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Pluto (
minor-planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur, trans-Neptunian object and dwarf planet but not comet). Such designation always features a leading number (catalog or ...
: 134340 Pluto) is a
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
in the
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
ive known object to directly orbit the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
. It is the largest known trans-Neptunian object by volume, by a small margin, but is slightly less massive than Eris. Like other Kuiper belt objects, Pluto is made primarily of ice and rock and is much smaller than the
inner planet The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
s. Compared to Earth's moon, Pluto has only one sixth its mass and one third its volume. Pluto has a moderately
eccentric Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
and inclined orbit, ranging from from the Sun. Light from the Sun takes 5.5 hours to reach Pluto at its average distance (). Pluto's eccentric orbit periodically brings it closer to the Sun than
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
, but a stable
orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
prevents them from colliding. Pluto has five known moons:
Charon In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the wo ...
, the largest, whose diameter is just over half that of Pluto;
Styx In Greek mythology, Styx (; grc, Στύξ ) is a river that forms the boundary between Earth (Gaia) and the Underworld. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the centre of the underworld on a great marsh, whic ...
;
Nix Nix or NIX may refer to: Places * Nix, Alabama, an unincorporated community, United States * Nix, Texas, a ghost town in southwestern Lampasas County, Texas, United States * Nix (moon), a moon of Pluto People * Nix (surname), listing people with ...
; Kerberos; and
Hydra Hydra generally refers to: * Lernaean Hydra, a many-headed serpent in Greek mythology * ''Hydra'' (genus), a genus of simple freshwater animals belonging to the phylum Cnidaria Hydra or The Hydra may also refer to: Astronomy * Hydra (constel ...
. Pluto and Charon are sometimes considered a
binary system A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that thi ...
because the
barycenter In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important conc ...
of their orbits does not lie within either body, and they are tidally locked. The ''
New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...
'' mission was the first spacecraft to visit Pluto and its moons, making a flyby on July 14, 2015 and taking detailed measurements and observations. Pluto was discovered in 1930, the first object in the Kuiper belt. It was immediately hailed as the ninth planet, but its planetary status was questioned when it was found to be much smaller than expected. These doubts increased following the discovery of additional objects in the Kuiper belt starting in the 1990s, and particularly the more massive
scattered disk The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objec ...
object Eris in 2005. In 2006 the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
(IAU) formally redefined the term ''planet'' to exclude
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
s such as Pluto. Many planetary astronomers, however, continue to consider Pluto and other dwarf planets to be planets.


History


Discovery

In the 1840s,
Urbain Le Verrier Urbain Jean Joseph Le Verrier FRS (FOR) HFRSE (; 11 March 1811 – 23 September 1877) was a French astronomer and mathematician who specialized in celestial mechanics and is best known for predicting the existence and position of Neptune using ...
used
Newtonian mechanics Newton's laws of motion are three basic laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows: # A body remains at rest, or in motion ...
to predict the position of the then-undiscovered planet
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
after analyzing perturbations in the orbit of
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune. In 1906,
Percival Lowell Percival Lowell (; March 13, 1855 – November 12, 1916) was an American businessman, author, mathematician, and astronomer who fueled speculation that there were canals on Mars, and furthered theories of a ninth planet within the Solar System. ...
—a wealthy Bostonian who had founded
Lowell Observatory Lowell Observatory is an astronomical observatory in Flagstaff, Arizona, United States. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark ...
in
Flagstaff, Arizona Flagstaff ( ) is a city in, and the county seat of, Coconino County, Arizona, Coconino County in northern Arizona, in the southwestern United States. In 2019, the city's estimated population was 75,038. Flagstaff's combined metropolitan area has ...
, in 1894—started an extensive project in search of a possible ninth planet, which he termed "
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 and continued at the start of the 20th with Percival Lowell's ...
". By 1909, Lowell and William H. Pickering 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, his surveys had captured two faint images of Pluto on March 19 and April 7, 1915, but they were not recognized for what they were. There are fourteen other known
precovery In astronomy, precovery (short for pre-discovery recovery) is the process of finding the image of an object in images or photographic plates predating its discovery, typically for the purpose of calculating a more accurate orbit. This happens mos ...
observations, with the earliest made by the
Yerkes Observatory Yerkes Observatory ( ) is an astronomical observatory located in Williams Bay, Wisconsin, United States. The observatory was operated by the University of Chicago Department of Astronomy and Astrophysics from its founding in 1897 to 2018. Owne ...
on August 20, 1909. Percival's widow, Constance Lowell, entered into a ten-year legal battle with the Lowell Observatory over her husband's legacy, and the search for Planet X did not resume until 1929. Vesto Melvin Slipher, the observatory director, gave the job of locating Planet X to 23-year-old
Clyde Tombaugh Clyde William Tombaugh (February 4, 1906 January 17, 1997) was an American astronomer. He discovered Pluto in 1930, the first object to be discovered in what would later be identified as the Kuiper belt. At the time of discovery, Pluto was cons ...
, who had just arrived at the 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, then examine each pair and determine whether any objects had shifted position. Using a
blink comparator A blink comparator is a viewing apparatus formerly used by astronomers to find differences between two photographs of the night sky. It permits rapid switching from viewing one photograph to viewing the other, "blinking" back and forth between th ...
, 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 29. 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 The Harvard College Observatory (HCO) 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, United St ...
on March 13, 1930. As one Plutonian year corresponds to 247.94 Earth years, Pluto will complete its first orbit since its discovery in 2178.


Name and symbol

The discovery made headlines around the globe. Lowell Observatory, which had the right to name the new object, received more than 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 or , , ; grc, Δῐός, ''Diós'', label=Genitive case, genitive Aeolic Greek, Boeotian Aeolic and Doric Greek#Laconian, Laconian grc-dor, Δεύς, Deús ; grc, Δέος, ''Déos'', label=Genitive case, genitive el, Δίας, ''D ...
'', then ''Percival'' and finally ''Constance''. These suggestions were disregarded. The name Pluto, after the Greek/Roman god of the underworld, was proposed by
Venetia Burney Venetia Katharine Douglas Burney (married name Phair, 11 July 1918 – 30 April 2009) was an English accountant and teacher. She is remembered as the first person to suggest the name Pluto for the dwarf planet discovered by Clyde Tombaugh in 193 ...
(1918–2009), an eleven-year-old schoolgirl in
Oxford Oxford () is a city in England. It is the county town and only city of Oxfordshire. In 2020, its population was estimated at 151,584. It is north-west of London, south-east of Birmingham and north-east of Bristol. The city is home to the ...
, England, who was interested in
classical mythology Classical mythology, Greco-Roman mythology, or Greek and Roman mythology is both the body of and the study of myths from the ancient Greeks and ancient Romans as they are used or transformed by cultural reception. Along with philosophy and polit ...
. She suggested it in a conversation with her grandfather
Falconer Madan Falconer Madan (15 April 1851 – 22 May 1935) was Librarian of the Bodleian Library of Oxford University. Early life and education Falconer Madan was born in Cam, Gloucestershire, the fifth son of George and Harriet Madan. He was educated at ...
, a former librarian at the
University of Oxford , mottoeng = The Lord is my light , established = , endowment = £6.1 billion (including colleges) (2019) , budget = £2.145 billion (2019–20) , chancellor ...
's
Bodleian Library The Bodleian Library () is the main research library of the University of Oxford, and is one of the oldest libraries in Europe. It derives its name from its founder, Sir Thomas Bodley. With over 13 million printed items, it is the second- ...
, who passed the name to astronomy professor
Herbert Hall Turner Herbert Hall Turner (13 August 1861 – 20 August 1930) was a British astronomer and seismologist. Biography Herbert Hall Turner was educated at the Leeds Modern School, Clifton College, Bristol and Trinity College, Cambridge. In 1884 h ...
, who cabled it to colleagues in the United States. Each member of the Lowell Observatory was allowed to vote on a short-list of three potential names:
Minerva Minerva (; ett, Menrva) is the Roman goddess of wisdom, justice, law, victory, and the sponsor of arts, trade, and strategy. Minerva is not a patron of violence such as Mars, but of strategic war. From the second century BC onward, the Roma ...
(which was already the name for an asteroid),
Cronus In Ancient Greek religion and mythology, Cronus, Cronos, or Kronos ( or , from el, Κρόνος, ''Krónos'') was the leader and youngest of the first generation of Titans, the divine descendants of the primordial Gaia (Mother Earth) and ...
(which had lost reputation through being proposed by the unpopular astronomer
Thomas Jefferson Jackson See Thomas Jefferson Jackson (T. J. J.) See (February 19, 1866 – July 4, 1962) was an American astronomer whose promulgated theories in astronomy and physics were eventually disproven. His educational and professional career were dogged by plagiar ...
), and Pluto. Pluto received a unanimous vote. The name was published on May 1, 1930. Upon the announcement, Madan gave Venetia £5 (equivalent to £ in , or
US$ The United States dollar (symbol: $; code: USD; also abbreviated US$ or U.S. Dollar, to distinguish it from other dollar-denominated currencies; referred to as the dollar, U.S. dollar, American dollar, or colloquially buck) is the official ...
in ) as a reward. The final choice of name was helped in part by the fact that the first two letters of ''Pluto'' are the initials of Percival Lowell. Pluto's
planetary symbol A planet symbol (or ''planetary symbol'') is a graphical symbol used in astrology and astronomy to represent a classical planet (including the Sun and the Moon) or one of the modern planets. The symbols were also used in alchemy Alchemy (from ...
was then created as a
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 o ...
of the letters "PL" (in
Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expre ...
: ), though it is rarely used in astronomy today. For example, occurs in a table of the planets identified by their symbols in a 2004 article written before the 2006 IAU definition, but not in a graph of planets, dwarf planets and moons from 2016, where only the eight IAU planets are identified by their symbols. (Planetary symbols in general are uncommon in astronomy, and are discouraged by the IAU.) The ♇ monogram is also used in astrology, but the most-common
astrological symbol Historically, astrological and astronomical symbols overlapped. Frequently used symbols include signs of the zodiac and classical planets. These originate from medieval Byzantine codices. Their current form is a product of the European Renaissan ...
for Pluto, at least in English-language sources, is an orb over Pluto's
bident A bident is a two-pronged implement resembling a pitchfork. In Greek mythology, the bident is a weapon associated with Hades (Pluto), the ruler of the underworld. Likewise, the three-pronged trident is the implement of his brother Poseidon (Nept ...
(). The bident symbol has seen some astronomical use as well since the IAU decision on dwarf planets, for example in a public-education poster on dwarf planets published by the NASA/JPL ''Dawn'' mission in 2015, in which each of the five dwarf planets announced by the IAU receives a symbol. There are in addition several other symbols for Pluto found in European astrological sources, including three accepted by Unicode: , ; , , used in
Uranian astrology The Hamburg School of Astrology originated in Hamburg, Germany, and revolved around the research and teachings of surveyor/ astrologer/amateur astronomer Alfred Witte. The term ''Hamburg School'' as an astrological method originated in 1923 a ...
; and /, , found in various orientations, showing Pluto's orbit cutting across that of Neptune. The name 'Pluto' was soon embraced by wider culture. In 1930,
Walt Disney Walter Elias Disney (; December 5, 1901December 15, 1966) was an American animator, film producer and entrepreneur. A pioneer of the American animation industry, he introduced several developments in the production of cartoons. As a film p ...
was apparently inspired by it when he introduced for
Mickey Mouse Mickey Mouse is an animated cartoon Character (arts), character co-created in 1928 by Walt Disney and Ub Iwerks. The longtime mascot of The Walt Disney Company, Mickey is an Anthropomorphism, anthropomorphic mouse who typically wears red sho ...
a canine companion named
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
, although
Disney The Walt Disney Company, commonly known as Disney (), is an American multinational mass media and entertainment conglomerate headquartered at the Walt Disney Studios complex in Burbank, California. Disney was originally founded on October ...
animator
Ben Sharpsteen Benjamin Sharpsteen (November 4, 1895 – December 20, 1980) was an American film director and producer for Disney The Walt Disney Company, commonly known as Disney (), is an American multinational mass media and entertainment con ...
could not confirm why the name was given. In 1941,
Glenn T. Seaborg Glenn Theodore Seaborg (; April 19, 1912February 25, 1999) was an American chemist whose involvement in the synthesis, discovery and investigation of ten transuranium elements earned him a share of the 1951 Nobel Prize in Chemistry. His work in ...
named the newly created element
plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...
after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following
uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
, which was named after Uranus, and
neptunium Neptunium is a chemical element with the Symbol (chemistry), symbol Np and atomic number 93. A radioactivity, radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after ...
, which was named after Neptune. Most languages use the name "Pluto" in various transliterations. In Japanese, Houei Nojiri suggested the calque , and this was borrowed into Chinese and Korean. Some
languages of India Languages spoken in India belong to several language families, the major ones being the Indo-European languages spoken by 78.05% of Indians and the Dravidian languages spoken by 19.64% of Indians, both families together are sometimes known ...
use the name Pluto, but others, such as
Hindi Hindi (Devanāgarī: or , ), or more precisely Modern Standard Hindi (Devanagari: ), is an Indo-Aryan language spoken chiefly in the Hindi Belt region encompassing parts of northern, central, eastern, and western India. Hindi has been de ...
, use the name of ''
Yama Yama (Devanagari: यम) or Yamarāja (यमराज), is a deity of death, dharma, the south direction, and the underworld who predominantly features in Hindu and Buddhist religion, belonging to an early stratum of Rigvedic Hindu deities ...
'', the God of Death in Hinduism.
Polynesian languages The Polynesian languages form a genealogical group of languages, itself part of the Oceanic branch of the Austronesian family. There are 38 Polynesian languages, representing 7 percent of the 522 Oceanic languages, and 3 percent of the Austron ...
also tend to use the indigenous god of the underworld, as in
Māori Māori or Maori can refer to: Relating to the Māori people * Māori people of New Zealand, or members of that group * Māori language, the language of the Māori people of New Zealand * Māori culture * Cook Islanders, the Māori people of the C ...
''
Whiro Whiro-te-tipua (Whiro, or Hiro in the Tuamotus) is the lord of darkness and embodiment of all evil in Māori mythology. He inhabits the underworld and is responsible for the ills of all people, a contrast to his brother and enemy Tāne. Acco ...
''. Vietnamese might be expected to follow Chinese, but does not because the Sino-Vietnamese word 冥 ''minh'' "dark" is homophonous with 明 ''minh'' "bright". Vietnamese instead uses Yama, which is also a Buddhist deity, in the form of ''Sao Diêm Vương'' 星閻王 "Yama's Star", derived from Chinese 閻王 '' Yán Wáng / Yìhm Wòhng'' "King Yama".


Planet X disproved

Once Pluto was found, its faintness and lack of a viewable disc cast doubt on the idea that it was Lowell's
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 and continued at the start of the 20th with Percival Lowell's ...
. Estimates of Pluto's mass were revised downward throughout the 20th century. Astronomers initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1931, Pluto was calculated to be roughly the mass of
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, with further calculations in 1948 bringing the mass down to roughly that of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
. In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the
University of Hawaii A university () is an institution of higher (or tertiary) education and research which awards academic degrees in several academic disciplines. Universities typically offer both undergraduate and postgraduate programs. In the United States, th ...
calculated Pluto's
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of Earth. (Pluto's albedo is times that of Earth.) In 1978, the discovery of Pluto's moon
Charon In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the wo ...
allowed the measurement of Pluto's mass for the first time: roughly 0.2% that of Earth, and far too small to account for the discrepancies in the orbit of Uranus. Subsequent searches for an alternative Planet X, notably by
Robert Sutton Harrington Robert Sutton Harrington (October 21, 1942 – January 23, 1993) was an American astronomer who worked at the United States Naval Observatory (USNO). Harrington was born near Newport News, Virginia. His father was an archaeologist. He was marri ...
, failed. In 1992,
Myles Standish Myles Standish (c. 1584 – October 3, 1656) was an English military officer and colonizer. He was hired as military adviser for Plymouth Colony in present-day Massachusetts, United States by the Pilgrims. Standish accompanied the Pilgrims on ...
used data from ''
Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a ...
s flyby of Neptune in 1989, which had revised the estimates of Neptune's mass downward by 0.5%—an amount comparable to the mass of Mars—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 orbit and position in 1915 that was fairly close to Pluto's actual orbit and its position at that time;
Ernest W. Brown Ernest William Brown FRS (29 November 1866 – 22 July 1938) was an English mathematician and astronomer, who spent the majority of his career working in the United States and became a naturalised American citizen in 1923. His life's work was ...
concluded soon after Pluto's discovery that this was a coincidence.


Classification

From 1992 onward, many bodies were discovered orbiting in the same volume as Pluto, showing that Pluto is part of a population of objects called the
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
. This made its official status as a planet 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. In February 2000 the
Hayden Planetarium The Rose Center for Earth and Space is a part of the American Museum of Natural History in New York City. The Center's complete name is The Frederick Phineas and Sandra Priest Rose Center for Earth and Space. The main entrance is located on the no ...
in New York City displayed a Solar System model of only eight planets, which made headlines almost a year later.
Ceres Ceres most commonly refers to: * Ceres (dwarf planet), the largest asteroid * Ceres (mythology), the Roman goddess of agriculture Ceres may also refer to: Places Brazil * Ceres, Goiás, Brazil * Ceres Microregion, in north-central Goiás ...
,
Pallas Pallas may refer to: Astronomy * 2 Pallas asteroid ** Pallas family, a group of asteroids that includes 2 Pallas * Pallas (crater), a crater on Earth's moon Mythology * Pallas (Giant), a son of Uranus and Gaia, killed and flayed by Athena * Pall ...
,
Juno Juno commonly refers to: *Juno (mythology), the Roman goddess of marriage and queen of the gods *Juno (film), ''Juno'' (film), 2007 Juno may also refer to: Arts, entertainment and media Fictional characters *Juno, in the film ''Jenny, Juno'' *Ju ...
and Vesta lost their planet status after the discovery of many other
asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...
s. Similarly, objects increasingly closer in size to Pluto were discovered in the Kuiper belt region. On July 29, 2005, astronomers at
Caltech The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...
announced the discovery of a new
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically, ...
, Eris, which was substantially more massive than Pluto and the most massive object discovered in the Solar System since
Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus' ...
in 1846. Its discoverers and the press initially called it the
tenth planet 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 and continued at the start of the 20th with Percival Lowell's ...
, 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.


IAU classification

The debate came to a head in August 2006, with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three conditions for an object in the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
to be considered a planet: * The object must be in orbit around the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
. * The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape defined by
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ...
. * It must have cleared the neighborhood around its orbit. Pluto fails to meet the third condition. Its mass is substantially less than the combined mass of the other objects in its orbit: 0.07 times, in contrast to Earth, which is 1.7 million times the remaining mass in its orbit (excluding the moon). The IAU further decided that bodies that, like Pluto, meet criteria 1 and 2, but do not meet criterion 3 would be called
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
s. In September 2006, the IAU included Pluto, and Eris and its moon Dysnomia, in their
Minor Planet Catalogue The following is a list of numbered minor planets in ascending numerical order. With the exception of comets, minor planets are all Small Solar System bodies, small bodies in the Solar System, including asteroids, Distant minor planet, distant o ...
, giving them the official
minor-planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur, trans-Neptunian object and dwarf planet but not comet). Such designation always features a leading number (catalog or ...
s "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia". Had Pluto been included upon its discovery in 1930, it would have likely been designated 1164, following 1163 Saga, which was discovered a month earlier. There has been some resistance within the astronomical community toward the reclassification.
Alan Stern Sol Alan Stern (born November 22, 1957) is an American engineer and planetary scientist. He is the principal investigator of the ''New Horizons'' mission to Pluto and the Chief Scientist at Moon Express. Stern has been involved in 24 suborbital ...
, principal investigator with
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
's ''New Horizons'' mission to Pluto, derided the IAU resolution, stating that "the definition stinks, for technical reasons". Stern contended that, by the terms of the new definition, Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded. He argued that all big spherical moons, including the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, should likewise be considered planets. He also stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community. Marc W. Buie, then at the Lowell Observatory, petitioned against the definition. Others have supported the IAU. Mike Brown, the astronomer who discovered Eris, 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." Public reception to the IAU decision was mixed. A resolution introduced in the
California State Assembly The California State Assembly is the lower house of the California State Legislature, the upper house being the California State Senate. The Assembly convenes, along with the State Senate, at the California State Capitol in Sacramento. The A ...
facetiously called the IAU decision a "scientific heresy". 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 most recent elections were held on November 3, 2020. Composition Leadership Cu ...
passed a resolution in honor of Tombaugh, a longtime resident of that state, that 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 Senate The Illinois Senate is the Upper house, upper chamber of the Illinois General Assembly, the legislative branch of the government of the U.S. state, State of Illinois in the United States. The body was created by the first state constitution adop ...
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. In 2006, in its 17th annual words-of-the-year vote, the
American Dialect Society The American Dialect Society (ADS), founded in 1889, is a learned society "dedicated to the study of the English language in North America, and of other languages, or dialects of other languages, influencing it or influenced by it." The Society p ...
voted ''
plutoed The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a ''planet'' is a celestial body that: # is in orbit around the Sun, # has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), an ...
'' as the word of the year. To "pluto" is to "demote or devalue someone or something". Researchers on both sides of the debate gathered in August 2008, at the Johns Hopkins University
Applied Physics Laboratory The Johns Hopkins University Applied Physics Laboratory (Applied Physics Laboratory, or APL) is a not-for-profit University Affiliated Research Center, university-affiliated research center (UARC) in Howard County, Maryland. It is affiliated w ...
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 planet. In June 2008, the IAU had announced in a press release that the term "
plutoid A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to pl ...
" would henceforth be used to refer to Pluto and other planetary-mass objects that have an orbital
semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...
greater than that of Neptune, though the term has not seen significant use.


Orbit

Pluto's orbital period is currently about 248 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(s) most often refers to: * Aero- or airplane, a powered, fixed-wing aircraft * Plane (geometry), a flat, 2-dimensional surface Plane or planes may also refer to: Biology * Plane (tree) or ''Platanus'', wetland native plant * Planes (gen ...
called the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...
. In contrast, Pluto's orbit is moderately
inclined Incline, inclined, inclining, or inclination may refer to: *Grade (slope), the tilt, steepness, or angle from horizontal of a topographic feature (hillside, meadow, etc.) or constructed element (road, railway, field, etc.) *Slope, the tilt, steepn ...
relative to the ecliptic (over 17°) and moderately
eccentric Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
(elliptical). This eccentricity means a small region of Pluto's orbit lies closer to the Sun than Neptune's. The Pluto–Charon barycenter came to
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
on September 5, 1989, and was last closer to the Sun than Neptune between February 7, 1979, and February 11, 1999. Although the 3:2 resonance with Neptune (see below) is maintained, Pluto's inclination and eccentricity behave in a
chaotic Chaotic was originally a Danish trading card game. It expanded to an online game in America which then became a television program based on the game. The program was able to be seen on 4Kids TV (Fox affiliates, nationwide), Jetix, The CW4Kid ...
manner. Computer simulations can be used to predict its position for several million years (both forward and backward in time), but after intervals much longer than the
Lyapunov time In mathematics, the Lyapunov time is the characteristic timescale on which a dynamical system is chaotic. It is named after the Russian mathematician Aleksandr Lyapunov. It is defined as the inverse of a system's largest Lyapunov exponent. Use T ...
of 10–20 million years, calculations become unreliable: Pluto is sensitive to immeasurably small details of the Solar System, hard-to-predict factors that will gradually change Pluto's position in its orbit. The
semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...
of Pluto's orbit varies between about 39.3 and 39.6  au with a period of about 19,951 years, corresponding to an orbital period varying between 246 and 249 years. The semi-major axis and period are presently getting longer.


Relationship with Neptune

Despite Pluto's orbit appearing to cross that of Neptune when viewed from directly above, the two objects' orbits do not intersect. When Pluto is closest to the Sun, and close to Neptune's orbit as viewed from above, it is also the farthest above Neptune's path. Pluto's orbit passes about 8 AU above that of Neptune, preventing a collision. This alone is not enough to protect Pluto; perturbations from the planets (especially Neptune) could alter Pluto's orbit (such as its orbital precession) over millions of years so that a collision could be possible. However, Pluto is also protected by its 2:3
orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
with
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
: for every two orbits that Pluto makes around the Sun, Neptune makes three. Each cycle lasts about 495 years. (There are many other objects in this same resonance, called
plutino In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the lar ...
s.) This pattern is such that, in each 495-year cycle, the first time Pluto is near
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
, 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 nearly 130° ahead of Pluto. Pluto and Neptune's minimum separation is over 17 AU, which is greater than Pluto's minimum separation from Uranus (11 AU). The minimum separation between Pluto and Neptune actually occurs near the time of Pluto's aphelion. The 2:3 resonance between the two bodies is highly stable and has been preserved over millions of years. This prevents their orbits from changing relative to one another, and so the two bodies can never pass near each other. Even if Pluto's orbit were not inclined, the two bodies could never collide. The long term stability of the mean-motion resonance is due to phase protection. When Pluto's period is slightly shorter than 3/2 of Neptune, its orbit relative to Neptune will drift, causing it to make closer approaches behind Neptune's orbit. The gravitational pull between the two then causes
angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly larger orbit, where it travels slightly more slowly, according to
Kepler's third law In astronomy, Kepler's laws of planetary motion, published by Johannes Kepler between 1609 and 1619, describe the orbits of planets around the Sun. The laws modified the heliocentric theory of Nicolaus Copernicus, replacing its circular orbits ...
. After many such repetitions, Pluto is sufficiently slowed that Pluto's orbit relative to Neptune drifts in the opposite direction until the process is reversed. The whole process takes about 20,000 years to complete.


Other factors

Numerical studies have shown that over millions of years, the general nature of the alignment between the orbits of Pluto and Neptune does not change. There are several other resonances and interactions that enhance Pluto's stability. These arise principally from two additional mechanisms (besides the 2:3 mean-motion resonance). First, Pluto's
argument of perihelion The argument of periapsis (also called argument of perifocus or argument of pericenter), symbolized as ''ω'', is one of the orbital elements of an orbiting body. Parametrically, ''ω'' is the angle from the body's ascending node to its periapsi ...
, the angle between the point where it crosses the ecliptic and the point where it is closest to the Sun, librates around 90°. This means that when Pluto is closest to the Sun, it is at its farthest above the plane of the Solar System, preventing encounters with Neptune. This is a consequence of the
Kozai mechanism In celestial mechanics, the Kozai mechanism is a dynamical phenomenon affecting the orbit of a binary system perturbed by a distant third body under certain conditions. It is also known as the von Zeipel-Kozai-Lidov, Lidov–Kozai mechanism, Kozai ...
, 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° . 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 hence it comes closest to the Sun when it is highest above Neptune's orbit. This is known as the ''1:1 superresonance''. All the
Jovian planets The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. Ther ...
, particularly Jupiter, play a role in the creation of the superresonance.


Quasi-satellite

In 2012, it was hypothesized that
15810 Arawn 15810 Arawn, provisional designation , is a trans-Neptunian object (TNO) from the inner regions of the Kuiper belt, approximately in diameter. It belongs to the plutinos, the largest class of resonant TNOs. It was named after Arawn, the ruler of ...
could be a
quasi-satellite A quasi-satellite is an object in a specific type of co-orbital configuration (1:1 orbital resonance) with a planet (or dwarf planet) where the object stays close to that planet over many orbital periods. A quasi-satellite's orbit around the Sun t ...
of Pluto, a specific type of co-orbital configuration. According to the hypothesis, the object would be a quasi-satellite of Pluto for about 350,000 years out of every two-million-year period. Measurements made by the ''New Horizons'' spacecraft in 2015 made it possible to calculate the orbit of Arawn more accurately. These calculations confirm the overall dynamics described in the hypothesis. However, it is not agreed upon among astronomers whether Arawn should be classified as a quasi-satellite of Pluto based on this motion, since its orbit is primarily controlled by Neptune with only occasional smaller perturbations caused by Pluto.


Rotation

Pluto's
rotation period The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) may refer to its sidereal rotation period, i.e. the time that the object takes to complete a single revolution around its axis of rotation relative to the ...
, its day, is equal to 6.387
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
days. Like
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
, Pluto rotates on its "side" in its orbital plane, with an axial tilt of 120°, and so its seasonal variation is extreme; at its
solstice A solstice is an event that occurs when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. In many countr ...
s, one-fourth of its surface is in continuous daylight, whereas another fourth is in continuous darkness. The reason for this unusual orientation has been debated. Research from the
University of Arizona The University of Arizona (Arizona, U of A, UArizona, or UA) is a public land-grant research university in Tucson, Arizona. Founded in 1885 by the 13th Arizona Territorial Legislature, it was the first university in the Arizona Territory. T ...
has suggested that it may be due to the way that a body's spin will always adjust to minimise energy. This could mean a body reorienting itself to put extraneous mass near the equator and regions lacking mass tend towards the poles. This is called ''
polar wander Polar wander is the motion of a pole in relation to some reference frame. It can be used, for example, to measure the degree to which Earth's magnetic poles have been observed to move relative to the Earth's rotation axis. It is also possible to use ...
''. According to a paper released from the University of Arizona, this could be caused by masses of frozen nitrogen building up in shadowed areas of the dwarf planet. These masses would cause the body to reorient itself, leading to its unusual axial tilt of 120°. The buildup of nitrogen is due to Pluto's vast distance from the Sun. At the equator, temperatures can drop to , causing nitrogen to freeze as water would freeze on Earth. The same effect seen on Pluto would be observed on Earth were the
Antarctic ice sheet The Antarctic ice sheet is one of the two polar ice caps of Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth, with an average thickness of over 2 kilometers. It covers an area of almost and ...
is several times larger.


Geology


Surface

The plains on Pluto's surface are composed of more than 98 percent
nitrogen ice Solid nitrogen is a number of solid forms of the element nitrogen, first observed in 1884. Solid nitrogen is mainly the subject of academic research, but low-temperature, low-pressure solid nitrogen is a substantial component of bodies in the ou ...
, with traces of methane and
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
.
Nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and carbon monoxide are most abundant on the anti-Charon face of Pluto (around 180° longitude, where
Tombaugh Regio Tombaugh Regio () is the largest bright surface feature of the dwarf planet Pluto. It is just north of the equator, to the northeast of Cthulhu Macula and to the northwest of Krun Macula, which are both dark features. Its western lobe, a -wide p ...
's western lobe,
Sputnik Planitia Sputnik Planitia , originally Sputnik Planum, is a high-albedo ice-covered basin on Pluto, about in size, named after Earth's first artificial satellite, Sputnik 1. It constitutes the western lobe of the heart-shaped Tombaugh Regio. Sputnik Pla ...
, is located), whereas methane is most abundant near 300° east. The mountains are made of water ice. Pluto's surface is quite varied, with large differences in both brightness and color. Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
's moon
Iapetus In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other ...
. The color varies from charcoal black, to dark orange and white. Pluto's color is more similar to that of Io with slightly more orange and significantly less red than
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
. Notable geographical features include Tombaugh Regio, or the "Heart" (a large bright area on the side opposite Charon),
Cthulhu Macula Cthulhu Macula (formerly ''Cthulhu Regio'') is a prominent surface feature of the dwarf planet Pluto, that is reminiscent of a whale in shape. It is an elongated dark region along Pluto's equator, long and one of the darkest features on Pluto. ...
, or the "Whale" (a large dark area on the trailing hemisphere), and the "
Brass Knuckles Brass knuckles (variously referred to as knuckles, knucks, brass knucks, knucklebusters, knuckledusters, knuckle daggers, English punch, iron fist, paperweight, or a classic) are "fist-load weapons" used in hand-to-hand combat. Brass knuckles ...
" (a series of equatorial dark areas on the leading hemisphere). Sputnik Planitia, the western lobe of the "Heart", is a 1,000 km-wide basin of frozen nitrogen and carbon monoxide ices, divided into polygonal cells, which are interpreted as
convection cell In the field of fluid dynamics, a convection cell is the phenomenon that occurs when density differences exist within a body of liquid or gas. These density differences result in rising and/or falling currents, which are the key characteristics ...
s that carry floating blocks of water ice crust and sublimation pits towards their margins; there are obvious signs of glacial flows both into and out of the basin. It has no craters that were visible to ''New Horizons'', indicating that its surface is less than 10 million years old. Latest studies have shown that the surface has an age of years. The New Horizons science team summarized initial findings as "Pluto displays a surprisingly wide variety of geological landforms, including those resulting from glaciological and surface–atmosphere interactions as well as impact,
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...
, possible
cryovolcanic A cryovolcano (sometimes informally called an ice volcano) is a type of volcano that erupts volatiles such as water, ammonia or methane into an extremely cold environment that is at or below their freezing point. The process of formation is known ...
, and mass-wasting processes." In Western parts of Sputnik Planitia there are fields of
transverse dunes A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, fl ...
formed by the winds blowing from the center of Sputnik Planitia in the direction of surrounding mountains. The dune wavelengths are in the range of 0.4–1 km and they are likely consists of methane particles 200–300 μm in size.


Internal structure

Pluto's density is . Because the decay of radioactive elements 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 Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the central ...
surrounded by a mantle of water ice. The pre–''New Horizons'' estimate for the diameter of the core is , 70% of Pluto's diameter. Pluto has no magnetic field. It is possible that such heating continues today, creating a
subsurface ocean Planetary oceanography also called exo-oceanography is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry and planetary geology, it only began after the discovery of undergr ...
of liquid water thick at the core–mantle boundary.Overlooked Ocean Worlds Fill the Outer Solar System
John Wenz, ''Scientific American''. October 4, 2017.
In September 2016, scientists at
Brown University Brown University is a private research university in Providence, Rhode Island. Brown is the seventh-oldest institution of higher education in the United States, founded in 1764 as the College in the English Colony of Rhode Island and Providenc ...
simulated the impact thought to have formed
Sputnik Planitia Sputnik Planitia , originally Sputnik Planum, is a high-albedo ice-covered basin on Pluto, about in size, named after Earth's first artificial satellite, Sputnik 1. It constitutes the western lobe of the heart-shaped Tombaugh Regio. Sputnik Pla ...
, and showed that it might have been the result of liquid water upwelling from below after the collision, implying the existence of a subsurface ocean at least 100 km deep. In June 2020, astronomers reported evidence that Pluto may have had a
subsurface ocean Planetary oceanography also called exo-oceanography is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry and planetary geology, it only began after the discovery of undergr ...
, and consequently may have been
habitable Habitability refers to the adequacy of an environment for human living. Where housing is concerned, there are generally local ordinances which define habitability. If a residence complies with those laws it is said to be habitable. In extreme e ...
, when it was first formed. In March 2022, they concluded that peaks on Pluto are actually a merger of "ice volcanoes", suggesting a source of heat on the body at levels previously thought not possible.


Mass and size

Pluto's diameter is and its mass is , 17.7% that of the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
(0.22% that of Earth). Its
surface area The surface area of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of arc ...
is , or roughly the same surface area as Russia. Its
surface gravity The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experien ...
is 0.063 ''g'' (compared to 1 ''g'' for Earth and 0.17 ''g'' for the Moon). The discovery of Pluto's satellite
Charon In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the wo ...
in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Newton's formulation of Kepler's third law. Observations of Pluto in
occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
with Charon allowed scientists to establish Pluto's diameter more accurately, whereas the invention of
adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
allowed them to determine its shape more accurately. With less than 0.2 lunar masses, Pluto is much less massive than the
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 accepted by the IAU are the inner planets closest to the Sun: Mercury, Ve ...
s, and also less massive than seven
moons A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
: Ganymede, Titan,
Callisto Callisto most commonly refers to: *Callisto (mythology), a nymph *Callisto (moon), a moon of Jupiter Callisto may also refer to: Art and entertainment *''Callisto series'', a sequence of novels by Lin Carter *''Callisto'', a novel by Torsten Kro ...
, Io, the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
,
Europa Europa may refer to: Places * Europe * Europa (Roman province), a province within the Diocese of Thrace * Europa (Seville Metro), Seville, Spain; a station on the Seville Metro * Europa City, Paris, France; a planned development * Europa Cliff ...
, and Triton. The mass is much less than thought before Charon was discovered. Pluto is more than twice the diameter and a dozen times the mass of
Ceres Ceres most commonly refers to: * Ceres (dwarf planet), the largest asteroid * Ceres (mythology), the Roman goddess of agriculture Ceres may also refer to: Places Brazil * Ceres, Goiás, Brazil * Ceres Microregion, in north-central Goiás ...
, the largest object in the
asteroid belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
. It is less massive than the dwarf planet Eris, a
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically, ...
discovered in 2005, though Pluto has a larger diameter of 2376.6 km compared to Eris's approximate diameter of 2326 km. Determinations of Pluto's size have been complicated by its atmosphere and
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
haze. In March 2014, Lellouch, de Bergh et al. published findings regarding methane mixing ratios in Pluto's atmosphere consistent with a Plutonian diameter greater than 2360 km, with a "best guess" of 2368 km. On July 13, 2015, images from NASA's ''New Horizons'' mission
Long Range Reconnaissance Imager Long Range Reconnaissance Imager (LORRI) is a telescope aboard the ''New Horizons'' spacecraft for imaging. LORRI has been used to image Jupiter, its moons, Pluto and its moons, and Arrokoth since its launch in 2006. LORRI is a reflecting telescope ...
(LORRI), along with data from the other instruments, determined Pluto's diameter to be , which was later revised to be on July 24, and later to . Using
radio occultation Radio occultation (RO) is a remote sensing technique used for measuring the physical properties of a planetary atmosphere or ring system. Atmospheric radio occultation Atmospheric radio occultation relies on the detection of a change in a radio ...
data from the ''New Horizons'' Radio Science Experiment (REX), the diameter was found to be .


Atmosphere

Pluto has a tenuous
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
consisting of
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
(N2),
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
(CH4), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto's surface. According to the measurements by ''New Horizons'', the surface pressure is about 1  Pa (10  μbar), roughly one million to 100,000 times less than Earth's atmospheric pressure. It was initially thought that, as Pluto moves away from the Sun, its atmosphere should gradually freeze onto the surface; studies of ''New Horizons'' data and ground-based occultations show that Pluto's atmospheric density increases, and that it likely remains gaseous throughout Pluto's orbit. ''New Horizons'' observations showed that atmospheric escape of nitrogen to be 10,000 times less than expected. Alan Stern has contended that even a small increase in Pluto's surface temperature can lead to exponential increases in Pluto's atmospheric density; from 18 hPa to as much as 280 hPa (three times that of Mars to a quarter that of the Earth). At such densities, nitrogen could flow across the surface as liquid. Just like sweat cools the body as it evaporates from the skin, the sublimation of Pluto's atmosphere cools its surface. Pluto has no or almost no
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
; observations by ''New Horizons'' suggest only a thin tropospheric
boundary layer In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary condi ...
. Its thickness in the place of measurement was 4 km, and the temperature was 37±3 K. The layer is not continuous.
Supplementary Material
In July 2019, an occultation by Pluto showed that its atmospheric pressure, against expectations, had fallen by 20% since 2016. In 2021, astronomers at the
Southwest Research Institute Southwest Research Institute (SwRI), headquartered in San Antonio, Texas, is an independent and nonprofit applied research and development (R&D) organization. Founded in 1947 by oil businessman Tom Slick, it provides contract research and develop ...
confirmed the result using data from an occultation in 2018, which showed that light was appearing less gradually from behind Pluto's disc, indicating a thinning atmosphere. The presence of methane, a powerful
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
, in Pluto's atmosphere creates a
temperature inversion In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to an inversion of the air temperature lapse rate, in which case it is called a temperature inversion. No ...
, with the average temperature of its atmosphere tens of degrees warmer than its surface, though observations by ''New Horizons'' have revealed Pluto's upper atmosphere to be far colder than expected (70 K, as opposed to about 100 K). Pluto's atmosphere is divided into roughly 20 regularly spaced haze layers up to 150 km high, thought to be the result of pressure waves created by airflow across Pluto's mountains.


Satellites

Pluto has five known
natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
s. The closest to Pluto is
Charon In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the wo ...
. First identified in 1978 by astronomer James Christy, Charon is the only moon of Pluto that may be in
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ...
. Charon's mass is sufficient to cause the barycenter of the Pluto–Charon system to be outside Pluto. Beyond Charon there are four much smaller circumbinary moons. In order of distance from Pluto they are Styx, Nix, Kerberos, and Hydra.
Nix Nix or NIX may refer to: Places * Nix, Alabama, an unincorporated community, United States * Nix, Texas, a ghost town in southwestern Lampasas County, Texas, United States * Nix (moon), a moon of Pluto People * Nix (surname), listing people with ...
and
Hydra Hydra generally refers to: * Lernaean Hydra, a many-headed serpent in Greek mythology * ''Hydra'' (genus), a genus of simple freshwater animals belonging to the phylum Cnidaria Hydra or The Hydra may also refer to: Astronomy * Hydra (constel ...
were both discovered in 2005, Kerberos was discovered in 2011, and
Styx In Greek mythology, Styx (; grc, Στύξ ) is a river that forms the boundary between Earth (Gaia) and the Underworld. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the centre of the underworld on a great marsh, whic ...
was discovered in 2012. The satellites' orbits are circular (eccentricity < 0.006) and coplanar with Pluto's equator (inclination < 1°), and therefore tilted approximately 120° relative to Pluto's orbit. The Plutonian system is highly compact: the five known satellites orbit within the inner 3% of the region where
prograde orbit Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession or ...
s would be stable. The orbital periods of all Pluto's moons are linked in a system of
orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
s and near resonances. When
precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In othe ...
is accounted for, the orbital periods of Styx, Nix, and Hydra are in an exact 18:22:33 ratio. There is a sequence of approximate ratios, 3:4:5:6, between the periods of Styx, Nix, Kerberos, and Hydra with that of Charon; the ratios become closer to being exact the further out the moons are. The Pluto–Charon system is one of the few in the Solar System whose barycenter lies outside the primary body; the Patroclus–Menoetius system is a smaller example, and the Sun–Jupiter system is the only larger one. The similarity in size of Charon and Pluto has prompted some astronomers to call it a double dwarf planet. The system is also unusual among planetary systems in that each is
tidally locked Tidal locking between a pair of co-orbiting astronomical body, astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where ...
to the other, which means that Pluto and Charon always have the same hemisphere facing each other — a property shared by only one other known system, Eris and Dysnomia. From any position on either body, the other is always at the same position in the sky, or always obscured. This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its barycenter. In 2007, observations by the
Gemini Observatory The Gemini Observatory is an astronomical observatory consisting of two 8.1-metre (26.6 ft) telescopes, Gemini North and Gemini South, which are located at two separate sites in Hawaii and Chile, respectively. The twin Gemini telescopes prov ...
of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers. Pluto's moons are hypothesized to have been formed by a collision between Pluto and a similar-sized body, early in the history of the Solar System. The collision released material that consolidated into the moons around Pluto. Pluto moon P5 discovery with moons' orbits.jpg, The Pluto system: Pluto, Charon,
Styx In Greek mythology, Styx (; grc, Στύξ ) is a river that forms the boundary between Earth (Gaia) and the Underworld. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the centre of the underworld on a great marsh, whic ...
,
Nix Nix or NIX may refer to: Places * Nix, Alabama, an unincorporated community, United States * Nix, Texas, a ghost town in southwestern Lampasas County, Texas, United States * Nix (moon), a moon of Pluto People * Nix (surname), listing people with ...
, Kerberos, and
Hydra Hydra generally refers to: * Lernaean Hydra, a many-headed serpent in Greek mythology * ''Hydra'' (genus), a genus of simple freshwater animals belonging to the phylum Cnidaria Hydra or The Hydra may also refer to: Astronomy * Hydra (constel ...
, imaged by the Hubble Space Telescope in July 2012. Pluto charon 150709 color final.png, Pluto and Charon, to scale. Image acquired by ''New Horizons'' on July 8, 2015. Nh-pluto moons family portrait.png, Family portrait of the five moons of Pluto, to scale. Charon in True Color - High-Res.jpg, Pluto's moon Charon as viewed by ''New Horizons'' on July 13, 2015


Origin

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 Neptune's largest current moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit. Pluto's true place in the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
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 thought to be the source of many
short-period comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are ...
s. Pluto is now known to be the largest member of the
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
, a stable belt of objects located between 30 and 50 AU from the Sun. As of 2011, surveys of the Kuiper belt to magnitude 21 were nearly complete and any remaining Pluto-sized objects are expected to be beyond 100 AU from the Sun. Like other Kuiper-belt objects (KBOs), Pluto shares features with
comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
s; for example, the
solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
is gradually blowing Pluto's surface into space. It has been claimed that if Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do. This claim has been disputed with the argument that Pluto's escape velocity is too high for this to happen. It has been proposed that Pluto may have formed as a result of the agglomeration of numerous comets and Kuiper-belt objects. Though Pluto is the largest Kuiper belt object discovered, Neptune's moon
Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus' ...
, which is larger than Pluto, is similar to it both geologically and atmospherically, and is thought to be a captured Kuiper belt object. Eris ( see above) is about the same size as Pluto (though more massive) 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 The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objec ...
. Many Kuiper belt objects, like Pluto, are in a 2:3 orbital resonance with Neptune. KBOs with this orbital resonance are called "
plutino In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the lar ...
s", after Pluto. Like other members of the Kuiper belt, Pluto is thought to be a residual
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...
; a component of the original
protoplanetary disc A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, be ...
around the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto owes its current position to a sudden migration 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 (Triton), locking others into resonances, and knocking others into chaotic orbits. The objects in the
scattered disc The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objec ...
, a dynamically unstable region overlapping the Kuiper belt, are thought to have been placed in their current positions by interactions with Neptune's migrating resonances. A computer model created in 2004 by Alessandro Morbidelli of the Observatoire de la Côte d'Azur in
Nice Nice ( , ; Niçard: , classical norm, or , nonstandard, ; it, Nizza ; lij, Nissa; grc, Νίκαια; la, Nicaea) is the prefecture of the Alpes-Maritimes department in France. The Nice agglomeration extends far beyond the administrative c ...
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 The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypoth ...
600 million years after the Solar System's formation and the origin of the
Jupiter trojan The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange poin ...
s. It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune's migration perturbed it into a resonant capture. The
Nice model The Nice () model is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Côte d'Azur Observatory—where it was initially developed in 2005—in Nice, France. It proposes the migration of the giant p ...
requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk, which included Triton and Eris.


Observation and exploration

Pluto's distance from Earth makes its in-depth study and
exploration Exploration refers to the historical practice of discovering remote lands. It is studied by geographers and historians. Two major eras of exploration occurred in human history: one of convergence, and one of divergence. The first, covering most ...
difficult. On July 14, 2015, NASA's ''
New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...
'' space probe flew through the Pluto system, providing much information about it.


Observation

Pluto's visual
apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
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 The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is ...
is maximum 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. Better maps were produced from images taken by the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
(HST), which offered higher
resolution Resolution(s) may refer to: Common meanings * Resolution (debate), the statement which is debated in policy debate * Resolution (law), a written motion adopted by a deliberative body * New Year's resolution, a commitment that an individual mak ...
, and showed considerably more detail, resolving variations several hundred kilometers across, including polar regions and large bright spots. These maps were produced by complex computer processing, which finds the best-fit projected maps for the few pixels of the Hubble images. These remained the most detailed maps of Pluto until the flyby of ''New Horizons'' in July 2015, because the two cameras on the HST used for these maps were no longer in service.


Exploration

The ''New Horizons'' spacecraft, which flew by Pluto in July 2015, is the first and so far only attempt to explore Pluto directly. Launched in 2006, it captured its first (distant) images of Pluto in late September 2006 during a test of the Long Range Reconnaissance Imager. The images, taken from a distance of approximately 4.2 billion kilometers, confirmed the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper belt objects. In early 2007 the craft made use of a
gravity assist In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the p ...
from
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
. ''New Horizons'' made its closest approach to Pluto on July 14, 2015, after a 3,462-day journey across the Solar System. Scientific observations of Pluto began five months before the closest approach and continued for at least a month after the encounter. Observations were conducted using a
remote sensing Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
package that included
imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of an image). Imaging technology is the application of materials and methods to create, preserve, or duplicate images. ...
instruments and a radio science investigation tool, as well as
spectroscopic Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wav ...
and other experiments. The scientific goals of ''New Horizons'' were to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition, and analyze Pluto's neutral atmosphere and its escape rate. On October 25, 2016, at 05:48 pm ET, the last bit of data (of a total of 50 billion bits of data; or 6.25 gigabytes) was received from ''New Horizons'' from its close encounter with Pluto. Since the ''New Horizons'' flyby, scientists have advocated for an orbiter mission that would return to Pluto to fulfill new science objectives. They include mapping the surface at per pixel, observations of Pluto's smaller satellites, observations of how Pluto changes as it rotates on its axis, investigations of a possible subsurface ocean, and topographic mapping of Pluto's regions that are covered in long-term darkness due to its axial tilt. The last objective could be accomplished using laser pulses to generate a complete topographic map of Pluto. ''New Horizons'' principal investigator Alan Stern has advocated for a ''Cassini''-style orbiter that would launch around 2030 (the 100th anniversary of Pluto's discovery) and use Charon's gravity to adjust its orbit as needed to fulfill science objectives after arriving at the Pluto system. The orbiter could then use Charon's gravity to leave the Pluto system and study more KBOs after all Pluto science objectives are completed. A conceptual study funded by the NASA Innovative Advanced Concepts ( NIAC) program describes a fusion-enabled Pluto orbiter and lander based on the Princeton field-reversed configuration reactor. NASA-Pluto-NotingUnseenFarSide-2019-20150714.png, A composite image of the sub-Charon hemisphere of Pluto. The region inside/below the white line was on the far side of Pluto when ''New Horizons'' made its closest approach, and was only imaged (at lower resolution) in the early days of the flyby. Black regions were not imaged at all. Pluto-FarSideView-2019.png, The low-resolution area, with named features labeled Pluto-FarSideFeatures-2019.png, The low-resolution area, with features classified by geological type Sources:


Southern hemisphere

''New Horizons'' imaged all of Pluto's northern hemisphere, and the equatorial regions down to about 30° South. Higher southern latitudes have only been observed, at very low resolution, from Earth. Images from the Hubble Space Telescope in 1996 cover 85% of Pluto and show large albedo features down to about 75° South. This is enough to show the extent of the temperate-zone maculae. Later images had slightly better resolution, due to minor improvements in Hubble instrumentation. Some albedo variations in the higher southern latitudes could be detected by ''New Horizons'' using Charon-shine (light reflected off Charon). The south polar region seems to be darker than the north polar region, but there is a high-albedo region in the southern hemisphere that may be a regional nitrogen or methane ice deposit. File:Pluto map (Hubble).jpg, A map of Pluto based on Hubble images from 1996, centered on the anti-Charon hemisphere (Sputnik Planitia), covering the southern hemisphere down to 75°S File:Pluto southern hemisphere psjac2743f10 hr.jpg, (a) Synthesized HST map of Pluto from Buie et al. (2010).
(b) Colorized New Horizons MVIC and LORRI mosaic.
(c–d) Destreaked Charon-illuminated image stack, shown to approximately the same stretch. The green line is the limit of the sub-Charon hemisphere.


Videos


See also

* ''
How I Killed Pluto and Why It Had It Coming ''How I Killed Pluto and Why It Had It Coming'' is the 2010 memoir by Mike Brown, the American astronomer most responsible for the reclassification of Pluto from planet to dwarf planet. Summary The memoir is an account of the events surround ...
'' *
List of geological features on Pluto This is a list of named geological features on Pluto, identified by scientists working with data from the '' New Horizons'' spacecraft. The International Astronomical Union (IAU) officially approved the first 14 names on 8 August 2017 (announced ...
* Pluto in astrology *
Pluto in fiction Pluto was discovered in 1930 and has made several appearances in fiction since. It was initially popular as it was newly discovered and thought to be the outermost object of the Solar System. Alien life, sometimes intelligent life and occasionally ...
* Stats of planets in the Solar System


Notes


References


Further reading

* Codex Regius (2016), ''Pluto & Charon'', CreateSpace Independent Publishing Platform * Stern, S A and Tholen, D J (1997), ''Pluto and Charon'', University of Arizona Press * *


External links


''New Horizons'' homepage

Pluto Profile
a
NASA's Solar System Exploration site



Website of the observatory that discovered Pluto

Earth telescope image of Pluto system


*
Video – Pluto – viewed through the years (GIF)
(NASA; animation; July 15, 2015).
Video – Pluto – "FlyThrough" (00:22; MP4)(YouTube)
(NASA; animation; August 31, 2015).
"A Day on Pluto Video made from July 2015 New Horizon Images"
Scientific American ''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many famous scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it. In print since 1845, it i ...
* NASA CG
video
of Pluto flyover (July 14, 2017)
CGI video
simulation of rotating Pluto by Seán Doran (se
album
for more)
Google Pluto 3D
interactive map of the dwarf planet * {{Featured article Articles containing video clips 19300218 Discoveries by Clyde Tombaugh 134340 Minor planets visited by spacecraft
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
134340 Plutinos 134340 Dwarf planets