 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 centre of the
UniverseThe Universe comprises everything that physically exists, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them...
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 senseCommon sense , based on a strict construction of the term, consists of what people in common would agree on: that which they "sense" as their common natural understanding...
perception that the Earth was solid and stable, and that it is not moving but at rest.
Babylon
The first Western civilisation known to possess a functional theory of the planets were the Babylonians, who lived in
MesopotamiaMesopotamia "land between the rivers" is a name for the Tigris–Euphrates region in the eastern Mediterranean, largely corresponding to Iraq, as well as northeastern Syria, some parts of southeastern Turkey, and some parts of the Khūzestān Province of southwestern...
in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian
Venus tablet of AmmisaduqaThe Venus tablet of Ammisaduqa recovered from the library at Nineveh, is a 7th century BC cuneiform tablet that bears much older records of the rise times of Venus and its first and last visibility on the horizon before or after sunrise and sunset in the form of lunar dates...
, 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 Babylonians also laid the foundations of what would eventually become
Western astrologyWestern astrology is the system of astrology most popular in Western countries. Western astrology was founded by Ptolemy's Tetrabiblos in the 2nd century AD, and forms a continuation of Hellenistic astrology and ultimately Babylonian astrology....
. The
Enuma anu enlilEnuma 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
omenAn omen is a phenomenon that is believed to foretell the future, often signifying the advent of change. Omens may be considered "good" or "bad", but the term is more often used in a foreboding sense, as with the word "ominous".-In ancient Rome:Ancient Roman religion employed two distinct types of...
s and their relationships with various celestial phenomena including the motions of the planets. The
SumerSumer was a civilization and historical region in southern Iraq . It is the earliest known civilization in the world and is known as the Cradle of Civilization...
ians, predecessors of the Babylonians who are considered as one of the
first civilizationsThe cradle of civilization is 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 Anatolia and the Persian Plateau,besides other Asian cultures situated...
and are credited with the invention of
writingWriting 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 the recording of language via a non-textual medium such as magnetic tape audio.In Eurasia writing began as a...
, had identified at least Venus by 1500 BC.
Ancient Greece to Medieval Europe
Ptolemy's "planetary spheres"
| Modern |
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
|
Mercury |
Venus |
the Sun The Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
|
Mars |
Jupiter |
Saturn |
| Medieval Europe |
☾ LVNA |
☿ MERCVRIVS |
♀VENVS |
☉ SOL |
♂ MARS |
♃ IVPITER |
♄ SATVRNVS |
The
ancient GreekAncient Greece is the civilisation belonging to the period of Greek history lasting from the Greek Dark Ages ca. 1100 BC and the Dorian invasion, to 146 BC and the Roman conquest of Greece after the Battle of Corinth. It is generally considered to be the seminal culture which provided the...
cosmological system was taken from that of the
BabyloniaBabylonia was a civilization in Lower Mesopotamia , with Babylon as its capital. Babylonia emerged when Hammurabi created an empire out of the territories of the former kingdoms of Sumer and Akkad...
ns, from whom they began to acquire astronomical learning from around 600 BC, including the
constellationIn modern astronomy, a constellation is an area of the celestial sphere, defined by exact boundaries.The term "constellation" can also be used loosely to refer to just the more prominent visible stars that seem to form a pattern in that area.-Definitions:...
s and the
zodiacIn astronomy, the zodiac is the ring of constellations that lines the ecliptic, which is the apparent path of the Sun across the sky over the course of the year. The Moon and planets also lie within the ecliptic, and so are also within the constellations of the zodiac. In astrology, the zodiac...
. In the 6th century BC, the Babylonians' astronomical knowledge at the time was far in advance of the Greeks. The earliest known Greek sources, such as the
IliadThe Iliad is an epic poem recounting significant events during a portion of the final year of the Trojan War — the Greek siege of the city of Ilion — hence the title...
and the
OdysseyThe Odyssey is one of two major ancient Greek epic poems attributed to Homer. It is, in part, a sequel to the Iliad, the other work traditionally ascribed to Homer. The poem is fundamental to the modern Western canon. Indeed it is the second—the Iliad being the first—extant work of...
, do not mention the planets.
By the first century BC, 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
AlmagestAlmagest is the Latin form of the Arabic name of a mathematical and astronomical treatise proposing the complex motions of the stars and planetary paths, originally written in Greek as by Ptolemy of Alexandria, Egypt, written in the 2nd century...
written by
PtolemyClaudius Ptolemaeus , known in English as Ptolemy , was a Roman citizen of Greek ancestry. He was a mathematician, astronomer, geographer, astrologer and a poet of a single epigram in the Greek Anthology. He lived in Egypt under the Roman Empire, and is believed to have been born in the town of...
in the 2nd century AD. 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 EarthIn astronomy, the geocentric model or the Ptolemaic worldview of the universe is the theory, now superseded, that the Earth is the center of the universe and other objects go around it. Belief in this system was common in ancient Greece...
according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
, Mercury, Venus, the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, Mars, Jupiter, and Saturn.
European Renaissance
Renaissance planets
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
{{See also|Heliocentrism}}
The five
naked-eye planetIn antiquity the classical planets were the non-fixed objects visible in the sky, known to various ancient cultures. The classical planets were therefore the Sun and Moon and the five non-earth planets of our solar system closest to the sun ; all easily visible without a telescope. They are the...
s may have been known since ancient times, and have had a significant impact on
mythologyMythology is the study of myths and or of a body of myths. For example, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece. The term "myth" is often used colloquially to refer to a false story;...
,
religious cosmologyReligious cosmologies are ways of explaining the history and evolution of the universe based, at least in part, on the acceptance of principles that cannot be justified by accepted scientific arguments...
, and ancient
astronomyAstronomy is the scientific study of celestial objects and phenomena that originate outside the Earth's atmosphere...
. As scientific knowledge progressed, however, 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
CopernicusNicolaus Copernicus was the first astronomer to formulate a comprehensive heliocentric cosmology, which displaced the Earth from the center of the universe...
,
GalileoGalileo 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
KeplerJohannes Kepler was a German mathematician, astronomer and astrologer, and 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...
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
Planets in early 1800s
| Mercury |
Venus |
Earth |
Mars |
Vesta |
Juno |
Ceres |
Pallas |
Jupiter |
Saturn |
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,
Pallas2 Pallas is one of the largest asteroids and is located in the main asteroid belt. It was the second asteroid to be discovered, by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802...
, and
Vesta4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt...
) were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the
Asteroid beltThe 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 "
asteroidthumb|260px|right|[[253 Mathilde]], a [[C-type asteroid]] measuring about across. Photograph taken in 1997 by the [[NEAR Shoemaker]] probe.Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are...
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 from late 1800s to 1930
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
However, in the 20th century,
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest 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 LyttletonRaymond Arthur Lyttleton was a British astronomer.- External links :*...
suggested that Pluto may be an escaped satellite of
NeptuneNeptune is the eighth planet from the Sun in our 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 Earth masses and...
, and
Fred WhippleFred 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 |
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
Pluto |
In 1992, astronomers
Aleksander WolszczanAleksander Wolszczan is a Polish astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets.- Scientific career :...
and
Dale FrailDale A. Frail is a Canadian radio astronomer working for the National Radio Astronomy Observatory in Socorro, New Mexico. He received his Ph.D. from the University of Toronto in 1989...
announced the discovery of planets around a
pulsarPulsars are highly magnetized, rotating neutron stars that emit a beam of electromagnetic radiation. The observed periods of their pulses range from 1.4 milliseconds to 8.5 seconds. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the...
,
PSR B1257+12PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located 980 light-years from the Sun. As of 2007, it is 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 MayorMichel G. E. Mayor is a Swiss professor in the Department of Astronomy at the University of Geneva.Together with Didier Queloz in 1995 he discovered 51 Pegasi b, the first extrasolar planet orbiting a sun-like star, 51 Pegasi....
and
Didier QuelozDidier Queloz is a Geneva-based astronomer with a prolific record in finding extrasolar planets. He is understudy to Michel Mayor....
of the
University of GenevaThe University of Geneva is a university in Geneva, Switzerland.Founded by John Calvin in 1559 as a theological seminary that also taught law, it remained focused on theology until the 17th century, when it became a center for Enlightenment scholarship. In 1873 it dropped its religious...
announced the first definitive detection of an exoplanet orbiting an ordinary
main-sequenceThe main sequence is a continuous and distinctive band of stars that appear 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 Pegasi51 Pegasi is a Sun-like star located 15.4 parsecs from Earth in the constellation Pegasus. It was the first Sun-like star, other than the Sun, found to have a planet orbiting it, a discovery that was announced in 1995....
).
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 dwarfBrown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which 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
deuteriumDeuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in of hydrogen...
, a heavier isotope of
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
. 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
| Planets 2006- |
| Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
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
beltThe 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
deuteriumDeuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in of hydrogen...
.
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 beltThe 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 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 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
Quaoar50000 Quaoar is a binary trans-Neptunian object and dwarf planet candidate orbiting the Sun in the Kuiper belt. It was 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...
,
Sedna90377 Sedna is a trans-Neptunian object and a likely dwarf planet discovered by Michael Brown , Chad Trujillo and David Rabinowitz on November 14, 2003. It is currently 88 AU from the Sun, about three times as distant as Neptune...
, and
ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
were heralded in the popular press as the
tenth planet, failing however to receive widespread scientific recognition. The discovery of Eris, an object more massive than Pluto, brought things to a head.
Acknowledging the problem, the IAU set about creating the
definition of planetFrom its beginnings denoting the "wandering stars" of the classical world, the definition of "planet" has been fraught with ambiguity. In its long life, the word has meant many different things, often simultaneously...
, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of
dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
s was created, initially containing three objects (Ceres, Pluto and Eris).
Extrasolar planet definition
| Dwarf Planets 2006- |
| Ceres |
Pluto |
Makemake |
Haumea |
Eris |
In 2003, The
International Astronomical UnionThe International Astronomical Union 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:
Image:EightTNOs.png|thumb|275px|Comparison of ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
, PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
, Makemake, Haumea, Sedna90377 Sedna is a trans-Neptunian object and a likely dwarf planet discovered by Michael Brown , Chad Trujillo and David Rabinowitz on November 14, 2003. It is currently 88 AU from the Sun, about three times as distant as Neptune...
, Orcus90482 Orcus is a Kuiper Belt object and a likely dwarf planet that was discovered by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. The discovery images of this object were acquired on February 17, 2004...
, Quaoar50000 Quaoar is a binary trans-Neptunian object and dwarf planet candidate orbiting the Sun in the Kuiper belt. It was 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...
, Varuna20000 Varuna is a large classical Kuiper Belt object and a potential dwarf planet. It previously had the provisional designation ' and has been precovered in plates dating back to 1953.- Name :...
, and EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
(all to scale).
- Earth
rect 646 1714 2142 1994 The EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
- Eris and Dysnomia
circle 226 412 16 DysnomiaDysnomia, officially Eris I Dysnomia, is the only known moon of the dwarf planet Eris. It was discovered in 2005 by Mike Brown and the laser guide star adaptive optics team at the W. M...
circle 350 626 197 ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
- Pluto and Charon
circle 1252 684 86 CharonCharon, discovered in 1978 at the United States Naval Observatory Flagstaff Station, is the largest satellite of the dwarf planet Pluto. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
circle 1038 632 188 PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
- Makemake
circle 1786 614 142 Makemake
- Haumea
circle 2438 616 155 Haumea
- Sedna
circle 342 1305 137 Sedna90377 Sedna is a trans-Neptunian object and a likely dwarf planet discovered by Michael Brown , Chad Trujillo and David Rabinowitz on November 14, 2003. It is currently 88 AU from the Sun, about three times as distant as Neptune...
- Orcus
circle 1088 1305 114 Orcus90482 Orcus is a Kuiper Belt object and a likely dwarf planet that was discovered by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. The discovery images of this object were acquired on February 17, 2004...
- Quaoar
circle 1784 1305 97 Quaoar50000 Quaoar is a binary trans-Neptunian object and dwarf planet candidate orbiting the Sun in the Kuiper belt. It was 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...
- Varuna
circle 2420 1305 58 Varuna20000 Varuna is a large classical Kuiper Belt object and a potential dwarf planet. It previously had the provisional designation ' and has been precovered in plates dating back to 1953.- Name :...
- link to image (under all other links)
rect 0 0 2749 1994
desc bottom-right
- - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
- Notes:
- Details on the new coding for clickable images is here: mw:Extension:ImageMap
- While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
- Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.
- Objects with 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 abundanceIn chemistry, natural abundance refers to the abundance 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.
- Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarf
Brown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth...
s", no matter how they formed or where they are located.
- Free-floating objects in young 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, a more loosely clustered group 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 journalAn 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. Content typically takes the...
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.
2006 definition
{{Main|2006 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 equilibriumHydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient wihch creates a pressure gradient force in the opposite direction...
(nearly round) shape, and (c) has
cleared the neighbourhoodIn the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence...
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 planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
s, provided they are not also
natural satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
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 SoterDr. 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. The next IAU
conferenceAn academic conference is a conference for researchers to present and discuss their work. Together with academic or scientific journals, conferences provide an important channel for exchange of information between researchers....
is not until 2009, when modifications could be made to the IAU definition, also possibly including extrasolar planets.
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
mediaMass media denotes a section of the media specifically designed to reach a very large audience such as the population of a nation state. The term was coined in the 1920s with the advent of nationwide radio networks, mass-circulation newspapers and magazines. However, some forms of mass media such...
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 SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
bodies formerly considered to be planets:
| Bodies |
Notes |
| Sun The Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
|
Classified as planets in antiquityAncient history is the study of the written past from the beginning of recorded human history in the Old World until the Early Middle Ages in Europe and the Qin Dynasty in China....
, in accordance with the definition then used. |
IoIo is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometres, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus.With over 400 active volcanoes, Io is the most...
, EuropaEuropa is the sixth moon of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei , and named after a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete...
, GanymedeGanymede is a moon of Jupiter and the largest moon in the Solar System. Completing an orbit in roughly seven days, it is the seventh moon and third Galilean moon from Jupiter. Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. It is larger in diameter...
, and CallistoCallisto is a moon of the planet Jupiter, 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 planet Mercury but only about a third of its mass...
|
The four largest moons of JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
, known as the Galilean moonsThe Galilean moons are the four moons of Jupiter discovered by Galileo Galilei on January 7, 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 GalileiGalileo 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 patronPatronage 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 familyThe House of Medici was a political dynasty, banking family and later royal house who 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, gradually rising until...
. |
TitanTitan , or Saturn VI, is the largest moon of Saturn, the only moon 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}} IapetusIapetus , occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the solar system, discovered by Giovanni Domenico Cassini in 1671...
,{{Ref_label|C|c|none}} RheaRhea 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}} TethysTethys is a moon of Saturn that was discovered by Giovanni Domenico Cassini in 1684.-Name:Tethys is named after the titan Tethys of Greek mythology...
,{{Ref_label|D|d|none}} and DioneDione 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 Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
's larger moonsThe moons of Saturn are numerous and diverse, ranging from tiny moonlets to the enormous Titan. Saturn has 61 moons with confirmed orbits, 52 of which have names, and most of which are quite small. There are also hundreds of known moonlets embedded within Saturn's rings...
, discovered by Christiaan HuygensChristiaan Huygens, FRS was a prominent Dutch mathematician, astronomer, physicist, horologist, and writer of early science fiction...
and Giovanni Domenico CassiniThis 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...
. |
E|e|none}} Pallas2 Pallas is one of the largest asteroids and is located in the main asteroid belt. It was the second asteroid to be discovered, by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802...
, JunoJuno , 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 Vesta4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt...
|
The first known asteroids, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s.
Ceres has subsequently been classified as a dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
. |
Astrea5 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...
, Hebe6 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...
, Iris7 Iris is a large main belt asteroid. Among S-type asteroids it ranks fifth in geometric mean diameter after Eunomia, Juno, Amphitrite and Herculina....
, Flora8 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...
, Metis9 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...
, Hygeia10 Hygiea is an asteroid located in the main 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 object in the region by volume and mass...
, Parthenope11 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...
, Victoria12 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 of the United Kingdom. The goddess Victoria was the daughter of Styx by the Titan Pallas...
, Egeria13 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...
, Irene14 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...
, Eunomia15 Eunomia is a very large asteroid in the inner main 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. |
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
{{Ref_label>F|f|none}} |
Trans-Neptunian objectA trans-Neptunian object is any object in the Solar System that orbits the Sun at a greater distance on average than Neptune. The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space....
with a semi-major axisIn geometry, the semi-major axis is used to describe the dimensions of ellipses and hyperbolae.- Ellipse :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 NeptuneNeptune is the eighth planet from the Sun in our 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 Earth masses and...
. In 2006, Pluto was reclassified as a dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
. |
Mythology
{{See also|Week-day 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 GreeceAncient Greece is the civilisation belonging to the period of Greek history lasting from the Greek Dark Ages ca. 1100 BC and the Dorian invasion, to 146 BC and the Roman conquest of Greece after the Battle of Corinth. It is generally considered to be the seminal culture which provided the...
, the two great luminaries the Sun and the Moon were called
HeliosIn Greek mythology the sun was personified as Helios 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
SeleneSelene is the Titan goddess of the moon.In Greek mythology, Seléne 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 of their pantheon of gods, the
OlympiansThe Twelve Olympians, also known as the Dodekatheon , in Greek mythology, were the principal gods of the Greek pantheon, residing atop Mount Olympus. The first ancient reference of religious ceremonies for them is found in the Homeric Hymn to Hermes...
: Helios and Selene were the names of both planets and gods; Phainon was sacred to
KronosCronus or Kronos was the leader and the youngest of the first generation of Titans, divine descendants of Gaia, the earth goddess, and Ouranos, the sky...
, the
TitanIn Greek mythology, the Titans , were a race of powerful deities that ruled during the legendary Golden Age...
who fathered the Olympians; Phaethon was sacred to
ZeúsIn Greek mythology, Zeus is the king of the gods, the ruler of Mount Olympus and the god of the sky and thunder. His symbols are the thunderbolt, eagle, bull, and oak. In addition to his Indo-European inheritance, the classical "cloud-gatherer" also derives certain iconographic traits from the...
, Kronos's son who deposed him as king; Pyroeis was given to
AresIn Greek mythology, Ares is the son of Zeus and Hera. Though often referred to as the Olympian god of warfare, he is more accurately the god of bloodlust, or slaughter personified: "Ares is apparently an ancient abstract noun meaning throng of battle, war."-Etymology:Ares is the god of war...
, son of Zeus and god of war; Phosphorus was ruled by
AphroditeAphrodite is the Greek goddess of love, beauty and raw sexuality. According to Greek poet Hesiod, she was born when Cronus cut off Ouranos's genitals and threw them into the sea, and from the aphros arose Aphrodite.Because of her beauty other gods feared that jealousy would interrupt the peace...
, the goddess of love; and
HermesHermes is the Messenger of the gods in Greek mythology as well as a guide to the Underworld. An Olympian god, he is also the patron of boundaries and of the travelers who cross them, of shepherds and cowherds, of thieves and road travelers, of orators and wit, of literature and poets, of...
, 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 Phosphorus after their goddess of love,
IshtarIshtar is the Assyrian and Babylonian counterpart to the Sumerian Inanna and to the cognate northwest Semitic goddess Astarte.-Characteristics:Ishtar is a goddess of fertility, love, war, and sex...
; Pyroeis after their god of war,
NergalThe name Nergal refers to a deity in Babylonia 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
NabuNabu is the 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,
MardukMarduk 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 slowly...
. 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 godsThe Twelve Olympians, also known as the Dodekatheon , in Greek mythology, were the principal gods of the Greek pantheon, residing atop Mount Olympus. The first ancient reference of religious ceremonies for them is found in the Homeric Hymn to Hermes...
. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the
Roman EmpireThe Roman Empire was the post-Republican phase of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean. The term is used to describe the Roman state during and after the time of the first emperor,...
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 pantheonRoman mythology, or Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its main city, Ancient Rome. It can be considered as having two parts; One part, largely later and literary, consists of borrowings from Greek mythology...
under different names but lacked the rich narrative traditions that Greek poetic culture had given
their godsGreek 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 RepublicThe Roman Republic was the phase of the ancient Roman civilization characterized by a republican form of government. It began with the overthrow of the Roman monarchy, c...
, 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:
MercuriusMercury was a messenger, and a god of trade, profit and commerce, the son of Maia Maiestas, also known as Ops, the Roman version of Rhea, and Jupiter. His name is related to the Latin word merx...
(for Hermes),
VenusVenus was a major Roman goddess principally associated with love, beauty and fertility, who played a key role in many Roman religious festivals and myths. From the third century BC, the increasing Hellenization of Roman upper classes identified her as the equivalent of the Greek goddess...
(Aphrodite),
MarsMars was the Roman god of war, the son of Juno and Jupiter, husband of Bellona, and the lover of Venus. He was the most prominent of the military gods that were worshipped by the Roman legions. The martial Romans considered him second in importance only to Jupiter...
(Ares),
IuppiterIn Roman mythology, Jupiter or Jove was the king of the gods, and the god of sky and thunder. He is the equivalent of Zeus in the Greek pantheon. He was called Iuppiter Optimus Maximus ; as the patron deity of the Roman state, he ruled over laws and social order...
(Zeus) and
SaturnusSaturn was a major Roman god of agriculture and harvest. In medieval times he was known as the Roman god of agriculture, justice and strength; he held a sickle in his left hand and a bundle of wheat in his right. His mother's name was Helen, or Hel...
(Kronos). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained:
Uranus (
OuranosUranus is the Latinized form of Ouranos , the Greek word for sky. In Greek mythology Ouranos or Father Sky, is personified as the son and husband of Gaia, Mother Earth...
) and
NeptūnusNeptune is the god of water and the sea in Roman mythology, a brother of Jupiter and Pluto. He is analogous with but not identical to the god Poseidon of Greek mythology. The Roman conception of Neptune owed a great deal to the Etruscan god Nethuns....
(
PoseidonIn Greek mythology, Poseidon was the god of the sea and, as "Earth-Shaker," of earthquakes. 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...
).
Some
RomansAncient Rome was a civilization that grew out of a small agricultural community founded on the Italian Peninsula as early as the 10th century BC. Located along the Mediterranean Sea, it became one of the largest empires in the ancient world....
, following a belief possibly originating in
MesopotamiaMesopotamia "land between the rivers" is a name for the Tigris–Euphrates region in the eastern Mediterranean, largely corresponding to Iraq, as well as northeastern Syria, some parts of southeastern Turkey, and some parts of the Khūzestān Province of southwestern...
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 in the
Roman calendarThe Roman calendar changed its form several times in the time between the foundation 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 many modern languages. Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after
TiwTyr is the god of single combat, victory and heroic glory in Norse mythology, portrayed as a one-handed man.Corresponding names in other Germanic languages are Gothic Teiws , Old English Tīw and Old High German Ziu, all from Proto-Germanic *Tîwaz.In the late Icelandic Eddas, Tyr is portrayed,...
, (Tuesday)
WódenWōden is a god in Anglo-Saxon paganism, together with Norse Odin representing a development of a Proto-Germanic god, *Wōdanaz. Other West Germanic forms of the name include Old High German Wuotan, Low German and Dutch Wodan....
(Wednesday),
ThunorThor is the red-haired and bearded god of thunder in Germanic mythology and Germanic paganism, and its subsets: Norse paganism, Anglo-Saxon paganism and Continental Germanic paganism....
(Thursday), and
Fríge*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.
Since Earth 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-SaxonOld English , also called Anglo-Saxon, is an early form of the English language that was spoken and written in parts of what are now England and south-eastern Scotland between at least the mid-5th century and the mid-12th century. What survives through writing represents primarily the literary...
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. It is the only planet whose name in English is not derived from
GrecoGreek 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...
-
Roman mythologyRoman mythology, or Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its main city, Ancient Rome. It can be considered as having two parts; One part, largely later and literary, consists of borrowings from Greek mythology...
. Many of the
Romance languagesThe Romance languages are a branch of the Indo-European language family comprising all the languages that descend from Latin, the language of ancient Rome...
retain the old Roman word
terraTerra Mater 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.-Form and function:...
(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,
ΓήGaia Gaia Gaia ( or ; "land" or "earth", from the Ancient Greek Γαῖα; also Gæa or Gea (Koine and Modern Greek Γῆ) is the primal Greek goddess personifying the Earth....
(
Ge or
Yi); the
Germanic languagesThe Germanic languages are a group of related languages that constitute a branch of the Indo-European language family. The common ancestor of all the languages in this branch is Proto-Germanic, spoken in approximately the mid-1st millennium BC in Iron Age northern Europe...
, including English, use a variation of an ancient Germanic word
ertho, "ground," as can be seen in the English
Earth, the German
Erde, the Dutch
Aarde, and the Scandinavian
Jorde.
Non-European cultures use other planetary naming systems.
IndiaIndia, 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, and the most populous democracy in the world. Bounded by the Indian Ocean on the south, the Arabian Sea on the west, and the Bay of Bengal...
uses a naming system based on the
NavagrahaGraha is a 'cosmic influencer' on the living beings of mother Bhumidevi . In Hindu Astrology, the Navagraha are some of these major influencers."The Sanskrit word captures the idea that these nine grahas are living energies which put out waves of energy...
, which incorporates the seven traditional planets (
SuryaIn Hinduism, Sūrya is the chief solar deity, 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,
ChandraIn 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
BudhaIn 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....
,
ShukraShukra , the Sanskrit for "clear, pure" or "brightness, clearness", is the name the son of Bhrgu and Urjaswathi, and preceptor of the Daityas, and the guru of the Asuras, identified with the planet Venus, one of the Navagrahas...
,
MangalaIn Jyotish astrology, Mangala is the name for Mars, the red planet. Mars is also called Angaraka or Bhauma in Sanskrit. He is the god of war and is celibate. He is considered the son of Prithvi or Bhumi, the Earth Goddess...
, {{IAST and
ShaniShani is one of the Navagraha which are the nine primary celestial beings in Hindu astrology, or Jyotiṣa. Shani is embodied in the planet Saturn...
for the traditional planets Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending
lunar nodeThe 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
RahuIn Hindu mythology, Rahu is a snake that swallows the sun or the moon causing eclipses. He is depicted in art as a dragon with no body riding a chariot drawn by eight black horses.Rahu is one of the navagrahas in Vedic astrology...
and
KetuKetu is the descending lunar node.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. In some special circumstances it helps someone achieve the zenith of fame...
.
ChinaChina is a cultural region, an ancient civilization, and, depending on perspective, a national or multinational entity extending over a large area in East Asia....
and the countries of eastern Asia influenced by it (such as
Japanis an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of 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...
,
KoreaKorea is a civilization and formerly unified nation currently divided into two states. Located on the Korean Peninsula, it borders China to the northwest, Russia to the northeast, and is separated from Japan to the east by the Korea Strait....
and
VietnamVietnam , 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 to the east...
) use a naming system based on the five Chinese elements:
waterWater has been important to all peoples of the earth, and it is rich in spiritual tradition.-Greek and Roman tradition:Water is one of the four classical elements in ancient Greek philosophy and science...
(Mercury),
metalMetal , 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 west and autumn, old age, the planet Venus and the color white. The archetypal metals...
(Venus),
fireFire 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 fashions throughout history.-Greek and Roman Tradition:...
(Mars),
woodTree , traditionally translated as Wood, is the growing of the matter, or the matter's growing stage. Tree is the first one of Wu Xing. Tree is yang in character...
(Jupiter) and
earthEarth, home and origin of humanity, has often been worshipped in its own right with its own unique spiritual traditio.-Greek and Roman tradition:...
(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
nebulaA nebula is an interstellar cloud of dust, hydrogen gas, helium gas and plasma...
into a thin disk of gas and dust. A
protostarA protostar is a large object 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 diskA protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star or Herbig star...
. Through
accretionIn 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
planetesimalPlanetesimals 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 dust grains that collide and stick to form larger and larger bodies...
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
protoplanetProtoplanets are moon-sized planets, or larger embryos within protoplanetary discs. 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 dragIn fluid dynamics, drag refers to forces that oppose the relative motion of an object through a fluid . Drag forces act in a direction opposite to the oncoming flow velocity...
.
When the protostar has grown such that it ignites to form a
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
, the surviving disk is removed from the inside outward by photoevaporation, the
solar windThe solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It consists mostly of electrons and protons with energies of about 1 keV. The stream of particles varies in temperature and speed with the passage of time...
,
Poynting-Robertson dragThe 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 inward...
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 satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
s of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or
small Solar System bodiesSmall Solar System Body is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neither planets nor dwarf planets:...
.
The energetic impacts of the smaller planetesimals (as well as
radioactive decayRadioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter...
) 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
cometA comet is a Small Solar System Body that has coma and is bigger than a meteoroid. When close enough to the Sun, a comet exhibits a visible coma , and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus...
s. (Smaller planets will lose any atmosphere they gain through various
escape mechanismsThere are several different processes that can lead to the escape of a planetary atmosphere. In some cases this can be a very important process; for example, both Venus and Mars have probably lost much of their water due to atmospheric escape since they have a weaker gravitational field strength...
.)
With the discovery and observation of
planetary systemA planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets, moons, 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
metallicityIn 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. Since stars, which comprise most of the visible matter in the universe, are composed mostly of hydrogen and helium, astronomers, for...
– an astronomical term describing the abundance of
chemical elementA 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. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons.Common examples of elements...
s with an
atomic numberIn 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 (
heliumHelium is the chemical element with atomic number 2, and 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 less likely that a metal-poor, population II star will possess a more substantial planetary system than a metal-rich population I star.
Solar System
{{Main|Solar System}}
{{See also|List of Solar System bodies in hydrostatic equilibrium}}
According to the
IAUThe International Astronomical Union 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 SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
. In increasing distance from the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, the planets are:
- Mercury
For the liquid metallic element, see Mercury .Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 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...
- Venus
Venus is the second-closest planet to 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...
- Earth
Earth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
- Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
- Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
- Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
- Uranus
Uranus is the seventh planet from the Sun, and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus the father of Kronos and grandfather of Zeus...
- Neptune
Neptune is the eighth planet from the Sun in our 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 Earth masses and...
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:
- Terrestrial
A terrestrial planet, telluric planet, rocky planet or inner planet is a planet that is primarily composed of silicate rocks. Within the solar system, the terrestrial planets are the closest planets to the Sun...
s: Planets that are similar to Earth, with bodies largely composed of rockIn geology, rock 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.
- 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 our Solar System: Jupiter, Saturn, Uranus, and Neptune...
s (Jovians): Planets with a composition largely made up of gasThis page is about the physical properties of gas as a state of matter. For the uses of gases, and other meanings, see Gas .A gas is one of four states of matter. Near absolute zero, a substance exists as a solid...
eous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass, and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
- 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 rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
s: Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAUThe International Astronomical Union is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
, 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 beltThe 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 beltThe 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 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 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 orbitsIn the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence...
. By definition, all dwarf planets are members of larger populationIn biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings. Individuals within a population share a factor may be reduced by statistical means, but such a generalization may be too vague to imply anything...
s. Ceres is the largest body in the asteroid beltThe 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 discThe scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets known as scattered disc objects , a subset of the broader family of trans-Neptunian objects...
. Scientists such as Mike BrownMichael E. Brown has been a professor of planetary astronomy at the California Institute of Technology since 2003. He was previously an associate professor at Caltech from 2002–2003 and an assistant professor at Caltech from 1997–2002.-Education:Brown is a Huntsville, Alabama native and graduated...
believe that there may soon be over forty 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 (AUAn astronomical unit is a unit of length roughly equal to the mean distance between the Earth and the Sun. It is approximately 150 million kilometres ....
) |
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) |
Inclination
to Sun's equatorInclination 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
eccent-ricityIn astrodynamics, under standard assumptions, any orbit must be of conic section shape. The eccentricity of this conic section, the orbit's eccentricity, is an important parameter of the orbit that defines its absolute shape...
|
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) |
Named
moonsA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
{{ref label|c|c|c}} |
Rings |
AtmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
|
| Terrestrials A terrestrial planet, telluric planet, rocky planet or inner planet is a planet that is primarily composed of silicate rocks. Within the solar system, the terrestrial planets are the closest planets to the Sun...
|
MercuryFor the liquid metallic element, see Mercury .Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 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...
|
0.382 |
0.06 |
0.39 |
0.24 |
3.38 |
0.206 |
58.64 |
— |
no |
minimal |
VenusVenus is the second-closest planet to 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...
|
0.949 |
0.82 |
0.72 |
0.62 |
3.86 |
0.007 |
-243.02 |
— |
no |
CO2 Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
, N2Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
|
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
{{ref label>b|b|b}} |
1.00 |
1.00 |
1.00 |
1.00 |
7.25 |
0.017 |
1.00 |
1The Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
|
no |
N2, O2Oxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
|
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
|
0.532 |
0.11 |
1.52 |
1.88 |
5.65 |
0.093 |
1.03 |
2 |
no |
CO2, N2 |
Gas giantA gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune...
s |
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
|
11.209 |
317.8 |
5.20 |
11.86 |
6.09 |
0.048 |
0.41 |
49 |
yes 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...
|
H2Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H 2...
, HeHelium is the chemical element with atomic number 2, and 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 is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
|
9.449 |
95.2 |
9.54 |
29.46 |
5.51 |
0.054 |
0.43 |
52 |
yes 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 |
UranusUranus is the seventh planet from the Sun, and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus the father of Kronos and grandfather of Zeus...
|
4.007 |
14.6 |
19.22 |
84.01 |
6.48 |
0.047 |
-0.72 |
27 |
yes 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 |
NeptuneNeptune is the eighth planet from the Sun in our 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 Earth masses and...
|
3.883 |
17.2 |
30.06 |
164.8 |
6.43 |
0.009 |
0.67 |
13 |
yes The rings of Neptune were discovered in 1989 by the Voyager 2 spacecraft and are tenuous, faint and dusty, and resemble the rings of Jupiter more closely than those of Saturn or Uranus. Neptune possesses five known rings, each named for an astronomer who contributed important work on the planet:...
|
H2, He |
Dwarf PlanetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
s |
| Ceres |
0.08 |
0.000 2 |
2.5–3.0 |
4.60 |
10.59 |
0.080 |
0.38 |
0 |
no |
none |
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
|
0.19 |
0.002 2 |
29.7–49.3 |
248.09 |
17.14 |
0.249 |
−6.39 |
3 |
no |
temporary |
| Haumea |
0.37×0.16 |
0.000 7 |
35.2–51.5 |
282.76 |
28.19 |
0.189 |
0.16 |
2 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 |
? |
d|d|d}} |
ErisEris, formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto....
|
0.19 |
0.002 5 |
37.8–97.6 |
~557 |
44.19 |
0.442 |
~0.3 |
1Dysnomia, officially Eris I Dysnomia, is the only known moon of the dwarf planet Eris. It was discovered in 2005 by Mike Brown and the laser guide star adaptive optics team at the W. M...
|
? |
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 MayorMichel G. E. Mayor is a Swiss professor in the Department of Astronomy at the University of Geneva.Together with Didier Queloz in 1995 he discovered 51 Pegasi b, the first extrasolar planet orbiting a sun-like star, 51 Pegasi....
and
Didier QuelozDidier Queloz is a Geneva-based astronomer with a prolific record in finding extrasolar planets. He is understudy to Michel Mayor....
of the
University of GenevaThe University of Geneva is a university in Geneva, Switzerland.Founded by John Calvin in 1559 as a theological seminary that also taught law, it remained focused on theology until the 17th century, when it became a center for Enlightenment scholarship. In 1873 it dropped its religious...
announced the detection of an exoplanet around
51 Pegasi51 Pegasi is a Sun-like star located 15.4 parsecs from Earth in the constellation Pegasus. It was the first Sun-like star, other than the Sun, found to have a planet orbiting it, a discovery that was announced in 1995....
. Of the 342 extrasolar planets discovered by February 2009, 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
pulsarPulsars are highly magnetized, rotating neutron stars that emit a beam of electromagnetic radiation. The observed periods of their pulses range from 1.4 milliseconds to 8.5 seconds. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the...
s, such as
PSR B1257+12PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located 980 light-years from the Sun. As of 2007, it is 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 AraeMu Arae , often referred to by its designation in the Henry Draper catalogue HD 160691, is a yellow average star approximately 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 Cancri55 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 436Gliese 436 is a red dwarf star approximately 33 light-years away in the constellation of Leo. As of 2004, the existence of an extrasolar planet orbiting the star has been confirmed...
. These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths). Another new category are the so-called "
super-EarthA super-Earth is an extrasolar planet more massive than the Earth, but theoretically less massive than a gas giant. The term super-Earth refers only to the mass of the planet and does not imply anything about the surface conditions or habitability: in particular it does not imply that the planet...
s", possibly
terrestrial planetA terrestrial planet, telluric planet, rocky planet or inner planet is a planet that is primarily composed of silicate rocks. Within the solar system, the terrestrial planets are the closest planets to the Sun...
s far larger than Earth but smaller than Neptune or Uranus. To date, six possible super-Earths have been found:
Gliese 876 dGliese 876 d is an extrasolar planet approximately 15 light-years away in the constellation of Aquarius . The planet was the third planet discovered orbiting the red dwarf star Gliese 876. At the time of its discovery, the planet had the lowest mass of any known extrasolar planet apart from the...
, which is roughly six times Earth's mass,
OGLE-2005-BLG-390LbOGLE-2005-BLG-390Lb or Schreiner Planet as it is commonly reffered , 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-192LbMOA-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 1.4 times that of Earth, it...
, frigid icy worlds discovered through
gravitational microlensingGravitational 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 ...
,
COROT-Exo-7bCOROT-7b is a reported exoplanet orbiting around the star COROT-7. It was first detected photometrically by the French-led COROT mission and reported in early 2009. It is the smallest exoplanet to have its diameter measured, at 1.7 times that of the Earth...
, a planet with a diameter estimated at around 1.7 times that of Earth, (making it the smallest
super-Earth