Pluto

Pluto

Overview
Pluto, formal designation
Minor planet names
Formal minor planet designations are number–name combinations overseen by the Minor Planet Center, a branch of the IAU. They are used for dwarf planets and small Solar System bodies such as asteroids, but not comets...

 134340 Pluto, is the second-most-massive known dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

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

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

) and the tenth-most-massive body observed directly orbiting the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. Originally classified as the ninth planet
Planets beyond Neptune
Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century but culminated at the start of the 20th with Percival Lowell's quest for Planet X...

 from the Sun, Pluto was recategorized as a dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

 and plutoid due to the discovery that it is one of several large bodies within the newly charted Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

.

Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

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

 and a third its volume.
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Pluto, formal designation
Minor planet names
Formal minor planet designations are number–name combinations overseen by the Minor Planet Center, a branch of the IAU. They are used for dwarf planets and small Solar System bodies such as asteroids, but not comets...

 134340 Pluto, is the second-most-massive known dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

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

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

) and the tenth-most-massive body observed directly orbiting the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. Originally classified as the ninth planet
Planets beyond Neptune
Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century but culminated at the start of the 20th with Percival Lowell's quest for Planet X...

 from the Sun, Pluto was recategorized as a dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

 and plutoid due to the discovery that it is one of several large bodies within the newly charted Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

.

Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

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

 and a third its volume. It has an eccentric
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 and highly inclined orbit that takes it from 30 to 49 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun than Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

. As of 2011, it is 32.1 AU from the Sun.

From its discovery in 1930 until 2006, Pluto was classified as a planet. In the late 1970s, following the discovery of minor planet 2060 Chiron
2060 Chiron
2060 Chiron is a minor planet in the outer Solar System. Discovered in 1977 by Charles T. Kowal , it was the first-known member of a new class of objects now known as centaurs, with an orbit between Saturn and Uranus.Although it was initially classified as an asteroid, it was later found to...

 in the outer Solar System and the recognition of Pluto's relatively low mass, its status as a major planet began to be questioned.
In the late 20th and early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...

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

 in 2005, which is 27% more massive than Pluto. On August 24, 2006, the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 (IAU) defined what it means to be a "planet" within the Solar System. This definition excluded Pluto as a planet and added it as a member of the new category "dwarf planet" along with Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

 and Ceres. After the reclassification, Pluto was added to the list of minor planet
Minor planet
An asteroid group or minor-planet group is a population of minor planets that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid...

s and given the number 134340. A number of scientists continue to hold that Pluto should be classified as a planet.

Pluto has four known moons, the largest being Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...

 discovered in 1978, along with Nix
Nix (moon)
Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...

 and Hydra
Hydra (moon)
Hydra is the second outermost known natural satellite of Pluto. It was discovered along with Nix in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...

, discovered in 2005, and the provisionally named S/2011 P 1
S/2011 P 1
S/2011 P 1 is a small natural satellite of Pluto whose existence was announced on July 20, 2011...

, discovered in 2011.
Pluto and Charon are sometimes described as a binary system
Binary system (astronomy)
A binary system is an astronomical term referring to two objects in space which are so close that their gravitational interaction causes them to orbit about a common center of mass. Some definitions A binary system is an astronomical term referring to two objects in space (usually stars, but also...

 because the barycenter of their orbits does not lie within either body. However, the IAU has yet to formalise a definition for binary dwarf planets, and as such Charon is officially classified as a moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 of Pluto.

Discovery



In the 1840s, using Newtonian mechanics
Classical mechanics
In physics, classical mechanics is one of the two major sub-fields of mechanics, which is concerned with the set of physical laws describing the motion of bodies under the action of a system of forces...

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

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

. Subsequent observations of Neptune in the late 19th century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet besides Neptune. In 1906, Percival Lowell
Percival Lowell
Percival Lawrence Lowell was a businessman, author, mathematician, and astronomer who fueled speculation that there were canals on Mars, founded the Lowell Observatory in Flagstaff, Arizona, and formed the beginning of the effort that led to the discovery of Pluto 14 years after his death...

, a wealthy Bostonian who had founded the Lowell Observatory
Lowell Observatory
Lowell Observatory is an astronomical observatory in Flagstaff, Arizona. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark in 1965....

 in Flagstaff, Arizona
Flagstaff, Arizona
Flagstaff is a city located in northern Arizona, in the southwestern United States. In 2010, the city's population was 65,870. The population of the Metropolitan Statistical Area was at 134,421 in 2010. It is the county seat of Coconino County...

 in 1894, started an extensive project in search of a possible ninth planet, which he termed "Planet X
Planet X
Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century but culminated at the start of the 20th with Percival Lowell's quest for Planet X...

". By 1909, Lowell and William H. Pickering
William Henry Pickering
William Henry Pickering was an American astronomer, brother of Edward Charles Pickering. He was elected a Fellow of the American Academy of Arts and Sciences in 1883.-Work:...

 had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916, but to no avail. Unknown to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto, but did not recognise them for what they were. Lowell was not the first to unknowingly photograph Pluto. There are sixteen known pre-discoveries, with the oldest being made by the Yerkes Observatory
Yerkes Observatory
Yerkes Observatory is an astronomical observatory operated by the University of Chicago in Williams Bay, Wisconsin. The observatory, which calls itself "the birthplace of modern astrophysics," was founded in 1897 by George Ellery Hale and financed by Charles T. Yerkes...

 on August 20, 1909.

Due to a ten-year legal battle with Constance Lowell, Percival's widow, who attempted to wrest the observatory's million-dollar portion of his legacy for herself, the search for Planet X did not resume until 1929, when its director, Vesto Melvin Slipher, summarily handed the job of locating Planet X to Clyde Tombaugh
Clyde Tombaugh
Clyde William Tombaugh was an American astronomer. Although he is best known for discovering the dwarf planet Pluto in 1930, the first object to be discovered in what would later be identified as the Kuiper Belt, Tombaugh also discovered many asteroids; he also called for serious scientific...

, a 23-year-old Kansas man who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.

Tombaugh's task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called a blink comparator
Blink comparator
A blink comparator was a viewing apparatus used by astronomers to find differences between two photographs of the night sky shot using optical telescopes such as astrographs. It permitted rapidly switching from viewing one photograph to viewing the other, "blinking" back and forth between the two...

, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photograph taken on January 21 helped confirm the movement. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory
Harvard College Observatory
The Harvard College Observatory is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, USA, and was founded in 1839...

 on March 13, 1930.

Name



The discovery made headlines across the globe. The Lowell Observatory
Lowell Observatory
Lowell Observatory is an astronomical observatory in Flagstaff, Arizona. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark in 1965....

, which had the right to name the new object, received over 1,000 suggestions from all over the world, ranging from Atlas to Zymal. Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did. Constance Lowell proposed Zeus
Zeus
In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...

, then Percival and finally Constance. These suggestions were disregarded.

The name Pluto was proposed by Venetia Burney (1918–2009), an eleven-year-old schoolgirl in Oxford
Oxford
The city of Oxford is the county town of Oxfordshire, England. The city, made prominent by its medieval university, has a population of just under 165,000, with 153,900 living within the district boundary. It lies about 50 miles north-west of London. The rivers Cherwell and Thames run through...

, England. Venetia was interested in classical mythology
Classical mythology
Classical mythology or Greco-Roman mythology is the cultural reception of myths from the ancient Greeks and Romans. Along with philosophy and political thought, mythology represents one of the major survivals of classical antiquity throughout later Western culture.Classical mythology has provided...

 as well as astronomy, and considered the name, a name for the god of the underworld
Pluto (mythology)
In ancient Greek religion and myth, Pluto was a name for the ruler of the underworld; the god was also known as Hades, a name for the underworld itself...

, appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfather Falconer Madan
Falconer Madan
Falconer Madan was Librarian of the Bodleian Library of Oxford University.Falconer was the fifth son of George and Harriet Madan. He was educated at Marlborough College and Brasenose College, Oxford, where he took part in Oxford and Cambridge Chess matches in 1873 and 1874, and won the University...

, a former librarian at the University of Oxford
University of Oxford
The University of Oxford is a university located in Oxford, United Kingdom. It is the second-oldest surviving university in the world and the oldest in the English-speaking world. Although its exact date of foundation is unclear, there is evidence of teaching as far back as 1096...

's Bodleian Library
Bodleian Library
The Bodleian Library , the main research library of the University of Oxford, is one of the oldest libraries in Europe, and in Britain is second in size only to the British Library...

. Madan passed the name to Professor Herbert Hall Turner
Herbert Hall Turner
Herbert Hall Turner was a British astronomer and seismologist.-Biography:Herbert Hall Turner was educated at Clifton College and Trinity College, Cambridge., In 1884 he accepted the post of Chief Assistant at Greenwich Observatory and stayed there for nine years...

, who then cabled it to colleagues in the United States.

The object was officially named on March 24, 1930. Each member of the Lowell Observatory was allowed to vote on a short-list of three: Minerva
Minerva
Minerva was the Roman goddess whom Romans from the 2nd century BC onwards equated with the Greek goddess Athena. She was the virgin goddess of poetry, medicine, wisdom, commerce, weaving, crafts, magic...

 (which was already the name for an asteroid), Cronus
Cronus
In Greek mythology, Cronus or Kronos was the leader and the youngest of the first generation of Titans, divine descendants of Gaia, the earth, and Uranus, the sky...

 (which had lost reputation through being proposed by the unpopular astronomer Thomas Jefferson Jackson See
Thomas Jefferson Jackson See
Thomas Jefferson Jackson See, was an American astronomer of high potential who ended a colorful life with no real accomplishment in astronomy or physics...

), and Pluto. Pluto received every vote. The name was announced on May 1, 1930. Upon the announcement, Madan gave Venetia five pounds
Pound sterling
The pound sterling , commonly called the pound, is the official currency of the United Kingdom, its Crown Dependencies and the British Overseas Territories of South Georgia and the South Sandwich Islands, British Antarctic Territory and Tristan da Cunha. It is subdivided into 100 pence...

 (£5) (£ as of ), as a reward.

It has been noted that the first two letters of Pluto are the initials of Percival Lowell, and Pluto's astronomical symbol (
) is a monogram
Monogram
A monogram is a motif made by overlapping or combining two or more letters or other graphemes to form one symbol. Monograms are often made by combining the initials of an individual or a company, used as recognizable symbols or logos. A series of uncombined initials is properly referred to as a...

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

 (), but has a circle in place of the middle prong of the trident ().

The name was soon embraced by wider culture. In 1930, Walt Disney
Walt Disney
Walter Elias "Walt" Disney was an American film producer, director, screenwriter, voice actor, animator, entrepreneur, entertainer, international icon, and philanthropist, well-known for his influence in the field of entertainment during the 20th century. Along with his brother Roy O...

 introduced for Mickey Mouse
Mickey Mouse
Mickey Mouse is a cartoon character created in 1928 by Walt Disney and Ub Iwerks at The Walt Disney Studio. Mickey is an anthropomorphic black mouse and typically wears red shorts, large yellow shoes, and white gloves...

 a canine companion, named Pluto
Pluto (Disney)
Pluto, also called Pluto the Pup, is a cartoon character created in 1930 by Walt Disney Productions. He is a light brown , medium-sized, short-haired dog. Unlike Goofy, Pluto is not anthropomorphic beyond some characteristics such as facial expression...

 apparently in the object's honour, although Disney
The Walt Disney Company
The Walt Disney Company is the largest media conglomerate in the world in terms of revenue. Founded on October 16, 1923, by Walt and Roy Disney as the Disney Brothers Cartoon Studio, Walt Disney Productions established itself as a leader in the American animation industry before diversifying into...

 animator Ben Sharpsteen
Ben Sharpsteen
Ben Sharpsteen was an American film director and producer for Disney. He directed 31 films between 1920 and 1980....

 could not confirm why the name was given. In 1941, Glenn T. Seaborg
Glenn T. Seaborg
Glenn Theodore Seaborg was an American scientist who won the 1951 Nobel Prize in Chemistry for "discoveries in the chemistry of the transuranium elements", contributed to the discovery and isolation of ten elements, and developed the actinide concept, which led to the current arrangement of the...

 named the newly created element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...

 after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

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

, and neptunium
Neptunium
Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a...

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

.

In Chinese
Chinese language
The Chinese language is a language or language family consisting of varieties which are mutually intelligible to varying degrees. Originally the indigenous languages spoken by the Han Chinese in China, it forms one of the branches of Sino-Tibetan family of languages...

, Japanese
Japanese language
is a language spoken by over 130 million people in Japan and in Japanese emigrant communities. It is a member of the Japonic language family, which has a number of proposed relationships with other languages, none of which has gained wide acceptance among historical linguists .Japanese is an...

 and Korean
Korean language
Korean is the official language of the country Korea, in both South and North. It is also one of the two official languages in the Yanbian Korean Autonomous Prefecture in People's Republic of China. There are about 78 million Korean speakers worldwide. In the 15th century, a national writing...

 the name was translated as underworld king star (冥王星), as suggested by Houei Nojiri
Houei Nojiri
was a Japanese essayist and astronomer.In 1930 he coined the Japanese word for the newly-discovered planet Pluto. And the name is used in China, Korea, and Japan.- See also :* 3008 Nojiri...

 in 1930. Many other non-European languages use a transliteration of "Pluto" as their name for the object; some Indian languages use a form of Yama
Yama (Buddhism and Chinese mythology)
Yama the name of the Buddhist dharmapala and judge of the dead, who presides over the Buddhist Narakas , "Hells" or "Purgatories". Although ultimately based on the god Yama of the Hindu Vedas, the Buddhist Yama has developed different myths and different functions from the Hindu deity...

, the Guardian of Hell in Hindu
Hindu
Hindu refers to an identity associated with the philosophical, religious and cultural systems that are indigenous to the Indian subcontinent. As used in the Constitution of India, the word "Hindu" is also attributed to all persons professing any Indian religion...

 mythology, such as the Gujarati
Gujarati language
Gujarati is an Indo-Aryan language, and part of the greater Indo-European language family. It is derived from a language called Old Gujarati which is the ancestor language of the modern Gujarati and Rajasthani languages...

 Yamdev.

Demise of Planet X



Mass estimates for Pluto
Year Mass Notes
1931 1 Earth Nicholson
Seth Barnes Nicholson
Seth Barnes Nicholson was an American astronomer.Nicholson was born in Springfield, Illinois and was raised in rural Illinois...

 & Mayall
1948 0.1 (1/10) Earth Kuiper
1976 0.01 (1/100) Earth Cruikshank, Pilcher, & Morrison
1978 0.002 (1/500) Earth Christy & Harrington

Once found, Pluto's faintness and lack of a resolvable disc cast doubt on the idea that it was Lowell's Planet X
Planet X
Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century but culminated at the start of the 20th with Percival Lowell's quest for Planet X...

. Estimates of Pluto's mass were revised downward throughout the 20th century. In 1978, the discovery of Pluto's moon Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...

 allowed the measurement of Pluto's mass for the first time. Its mass, roughly 0.2% that of the Earth, was far too small to account for the discrepancies in the orbit of Uranus. Subsequent searches for an alternate Planet X, notably by Robert Sutton Harrington
Robert Sutton Harrington
Robert Sutton Harrington was an American astronomer who worked at the United States Naval Observatory . Harrington was born near Newport News, Virginia. His father was an archaeologist. He was married to Betty-Jean Maycock in 1976, with two daughters, Amy and Ann.Harrington worked at the USNO....

, failed. In 1992, Myles Standish used data from Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

s 1989 flyby of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

, which had revised the planet's total mass downward by 0.5%, to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished. Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist. Lowell had made a prediction of Planet X's position in 1915 that was fairly close to Pluto's position at that time; Ernest W. Brown concluded almost immediately that this was a coincidence, a view still held today.

Orbit and rotation


Pluto's orbital period is 248 Earth years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun close to a flat reference plane
Plane (mathematics)
In mathematics, a plane is a flat, two-dimensional surface. A plane is the two dimensional analogue of a point , a line and a space...

 called the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...

. In contrast, Pluto's orbit is highly inclined relative to the ecliptic (over 17°) and highly eccentric
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 (elliptical). This high eccentricity means a small region of Pluto's orbit lies nearer the Sun than Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

's. The Pluto–Charon barycentre came to perihelion
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...

 on September 5, 1989, and was last closer to the Sun than Neptune between February 7, 1979 and February 11, 1999. Detailed calculations indicate that the previous such occurrence lasted only fourteen years, from July 11, 1735 to September 15, 1749, whereas between April 30, 1483 and July 23, 1503, it had also lasted 20 years. The durations of the relationship vary because of the varying speed of motion of Neptune along its orbit; the relationship between the orbits themselves varies much more slowly, as Neptune's orbit precesses and otherwise evolves.

Although this repeating pattern may suggest a regular structure, in the long term Pluto's orbit is in fact chaotic
Chaos theory
Chaos theory is a field of study in mathematics, with applications in several disciplines including physics, economics, biology, and philosophy. Chaos theory studies the behavior of dynamical systems that are highly sensitive to initial conditions, an effect which is popularly referred to as the...

. While computer simulations can be used to predict its position for several million years (both forward and backward
Time reversibility
Time reversibility is an attribute of some stochastic processes and some deterministic processes.If a stochastic process is time reversible, then it is not possible to determine, given the states at a number of points in time after running the stochastic process, which state came first and which...

 in time), after intervals longer than the Lyapunov time
Lyapunov time
In mathematics, the Lyapunov time is the length of time for a dynamical system to become chaotic. The Lyapunov time reflects the limits of the predictability of the system. By convention, it is defined as the time for the distance between nearby trajectories of the system to increase by a factor...

 of 10–20 million years, calculations become speculative: Pluto's tiny size makes it sensitive to unmeasurably small details of the Solar System, hard-to-predict factors that will gradually disrupt its orbit. Millions of years from now, Pluto may well be at aphelion, at perihelion or anywhere in between, with no way for us to predict which. This does not mean Pluto's orbit itself is unstable, but its position on that orbit is impossible to determine so far ahead. Several resonances and other dynamical effects keep Pluto's orbit stable, safe from planetary collision or scattering.

Relationship with Neptune


Despite Pluto's orbit appearing to cross that of Neptune when viewed from directly above, the two objects' orbits are aligned so that they can never collide or even approach closely. There are several reasons why.

At the simplest level, one can examine the two orbits and see that they do not intersect. When Pluto is closest to the Sun, and hence closest to Neptune's orbit as viewed from above, it is also the farthest above Neptune's path. Pluto's orbit passes about 8 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 above that of Neptune, preventing a collision. Pluto's ascending and descending nodes
Orbital node
An orbital node is one of the two points where an orbit crosses a plane of reference to which it is inclined. An orbit which is contained in the plane of reference has no nodes.-Planes of reference:...

, the points at which its orbit crosses the ecliptic, are currently separated from Neptune's by over 21°.

This alone is not enough to protect Pluto; perturbations
Perturbation (astronomy)
Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....

 from the planets (especially Neptune) could alter aspects of Pluto's orbit (such as its orbital precession
Apsidal precession
In celestial mechanics, perihelion precession, apsidal precession or orbital precession is the precession of the orbit of a celestial body. More precisely it is the gradual rotation of the line joining the apsides of an orbit, which are the points of closest and farthest approach...

) over millions of years so that a collision could be possible. Some other mechanism or mechanisms must therefore be at work. The most significant of these is that Pluto lies in the 3:2 mean motion resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

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

: for every three of Neptune's orbits around the Sun, Pluto makes two. The two objects then return to their initial positions and the cycle repeats, each cycle lasting about 500 years. This pattern is configured so that, in each 500-year cycle, the first time Pluto is near perihelion Neptune is over 50° behind Pluto. By Pluto's second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be a similar distance ahead of Pluto. Pluto and Neptune's minimum separation is over 17 AU. Pluto comes closer to Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 (11 AU) than it does to Neptune.

The 3:2 resonance between the two bodies is highly stable, and is preserved over millions of years. This prevents their orbits from changing relative to one another; the cycle always repeats in the same way, and so the two bodies can never pass near to each other. Thus, even if Pluto's orbit were not highly inclined the two bodies could never collide.

Other factors


Numerical studies have shown that over periods of millions of years, the general nature of the alignment between Pluto and Neptune's orbits does not change. There are several other resonances and interactions that govern the details of their relative motion, and enhance Pluto's stability. These arise principally from two additional mechanisms (besides the 3:2 mean motion resonance).

First, Pluto's argument of perihelion, the angle between the point where it crosses the ecliptic and the point where it is closest to the Sun, librates
Libration
In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...

 around 90°. This means that when Pluto is nearest the Sun, it is at its farthest above the plane of the Solar System, preventing encounters with Neptune. This is a direct consequence of the Kozai mechanism
Kozai mechanism
In celestial mechanics, the Kozai mechanism, or the Lidov-Kozai mechanism, causes a periodic exchange between the inclination and eccentricity of an orbit...

, which relates the eccentricity of an orbit to its inclination to a larger perturbing body—in this case Neptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion to the orbit of Neptune is always greater than 52° (= 90°–38°). The closest such angular separation occurs every 10,000 years.

Second, the longitudes of ascending nodes of the two bodies—the points where they cross the ecliptic—are in near-resonance with the above libration. When the two longitudes are the same—that is, when one could draw a straight line through both nodes and the Sun—Pluto's perihelion lies exactly at 90°, and it comes closest to the Sun at its peak above Neptune's orbit. In other words, when Pluto most closely intersects the plane of Neptune's orbit, it must be at its farthest beyond it. This is known as the 1:1 superresonance, and is controlled by all the Jovian planets.

To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distant vantage point where the planets orbit counter-clockwise. After passing the ascending node, Pluto is interior to Neptune's orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the two causes angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

 to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly larger orbit, where it travels slightly slower, according to Kepler's third law. As its orbit changes, this has the gradual effect of changing the pericentre and longitudes of Pluto (and, to a lesser degree, of Neptune). After many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch Pluto at the opposite side of its orbit (near the opposing node to where we began). The process is then reversed, and Pluto loses angular momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptune again at the original node. The whole process takes about 20,000 years to complete.

Rotation


Pluto's rotation period
Rotation period
The rotation period of an astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars...

, its day, is equal to 6.39 Earth day
Earth Day
Earth Day is a day that is intended to inspire awareness and appreciation for the Earth's natural environment. The name and concept of Earth Day was allegedly pioneered by John McConnell in 1969 at a UNESCO Conference in San Francisco. The first Proclamation of Earth Day was by San Francisco, the...

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

, Pluto rotates on its "side" on its orbital plane, with an axial tilt of 120°, and so its seasonal variation is extreme; at its solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...

s, one-fourth of its surface is in permanent daylight, while another fourth is in permanent darkness.

Physical characteristics


Pluto's distance from Earth makes in-depth investigation difficult. Many details about Pluto will remain unknown until 2015, when the New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...

 spacecraft is expected to arrive there.

Appearance and surface


Pluto's visual apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 averages 15.1, brightening to 13.65 at perihelion. To see it, a telescope is required; around 30 cm (12 in) aperture being desirable. It looks star-like and without a visible disk even in large telescopes, because its angular diameter
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...

 is only 0.11".

The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observations of eclipses by its largest moon, Charon. Observations were made of the change in the total average brightness of the Pluto–Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time.

Current maps have been produced from images from the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 (HST), which offers the highest resolution
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...

 currently available, and show considerably more detail, resolving variations several hundred kilometres across, including polar regions and large bright spots. The maps are produced by complex computer processing, which find the best-fit projected maps for the few pixels of the Hubble images. The two cameras on the HST used for these maps are no longer in service, so these will likely remain the most detailed maps of Pluto until the 2015 flyby of New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...

.

These maps, together with Pluto's lightcurve and the periodic variations in its infrared spectra, reveal that Pluto's surface is remarkably varied, with large changes in both brightness and colour. Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

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

. The colour varies between charcoal black, dark orange and white: Buie et al. term it "significantly less red than Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 and much more similar to the hues seen on Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

 with a slightly more orange cast".

Pluto's surface has changed between 1994 and 2002-3: the northern polar region has brightened and the southern hemisphere darkened. Pluto's overall redness has also increased substantially between 2000 and 2002. These rapid changes are probably related to seasonal condensation and sublimation of portions of Pluto's atmosphere, amplified by Pluto's extreme axial tilt and high orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

.

Spectroscopic analysis of Pluto's surface reveals it to be composed of more than 98 percent nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 ice, with traces of methane and carbon monoxide. The face of Pluto oriented toward Charon contains more methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 ice, while the opposite face contains more nitrogen and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 ice.

Structure



Observations by the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 place Pluto's density at between 1.8 and 2.1 g/cm3, suggesting its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass. Because decay of radioactive minerals would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

 of ice. The diameter of the core should be around 1,700 km, 70% of Pluto's diameter. It is possible that such heating continues today, creating a subsurface ocean layer of liquid water some thick at the core–mantle boundary. The DLR
German Aerospace Center
The German Aerospace Center is the national centre for aerospace, energy and transportation research of the Federal Republic of Germany. It has multiple locations throughout Germany. Its headquarters are located in Cologne. It is engaged in a wide range of research and development projects in...

 Institute of Planetary Research calculated that Pluto's density-to-radius ratio lies in a transition zone, along with Neptune's moon Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

, between icy satellites like the mid-sized moons of Uranus and Saturn, and rocky satellites such as Jupiter's Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

.

Mass and size


Pluto's mass is 1.31×1022 kg, less than 0.24 percent that of the Earth, while its diameter is 2,306 (+/- 20) km, or roughly 66% that of the Moon. Pluto's atmosphere complicates determining its true solid size within a certain margin.

Astronomers, assuming Pluto to be Lowell's Planet X, initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1955 Pluto was calculated to be roughly the mass of the Earth, with further calculations in 1971 bringing the mass down to roughly that of Mars. In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii
University of Hawaii
The University of Hawaii System, formally the University of Hawaii and popularly known as UH, is a public, co-educational college and university system that confers associate, bachelor, master, and doctoral degrees through three university campuses, seven community college campuses, an employment...

 calculated Pluto's albedo for the first time, finding that it matched that for methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of the Earth. Pluto's albedo is 1.3–2.0 times greater than that of Earth.

The discovery of Pluto's satellite Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...

 in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Newton's formulation of Kepler's third law. Once Charon's gravitational effect was measured, Pluto's true mass could be determined. Observations of Pluto in occultation with Charon allowed scientists to establish Pluto's diameter more accurately, while the invention of adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

 allowed them to determine its shape more accurately.
Selected size estimates for Pluto
Year Radius (Diameter) Notes
1993 1195 (2390) km Millis, et al. (If no haze)
1993 1180 (2360) km Millis, et al. (surface & haze)
1994 1164 (2328) km Young & Binzel
2006 1153 (2306) km Buie, et al.
2007 1161 (2322) km Young, Young, & Buie


Among the objects of the Solar System, Pluto is much less massive than the terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

s, and at less than 0.2 lunar masses, it is also less massive than seven moons
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

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

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

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

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

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

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

 and Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

. Pluto is more than twice the diameter and a dozen times the mass of the dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

 Ceres, the largest object in the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

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

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

 discovered in 2005. Given the error bar
Error bar
Error bars are a graphical representation of the variability of data and are used on graphs to indicate the error, or uncertainty in a reported measurement. They give a general idea of how accurate a measurement is, or conversely, how far from the reported value the true value might be...

s in the different size estimates, it is currently unknown whether Eris or Pluto has the larger diameter. Both Pluto and Eris are estimated to have solid-body diameters of about 2330 km. Determinations of Pluto's size are complicated by its atmosphere, and possible hydrocarbon haze.

Atmosphere



Pluto's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 consists of a thin envelope of nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

, methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

, and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 gases, which are derived from the ices of these substances on its surface. Its surface pressure ranges from 6.5 to 24 μbar. Pluto's elongated orbit is predicted to have a major effect on its atmosphere: as Pluto moves away from the Sun, its atmosphere should gradually freeze out, and fall to the ground. When Pluto is closer to the Sun, the temperature of Pluto's solid surface increases, causing the ices to sublimate into gas. This creates an anti-greenhouse effect
Anti-Greenhouse Effect
The anti-greenhouse effect is a neologism used to describe two different effects that describe a cooling effect an atmosphere has on the ambient temperature of the planet. Unlike the greenhouse effect, which is common, an anti-greenhouse effect is only known to exist in one situation in our Solar...

; much as sweat
SWEAT
SWEAT is an OLN/TSN show hosted by Julie Zwillich that aired in 2003-2004.Each of the 13 half-hour episodes of SWEAT features a different outdoor sport: kayaking, mountain biking, ice hockey, beach volleyball, soccer, windsurfing, rowing, Ultimate, triathlon, wakeboarding, snowboarding, telemark...

 cools the body as it evaporates from the surface of the skin, this sublimation cools the surface of Pluto. Scientists using the Submillimeter Array
Submillimeter Array
The Submillimeter Array consists of eight diameter radio telescopes arranged as an interferometer for submillimeter wavelength observations. It is the first purpose-built submillimeter interferometer, constructed after successful interferometry experiments using the pre-existing James Clerk...

 have recently discovered that Pluto's temperature is about 43 K (-230 °C), 10 K colder than would otherwise be expected.

The presence of methane, a powerful greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

, in Pluto's atmosphere creates a temperature inversion
Inversion (meteorology)
In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to a temperature inversion, i.e...

, with average temperatures 36 K warmer 10 km above the surface. The lower atmosphere contains a higher concentration of methane than its upper atmosphere.

The first evidence of Pluto's atmosphere was found by the Kuiper Airborne Observatory
Kuiper Airborne Observatory
The Gerard P. Kuiper Airborne Observatory was a national facility operated by NASA to support research in infrared astronomy. The observation platform was a highly modified C-141A jet transport aircraft with a range of 6,000 nautical miles , capable of conducting research operations up to 48,000...

 in 1985, from observations of the occultation
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

 of a star behind Pluto. When an object with no atmosphere moves in front of a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually. From the rate of dimming, the atmospheric pressure was determined to be 0.15 pascal
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

, roughly 1/700,000 that of Earth. The conclusion was confirmed and significantly strengthened by extensive observations of another similar occultation
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

 in 1988.

In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory
Paris Observatory
The Paris Observatory is the foremost astronomical observatory of France, and one of the largest astronomical centres in the world...

, James L. Elliot
James L. Elliot
James Ludlow Elliot was an American astronomer and scientist who, as part of a team, discovered the rings around the planet Uranus. Elliot was also part of a team that observed global warming on Triton, the largest moon of Neptune....

 of MIT, and Jay Pasachoff
Jay Pasachoff
Jay Myron Pasachoff is an American astronomer. Pasachoff is Field Memorial Professor of Astronomy at Williams College and the author of textbooks and tradebooks in astronomy, physics, mathematics, and other sciences.-Biography:...

 of Williams College
Williams College
Williams College is a private liberal arts college located in Williamstown, Massachusetts, United States. It was established in 1793 with funds from the estate of Ephraim Williams. Originally a men's college, Williams became co-educational in 1970. Fraternities were also phased out during this...

. Surprisingly, the atmospheric pressure was estimated to be 0.3 pascal, even though Pluto was farther from the Sun than in 1988 and thus should have been colder and had a more rarefied atmosphere. One explanation for the discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimate from the polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere as it freezes onto the north pole's now permanently dark ice cap. Spikes in the data from the same study revealed what may be the first evidence of wind in Pluto's atmosphere. Another stellar occultation was observed by the MIT-Williams College team of James Elliot, Jay Pasachoff
Jay Pasachoff
Jay Myron Pasachoff is an American astronomer. Pasachoff is Field Memorial Professor of Astronomy at Williams College and the author of textbooks and tradebooks in astronomy, physics, mathematics, and other sciences.-Biography:...

, and a Southwest Research Institute
Southwest Research Institute
Southwest Research Institute , headquartered in San Antonio, Texas, is one of the oldest and largest independent, nonprofit, applied research and development organizations in the United States...

 team led by Leslie Young on June 12, 2006 from sites in Australia.

In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery of ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....

 on Pluto's surface. This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto's surface and suspended in its atmosphere.

Satellites




Pluto has four known natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

s: Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...

, first identified in 1978 by astronomer James Christy
James W. Christy
James Walter Christy is an American astronomer.On June 22, 1978 while working at the United States Naval Observatory, he discovered that Pluto had a moon, which he named Charon shortly afterwards...

; Nix
Nix (moon)
Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...

 and Hydra
Hydra (moon)
Hydra is the second outermost known natural satellite of Pluto. It was discovered along with Nix in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...

, both discovered in 2005, and S/2011 P 1
S/2011 P 1
S/2011 P 1 is a small natural satellite of Pluto whose existence was announced on July 20, 2011...

 (provisional name, also known as P4), identified by the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 in 2011.

The Plutonian moons are unusually close to Pluto, compared to other observed systems. Moons could potentially orbit Pluto up to 53% (or 69%, if retrograde) of the Hill sphere
Hill sphere
An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sphere of its own...

 radius, the stable gravitational zone of Pluto's influence. For example, Psamathe
Psamathe (moon)
Psamathe , also known as Neptune X, is a retrograde irregular satellite of Neptune. It is named after Psamathe, one of the Nereids. This moon was discovered by Scott S. Sheppard and David C. Jewitt in 2003 using the 8.2 meter Subaru telescope...

 orbits Neptune at 40% of the Hill radius. In the case of Pluto, only the inner 3% of the zone is known to be occupied by satellites. In the discoverers’ terms, the Plutonian system appears to be "highly compact and largely empty", although others have pointed out the possibility of additional objects, including a small ring system.

Charon


The Pluto–Charon system is noteworthy for being the largest of the Solar System's few binary systems, defined as those whose barycentre lies above the primary's surface (617 Patroclus
617 Patroclus
617 Patroclus is a binary minor planet made up of two similarly-sized objects orbiting their common centre of gravity. It is a Trojan asteroid, sharing an orbit with Jupiter. It was discovered in 1906 by August Kopff, and was the second trojan to be discovered...

 is a smaller example). This and the large size of Charon relative to Pluto has led some astronomers to call it a dwarf double planet
Double planet
In astronomy, double planet and binary planet are informal terms used to describe a binary system of two astronomical objects that each satisfy the definition of planet and that are near enough to each other to have a significant gravitational effect on each other compared with the effect of the...

. The system is also unusual among planetary systems in that each is tidally locked
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...

 to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon: from any position on either body, the other is always at the same position in the sky, or always obscured. Because of this, the rotation period of each is equal to the time it takes the entire system to rotate around its common centre of gravity. Just as Pluto revolves on its side relative to the orbital plane, so the Pluto–Charon system does also. In 2007, observations by the Gemini Observatory
Gemini Observatory
The Gemini Observatory is an astronomical observatory consisting of two telescopes at sites in Hawai‘i and Chile. Together, the twin Gemini telescopes provide almost complete coverage of both the northern and southern skies...

 of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers.

Nix and Hydra




Two additional moons of Pluto were imaged by astronomers working with the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 on May 15, 2005, and received provisional designations
Provisional designation in astronomy
Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calculated...

 of S/2005 P 1 and S/2005 P 2. The International Astronomical Union officially named Pluto's newest moons Nix
Nix (moon)
Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...

 (or Pluto II, the inner of the two moons, formerly P 2) and Hydra
Hydra (moon)
Hydra is the second outermost known natural satellite of Pluto. It was discovered along with Nix in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...

 (Pluto III, the outer moon, formerly P 1), on June 21, 2006.

These small moons orbit Pluto at approximately two and three times the distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800 kilometres from the barycenter of the system. They have nearly circular prograde orbits in the same orbital plane as Charon.

Observations of Nix and Hydra to determine individual characteristics are ongoing. Hydra is sometimes brighter than Nix, suggesting either that it is larger or that different parts of its surface may vary in brightness. Sizes are estimated from albedos. The moons' spectral similarity to Charon suggests a 35% albedo similar to Charon's; this value results in diameter estimates of 46 kilometres for Nix and 61 kilometres for the brighter Hydra. Upper limits on their diameters can be estimated by assuming the 4% albedo of the darkest Kuiper Belt objects; these bounds are 137 ± 11 km and 167 ± 10 km, respectively. At the larger end of this range, the inferred masses are less than 0.3% that of Charon, or 0.03% of Pluto's.

The discovery of the two small moons suggests that Pluto may possess a variable ring system. Small body impacts can create debris that can form into planetary rings. Data from a deep optical survey by the Advanced Camera for Surveys
Advanced Camera for Surveys
The Advanced Camera for Surveys is a third generation axial instrument aboard the Hubble Space Telescope . The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University. ACS was assembled and tested extensively at Ball Aerospace & Technologies Corp...

 on the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 suggest that no ring system is present. If such a system exists, it is either tenuous like the rings of Jupiter
Rings of Jupiter
The planet Jupiter has a system of rings, known as the rings of Jupiter or the Jovian ring system. It was the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. It was first observed in 1979 by the Voyager 1 space probe and thoroughly investigated in the 1990s...

 or is tightly confined to less than 1,000 km in width.

Similar conclusions have been made from occultation studies. In imaging the Plutonian system, observations from Hubble placed limits on any additional moons. With 90% confidence, no additional moons larger than 12 km (or a maximum of 37 km with an albedo of 0.041) exist beyond the glare of Pluto 5 arcseconds from the dwarf planet. This assumes a Charon-like albedo of 0.38; at a 50% confidence level the limit is 8 kilometres.

S/2011 P 1


On July 20, 2011 Mark Showalter of the SETI Institute
SETI Institute
The SETI Institute is a not-for-profit organization whose mission is to “explore, understand and explain the origin, nature and prevalence of life in the universe”. SETI stands for the "search for extraterrestrial intelligence". One program is the use of both radio and optical telescopes to search...

 announced the discovery of a fourth moon of Pluto, provisionally named S/2011 P 1
S/2011 P 1
S/2011 P 1 is a small natural satellite of Pluto whose existence was announced on July 20, 2011...

 or P4. It was noticed by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Hubble Space Telescope during a survey searching for rings
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...

 around the dwarf planet. It has an estimated diameter of 8 to 21 miles (13 to 34 km) and is located between the orbits of Nix and Hydra.

S/2011 P 1 was first seen in a photo taken with Hubble's Wide Field Camera 3 on June 28. It was confirmed in subsequent Hubble pictures taken on July 3 and July 18.

Near resonances


Nix and Hydra and are very close to (but not in) 4:1 and 6:1 mean motion orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

s with Charon. S/2011 P 1 fits neatly into this arrangement with a near 5:1 resonance with Charon. Determining how close any of these near integer orbital period ratios might actually be to a true resonance requires accurate knowledge of the satellites' precession
Apsidal precession
In celestial mechanics, perihelion precession, apsidal precession or orbital precession is the precession of the orbit of a celestial body. More precisely it is the gradual rotation of the line joining the apsides of an orbit, which are the points of closest and farthest approach...

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

 comparison
Name

(Pronunciation)
Discovery
Year
Diameter
(km)
Mass
(kg)
Orbital radius (km)
(barycentric)
Orbital period (d)
Pluto /ˈpluːtoʊ/ 1930 2,306
(66% Moon)
13,050
(18% Moon)
2,040
(0.6% Moon)
Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...

/ˈʃærən/,
/ˈkɛərən/
1978 1,205
(35% Moon)
1,520
(2% Moon)
17,530
(5% Moon)
6.3872
(25% Moon)
Nix
Nix (moon)
Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...

/ˈnɪks/ 2005 91 4 48,708 24.856
S/2011 P 1
S/2011 P 1
S/2011 P 1 is a small natural satellite of Pluto whose existence was announced on July 20, 2011...

2011 13–34 ~59,000 32.1
Hydra
Hydra (moon)
Hydra is the second outermost known natural satellite of Pluto. It was discovered along with Nix in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...

/ˈhaɪdrə/ 2005 114 8 64,749 38.206


Mass of Nix and Hydra assumes icy/porous density of 1.0 g/cm3

Origins



Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

. This notion has been heavily criticised because Pluto never comes near Neptune in its orbit.

Pluto's true place in the Solar System began to reveal itself only in 1992, when astronomers began to find small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This trans-Neptunian population is believed to be the source of many short-period comets. Astronomers now believe Pluto to be the largest member of the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

, a somewhat stable ring of objects located between 30 and 50 AU from the Sun. Like other Kuiper-belt objects (KBOs), Pluto shares features with comets; for example, the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

 is gradually blowing Pluto's surface into space, in the manner of a comet. If Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.

Though Pluto is the largest of the Kuiper belt objects discovered so far, Neptune's moon Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

, which is slightly larger than Pluto, is similar to it both geologically and atmospherically, and is believed to be a captured Kuiper belt object. Eris (see below) is also larger than Pluto but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called the scattered disc
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...

.

A large number of Kuiper belt objects, like Pluto, possess a 3:2 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutino
Plutino
In astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino...

s", after Pluto.

Like other members of the Kuiper belt, Pluto is thought to be a residual planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...

; a component of the original protoplanetary disc around the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto owes its current position to a sudden migration
Planetary migration
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis...

 undergone by Neptune early in the Solar System's formation. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbit around itself, which became its moon Triton, locking others into resonances and knocking others into chaotic orbits. The objects in the scattered disc
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...

, a dynamically unstable region overlapping the Kuiper belt, are believed to have been placed in their current positions by interactions with Neptune's migrating resonances. A 2004 computer model by Alessandro Morbidelli of the Observatoire de la Côte d'Azur
Côte d'Azur Observatory
The Observatoire de la Côte d'Azur originated in 1988 with the merger of two observatories:# Observatoire de Nice# The CERGA - External links :*...

 in Nice
Nice
Nice is the fifth most populous city in France, after Paris, Marseille, Lyon and Toulouse, with a population of 348,721 within its administrative limits on a land area of . The urban area of Nice extends beyond the administrative city limits with a population of more than 955,000 on an area of...

 suggested that the migration of Neptune into the Kuiper belt may have been triggered by the formation of a 1:2 resonance between Jupiter and Saturn, which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places, ultimately doubling Neptune's distance from the Sun. The resultant expulsion of objects from the proto-Kuiper belt could also explain the Late Heavy Bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...

 600 million years after the Solar System's formation and the origin of Jupiter's Trojan asteroid
Trojan asteroid
The Jupiter Trojans, commonly called Trojans or Trojan asteroids, are a large group of objects that share the orbit of the planet Jupiter around the Sun. Relative to Jupiter, each Trojan librates around one of the planet's two Lagrangian points of stability, and , that respectively lie 60° ahead...

s.
It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune's migration perturbed
Perturbation (astronomy)
Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....

 it into a resonant capture. The Nice model requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk; these may have included the bodies which became Triton and Eris.

Exploration



Pluto presents significant challenges for spacecraft because of its small mass and great distance from Earth. Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...

 could have visited Pluto, but controllers opted instead for a close flyby of Saturn's
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 moon Titan, resulting in a trajectory incompatible with a Pluto flyby. Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

 never had a plausible trajectory for reaching Pluto. No serious attempt to explore Pluto by spacecraft occurred until the last decade of the 20th century. In August 1992, JPL scientist Robert Staehle telephoned Pluto's discoverer, Clyde Tombaugh
Clyde Tombaugh
Clyde William Tombaugh was an American astronomer. Although he is best known for discovering the dwarf planet Pluto in 1930, the first object to be discovered in what would later be identified as the Kuiper Belt, Tombaugh also discovered many asteroids; he also called for serious scientific...

, requesting permission to visit his planet. "I told him he was welcome to it," Tombaugh later remembered, "though he's got to go one long, cold trip." Despite this early momentum, in 2000, NASA cancelled the Pluto Kuiper Express
Pluto Kuiper Express
The Pluto Kuiper Express mission was a space mission designed to fly by the Pluto-Charon system and at least one large object in the Kuiper belt beyond Pluto's orbit. Originally designated the Pluto Fast Flyby, it was scheduled to reach Pluto by 2012...

 mission, citing increasing costs and launch vehicle delays.


After an intense political battle, a revised mission to Pluto, dubbed New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...

, was granted funding from the US government in 2003. New Horizons was launched successfully on January 19, 2006. The mission leader, S. Alan Stern, confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft.

In early 2007 the craft made use of a gravity assist from Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

. Its closest approach to Pluto will be on July 14, 2015; scientific observations of Pluto will begin 5 months before closest approach and will continue for at least a month after the encounter. New Horizons captured its first (distant) images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager (LORRI). The images, taken from a distance of approximately 4.2 billion kilometres, confirm the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper Belt objects.

New Horizons will use a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterise the global geology and morphology of Pluto and its moon Charon, map their surface composition and analyse Pluto's neutral atmosphere and its escape rate. New Horizons will also photograph the surfaces of Pluto and Charon.

The discovery of Pluto's two small moons, Nix and Hydra, may present unforeseen challenges for the probe. Debris from collisions between Kuiper belt objects and the smaller moons, with their relatively low escape velocities, may produce a tenuous dusty ring. Were New Horizons to fly through such a ring system, there would be an increased potential for damage that could disable the probe.

Concepts


A Pluto orbiter/lander/sample return mission using the MITEE nuclear engine was proposed in 2003. The plan included a twelve year trip to Pluto from Earth, then mapping from orbit, multiple landings, warm water probe, and possible in situ propellant production for another twelve year trip back to Earth with samples. Power and propulsion would come from the bi-modal nuclear reactor system.

Classification


After Pluto's place within the Kuiper belt was determined, its official status as a planet became controversial, with many questioning whether Pluto should be considered together with or separately from its surrounding population.

Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the Solar System. The Hayden Planetarium
Hayden Planetarium
The Hayden Planetarium is a public planetarium, part of the Rose Center for Earth and Space of the American Museum of Natural History in New York City, currently directed by astrophysicist Dr. Neil deGrasse Tyson....

 reopened after renovation in 2000 with a model of only eight planets. The controversy made headlines at the time.

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

 was discovered, with a diameter then thought to be roughly 1280 kilometres, about half that of Pluto. In 2004, the discoverers of 90377 Sedna
90377 Sedna
90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...

 placed an upper limit of 1800 km on its diameter, nearer to Pluto's diameter of 2320 km, although Sedna's diameter was revised downward to less than 1600 km by 2007. Just as Ceres, Pallas
2 Pallas
Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered , and one of the largest. It is estimated to constitute 7% of the mass of the asteroid belt, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta. It is however 20%...

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

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

 eventually lost their planet status after the discovery of many other asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s, so, it was argued, Pluto should be reclassified as one of the Kuiper belt objects.

On July 29, 2005, the discovery of a new Trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

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

, it is now known to be approximately the same size as Pluto. This was the largest object discovered in the Solar System since Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

 in 1846. Its discoverers and the press initially called it the tenth planet, although there was no official consensus at the time on whether to call it a planet. Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.

2006: IAU classification


The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':
  1. The object must be in orbit around the Sun
    Sun
    The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

    .
  2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium
    Hydrostatic equilibrium
    Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...

    .
  3. It must have cleared the neighbourhood
    Clearing the neighbourhood
    "Clearing the neighbourhood of its orbit" is a criterion for a celestial body to be considered a planet in the Solar System. This was one of the three criteria adopted by the International Astronomical Union in its 2006 definition of planet....

     around its orbit.


Pluto fails to meet the third condition, since its mass is only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit). The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

 category, and that it act as the prototype for the plutoid category of trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

s, in which it would be separately, but concurrently, classified.

On September 13, 2006, the IAU included Pluto, Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

, and the Eridian moon Dysnomia
Dysnomia (moon)
- References :...

 in their Minor Planet Catalogue, giving them the official minor planet designations "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia". If Pluto had been given a minor planet name upon its discovery, the number would have been ca. 1,164 rather than 134,340.

There has been some resistance within the astronomical community toward the reclassification. Alan Stern
Alan Stern
S. Alan Stern is an American planetary scientist. He is the principal investigator of the New Horizons mission to Pluto....

, principal investigator with NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...

 mission to Pluto, has publicly derided the IAU resolution, stating that "the definition stinks, for technical reasons." Stern's contention is that by the terms of the new definition Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded. His other claim is that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community. Marc W. Buie
Marc W. Buie
Marc W. Buie is an astronomer at Lowell Observatory in Flagstaff, Arizona. He grew up in Baton Rouge, Louisiana and went on to get a B.S. in Physics from Louisiana State University in 1980. After that he switched fields and earned his Ph.D. in Planetary Science from the University of Arizona in...

 of the Lowell observatory has voiced his opinion on the new definition on his website and is one of the petitioners against the definition. Others have supported the IAU. Mike Brown, the astronomer who discovered Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

, said "through this whole crazy circus-like procedure, somehow the right answer was stumbled on. It’s been a long time coming. Science is self-correcting eventually, even when strong emotions are involved."

Researchers on both sides of the debate gathered on August 14–16, 2008, at The Johns Hopkins University Applied Physics Laboratory for a conference that included back-to-back talks on the current IAU definition of a planet. Entitled "The Great Planet Debate", the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of a planet. Just before the conference, on June 11, 2008, the IAU announced in a press release that the term "plutoid" would henceforth be used to describe Pluto and other objects similar to Pluto which have an orbital semimajor axis greater than that of Neptune and enough mass to be of near-spherical shape.

Reaction


Reception to the IAU decision was mixed. While some accepted the reclassification, others seek to overturn the decision with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California state assembly light-heartedly denounces the IAU for "scientific heresy", among other crimes. The U.S. state of New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...

's House of Representatives
New Mexico House of Representatives
The New Mexico House of Representatives is the lower house of the New Mexico State Legislature.There are 70 members of the House. Each member represents roughly 25,980 residents of New Mexico.The Speaker of the House is Ben Luján .-Composition:...

 passed a resolution in honor of Tombaugh, a longtime resident of that state, which declared that Pluto will always be considered a planet while in New Mexican skies and that March 13, 2007 was Pluto Planet Day. The Illinois
Illinois
Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

 State Senate
Illinois Senate
The Illinois Senate is the upper chamber of the Illinois General Assembly, the legislative branch of the government of the state of Illinois in the United States. The body was created by the first state constitution adopted in 1818. The Illinois Senate is made up of 59 senators elected from...

 passed a similar resolution in 2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, was born in Illinois. The resolution asserted that Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU.

Some members of the public have also rejected the change, citing the disagreement within the scientific community on the issue, or for sentimental reasons, maintaining that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision.

See also


  • Moons of Pluto
  • Pluto in astrology
  • Pluto in fiction
    Pluto in fiction
    Pluto has been featured in many instances of science fiction and popular culture. Initially classified as a planet upon its discovery in 1930, Pluto has also received considerable publicity following a 2006 definition of planet decree, which dubbed it a dwarf planet.- Literature :* "The Whisperer...

  • Solar eclipses on Pluto
    Solar eclipses on Pluto
    Solar eclipses on Pluto are caused when one of its four natural satellites – Charon, Nix, Hydra, and P4 – passes in front of the Sun, blocking its light....

  • Dwarf planet
    Dwarf planet
    A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...


External links