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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 (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet). Such designation always features a le ...
: 134340 Pluto) is a
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something 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 d ...
in the
Kuiper belt The Kuiper belt () is a circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments in orbit ar ...
, a ring of bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered. After Pluto was discovered in 1930, it was declared to be the ninth planet from the Sun. Beginning in the 1990s, its status as a
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
was questioned following the discovery of several objects of similar size in the Kuiper belt and the
scattered disc The scattered disc (or scattered disk) is a distant circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or colli ...
, including the dwarf planet Eris. This led the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
(IAU) in 2006 to formally define the term ''planet''''—''excluding Pluto and reclassifying it as a dwarf planet. Pluto is the ninth-largest and tenth-most-
mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value ...
ive known object directly orbiting the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
. It is the largest known trans-Neptunian object by volume but is less massive than Eris. Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—one-sixth the mass of the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
and one-third its volume. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric system, metri ...

astronomical unit
s or AU (4.4–7.4 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun than
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
, but a stable
orbital resonance . Conjunctions ''Conjunctions'' is a biannual American literature, American literary journal based at Bard College. It was founded in 1981 and is currently edited by Bradford Morrow. Morrow received the PEN/Nora Magid Award for Magazine Editing ...
with Neptune prevents them from colliding. Light from the Sun takes 5.5 hours to reach Pluto at its average distance (39.5 AU). Pluto has
five known moons
five known moons
:
Charon In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. M ...

Charon
(the largest, with a diameter just over half that of Pluto),
Styx In Greek mythology, Styx (; grc, Στύξ ) is a deity and a river that forms the boundary between Gaia, Earth (Gaia) and the Greek underworld, Underworld. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the centre of the ...

Styx
, Nix,
Kerberos
Kerberos
, and
Hydra
Hydra
. 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 Barycenter#Gallery, animated examples)''. More restrictive definiti ...
because the
barycenter In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ...
of their orbits does not lie within either body. 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 ...

New Horizons
'' spacecraft performed a flyby of Pluto on July 14, 2015, becoming the first and, to date, only spacecraft to do so. During its brief flyby, ''New Horizons'' made detailed measurements and observations of Pluto and its moons. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of
tholin Tholins (after the Greek (') "hazy" or "muddy"; from the ancient Greek Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: (), Dark Ages () ...
s,
organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or ...
macromolecule macromolecule A macromolecule is a very large molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neu ...
s that may be ingredients for the emergence of life, and produced from
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
,
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
and other gases released from the
atmosphere of Pluto The atmosphere of Pluto is the tenuous layer of gases surrounding Pluto. It consists mainly of nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. It was first discovered and isolated by Scott ...
and transferred to the orbiting moon.


History


Discovery

In the 1840s,
Urbain Le Verrier Urbain Jean Joseph Le Verrier FRS (FOR) HFRSE Fellowship of the Royal Society of Edinburgh (FRSE) is an award granted to individuals that the Royal Society of Edinburgh, Scotland's national academy of science and Literature, letters, judged ...

Urbain Le Verrier
used
Newtonian mechanics Newton's laws of motion are three Scientific law, 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: ''Law 1''. A body continues ...
to predict the position of the then-undiscovered planet
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
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, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
. 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 Lawrence Lowell (; March 13, 1855 – November 12, 1916) was an American businessman, author, mathematician, and astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or fie ...

Percival Lowell
—a wealthy Bostonian who had founded
Lowell Observatory Lowell Observatory is an astronomical observatory in Flagstaff, Arizona Flagstaff ( ; nv, Kinłání Dookʼoʼoosłííd Biyaagi, ) is a city in, and the county seat of, Coconino County, Arizona, Coconino County in northern Arizona, in the s ...

Lowell Observatory
in
Flagstaff, Arizona Flagstaff ( ) is a city in, and the county seat of, Coconino County, Arizona, Coconino County in Northern Arizona, northern Arizona, in the southwestern United States. In 2019, the city's estimated population was 75,038. Flagstaff's combined metr ...
, 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 ...

Planet X
". By 1909, Lowell and 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
precoveryIn astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathem ...
observations, with the earliest made by the
Yerkes Observatory Yerkes Observatory ( ) is an Observatory#Astronomical observatories, astronomical observatory located in Williams Bay, Wisconsin, U.S.A. It was operated by the University of Chicago Department of Astronomy and Astrophysics from its founding in ...

Yerkes Observatory
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 co ...
, 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 Blinking is a bodily function; it is a semi-autonomic rapid closing of the eyelid An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outsi ...
, 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 Harvard University is a Private university, private Ivy League research ...
on March 13, 1930. Pluto has yet to complete a full orbit of the Sun since its discovery, as one Plutonian year is 247.68 years long.


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 In grammar In linguistics Linguistics is the scientific study of language, meaning that it is a comprehensive, systematic, objective, and precise study of language. Ling ...

Zeus
'', 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), as an English girl of 11 years old, was credited by Clyde Tombaugh with first suggesting the name Pluto for the planet he discovered in 1930. She was living in ...
(1918–2009), an eleven-year-old schoolgirl in
Oxford Oxford () is a city in England. It is the county town In the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' u ...

Oxford
, England, who was interested in
classical mythology Classical mythology, classical Greco-Roman mythology, Greek and Roman mythology or Greco-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 reception theory, cu ...
. She suggested it in a conversation with her grandfather
Falconer Madan Falconer Madan (15 April 1851 – 22 May 1935) was Librarian A librarian is a person who works professionally in a library A library is a curated collection of sources of information and similar resources, made accessible to a defined co ...

Falconer Madan
, a former librarian at the
University of Oxford The University of Oxford is a collegiate university, collegiate research university in Oxford, England. There is evidence of teaching as early as 1096, making it the oldest university in the English-speaking world and the List of oldest universit ...
's
Bodleian Library The Bodleian Library () is the main research library A research library is a library A library is a collection of materials, books or media that are easily accessible for use and not just for display purposes. It is responsible for housi ...

Bodleian Library
, who passed the name to astronomy professor
Herbert Hall Turner Herbert Hall Turner FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Res ...
, 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 Roman mythology is the body of of as represented in the and . One of a wide variety of genres of , ''Roman mythology'' may also refer to the modern study of these representations, and to ...

Minerva
(which was already the name for an asteroid),
Cronus In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practice ...
(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 An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. ...
), and Pluto. Pluto received a unanimous vote. The name was published on May 1, 1930. Upon the announcement, Madan gave Venetia £5 (equivalent to
GBP The pound sterling (symbol: £; ISO code: GBP), known in some contexts simply as the pound or sterling, is the official currency A currency, "in circulation", from la, currens, -entis, literally meaning "running" or "traversing" in the ...

GBP
, or 450
USD The United States dollar (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, ...

USD
in 2014) 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
astronomical symbol Astronomical symbols are abstract pictorial symbols used to represent astronomical objects, theoretical constructs and observational events in Western culture, European astronomy. The earliest forms of these symbols appear in Greek papyrus tex ...
Unicode Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

Unicode
U+2647: ♇) was then created as a
monogram A monogram or wenzel ( pl, Węzeł, "knot") is a motif Motif may refer to: General concepts * Motif (chess composition), an element of a move in the consideration of its purpose * Motif (folkloristics), a recurring element that creates recogn ...

monogram
constructed from the letters "PL". The most-common
astrological symbol Symbol A symbol is a mark, sign, or word that indicates, signifies, or is understood as representing an idea, Object (philosophy), object, or wikt:relationship, relationship. Symbols allow people to go beyond what is known or seen by creating ...
for Pluto in English-language sources is an orb over Pluto's
bident A bident is a two-pronged implement resembling a . In , the bident is a weapon associated with (), the ruler of the underworld. Likewise, the three-pronged is the implement of his brother (), god of the and , and the lightning bolt, which supe ...
Unicode U+2BD3: ⯓). This symbol was used by NASA after Pluto was reclassified as a dwarf planet. There are various other symbols in European astrological sources, including ⯔ (, U+2BD4), ⯕ (/, U+2BD5, used in Uranian astrology; also used for Pluto's moon
Charon In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. M ...

Charon
), ⯖ (/, U+2BD6, found in various orientations, showing Pluto's orbit cutting across that of Neptune). The name was soon embraced by wider culture. In 1930,
Walt Disney Walter Elias Disney (; December 5, 1901December 15, 1966) was an American entrepreneur, animator, writer, voice actor, and film producer. A pioneer of the American animation industry, he introduced several developments in the production of ...
was apparently inspired by it when he introduced for
Mickey Mouse Mickey Mouse is a cartoon A cartoon is a type of illustration that is typically drawn, sometimes animated, in an unrealistic or semi-realistic style. The specific meaning has evolved over time, but the modern usage usually refers to ei ...
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 was the first and the largest Kuiper belt object to be discovered. After Pluto wa ...
, although
Disney The Walt Disney Company, commonly just Disney (), is an American multinational entertainment and media conglomerate headquartered at the Walt Disney Studios (Burbank), Walt Disney Studios complex in Burbank, California. Disney was originall ...
animator
Ben Sharpsteen Benjamin Sharpsteen (November 4, 1895 – December 20, 1980) was an American film director A film director controls a film A film, also called a movie, motion picture or moving picture, is a work of visual art used to simulate ...
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 Synthesis or synthesize may also refer to: Science Chemistry and biochemistry *Chemical synthesis, the execution of ch ...
named the newly created
element Element may refer to: Science * Chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all ...
plutonium Plutonium is a radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material co ...

plutonium
after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
, which was named after Uranus, and
neptunium Neptunium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elemen ...

neptunium
, which was named after Neptune. Most languages use the name "Pluto" in various transliterations. In Japanese, Houei Nojiri suggested the translation , and this was borrowed into Chinese and Korean. Some
languages of India Languages spoken in India India, officially the Republic of India (: ), is a country in . It is the by area, the country, and the most populous in the world. Bounded by the on the south, the on the southwest, and the on the south ...

languages of India
use the name Pluto, but others, such as
Hindi Hindi (Devanagari: , हिंदी, ISO 15919, ISO: ), or more precisely Modern Standard Hindi (Devanagari: , ISO 15919, ISO: ), is an Indo-Aryan language spoken chiefly in Hindi Belt, North India. Hindi has been described as a Standard la ...

Hindi
, use the name of ''
Yama Yama(यम) (the last a is silent,in hindi)or Yamarāja(यमराज) is a deity of death, dharma, the south direction, and the underworld who predominantly features in deva (Hinduism), Hindu and deva (Buddhism), Buddhist mythology, belongi ...

Yama
'', the God of Death in Hinduism.
Polynesian languages The Polynesian languages form a genealogical group of languages, itself part of the Oceanic Oceanic may refer to: *Of or relating to the ocean *Of or relating to Oceania **Oceanic climate **Oceanic languages **Oceanic person or people, also cal ...

Polynesian languages
also tend to use the indigenous god of the underworld, as in Māori '' Whiro''. 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 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 harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
, 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, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
. In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the
University of Hawaii A university ( la, universitas, 'a whole') is an institution Institutions, according to Samuel P. Huntington, are "stable, valued, recurring patterns of behavior". Institutions can refer to mechanisms which govern the behavior Behavio ...

University of Hawaii
calculated Pluto's
albedo Albedo (prounounced ; la, albedo, meaning 'whiteness') is the measure of the diffuse reflection Diffuse reflection is the reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a comm ...
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 Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. M ...

Charon
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, failed. In 1992,
Myles Standish Myles Standish (c. 1584 – October 3, 1656) was an English military officer. He was hired as military adviser for Plymouth Colony Plymouth Colony (sometimes Plimouth) was an British America, English colonial venture in America from 1620 t ...
used data from ''
Voyager 2 ''Voyager 2'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel thro ...
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 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 A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments in orbit ar ...
. 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 New York City (NYC), often simply called New York, is the List of United States cities by population, most populous city in the United ...

Hayden Planetarium
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) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
, ,
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'' *Jun ...
and Vesta lost their planet status after the discovery of many other
asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

asteroid
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 (Caltech)The university itself only spells its short form as "Caltech"; other spellings such a"Cal Tech" and "CalTech" are incorrect. The Institute is also occasionally referred to as "CIT", most notably i ...
announced the discovery of a new
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet A minor planet is an astronomical object in direct orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, objec ...
, 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'', ...

Triton
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 que ...
, 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 definition of planet, 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 to be considered a planet: # The object must be in orbit around the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
. # 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. # It must have Clearing the neighbourhood, 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 – something 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 d ...
s. In September 2006, the IAU included Pluto, and Eris and its moon Dysnomia (moon), Dysnomia, in their Minor Planet Catalogue, 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 (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet). Such designation always features a le ...
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, principal investigator with NASA'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. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
, 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. Michael E. Brown, 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 facetiously called the IAU decision a "scientific heresy". The New Mexico House of Representatives 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 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 voted ''plutoed'' 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 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" would henceforth be used to refer to Pluto and other planetary-mass objects that have an orbital semi-major axis 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 (mathematics), plane called the ecliptic. In contrast, Pluto's orbit is moderately orbital inclination, inclined relative to the ecliptic (over 17°) and moderately eccentric (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 apsis, perihelion 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 chaos theory, chaotic manner. Computer simulations can be used to predict its position for several million years (both time reversibility, forward and backward in time), but after intervals much longer than the Lyapunov time 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 of Pluto's orbit varies between about 39.3 and 39.6 astronomical unit, 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 astronomical unit, AU above that of Neptune, preventing a collision. This alone is not enough to protect Pluto; perturbation (astronomy), perturbations from the planets (especially Neptune) could alter Pluto's orbit (such as its apsidal precession, orbital precession) over millions of years so that a collision could be possible. However, Pluto is also protected by its 2:3
orbital resonance . Conjunctions ''Conjunctions'' is a biannual American literature, American literary journal based at Bard College. It was founded in 1981 and is currently edited by Bradford Morrow. Morrow received the PEN/Nora Magid Award for Magazine Editing ...
with
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
: 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 plutinos.) This pattern is such that, in each 495-year cycle, the first time Pluto is near perihelion, Neptune is over 50° behind Pluto. By Pluto's second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be 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 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. 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 angle between the point where it crosses the ecliptic and the point where it is closest to the Sun, libration, 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, 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, particularly Jupiter, play a role in the creation of the superresonance.


Quasi-satellite

In 2012, it was hypothesized that 15810 Arawn could be a quasi-satellite 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, its day, is equal to 6.387
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
days. Like
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
, Pluto rotates on its "side" in its orbital plane, with an axial tilt of 120°, and so its seasonal variation is extreme; at its solstices, 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 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''. 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 several times larger.


Geology


Surface

The plains on Pluto's surface are composed of more than 98 percent solid nitrogen, nitrogen ice, with traces of methane and carbon monoxide. Nitrogen and carbon monoxide are most abundant on the anti-Charon face of Pluto (around 180° longitude, where Tombaugh Regio's western lobe, Sputnik Planitia, 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's moon Iapetus (moon), Iapetus. The color varies from charcoal black, to dark orange and white. Pluto's color is more similar to that of Io (moon), 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, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
. Geography of Pluto, Notable geographical features include Tombaugh Regio, or the "Heart" (a large bright area on the side opposite Charon), Cthulhu Regio, Cthulhu Macula, or the "Whale" (a large dark area on the trailing hemisphere), and the "Brass Knuckles (Pluto), 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 cells that carry floating blocks of water ice crust and Sublimation (phase transition), 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 glaciology, glaciological and surface–atmosphere interactions as well as impact, plate tectonics, tectonic, possible cryovolcano, cryovolcanic, and mass wasting, mass-wasting processes." In Western parts of Sputnik Planitia there are fields of transverse dunes 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 (geology), core surrounded by a mantle (geology), 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 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 simulated the impact thought to have formed Sputnik Planitia, 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 Ocean#Extraterrestrial oceans, subsurface ocean, and consequently may have been Planetary habitability, habitable, when it was first formed.


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. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
(0.22% that of Earth). Its surface area is , or roughly the same surface area as Russia. Its surface gravity 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 Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. M ...

Charon
in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Kepler's laws of planetary motion#Third law, Newton's formulation of Kepler's third law. Observations of Pluto in occultation with Charon allowed scientists to establish Pluto's diameter more accurately, whereas the invention of adaptive optics allowed them to determine its shape more accurately. With less than 0.2 lunar masses, Pluto is much less massive than the terrestrial planets, and also less massive than seven natural satellite, moons: Ganymede (moon), Ganymede, Titan (moon), Titan, Callisto (moon), Callisto, Io (moon), Io, the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
, Europa (moon), Europa, 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) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
, the largest object in the asteroid belt. 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 A minor planet is an astronomical object in direct orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, objec ...
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 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 (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 data from the ''New Horizons'' Radio Science Experiment (REX), the diameter was found to be .


Atmosphere

Pluto has a tenuous atmosphere consisting of
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
(N2),
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
(CH4), and carbon monoxide (CO), which are in equilibrium vapor pressure, equilibrium with their ices on Pluto's surface. According to the measurements by ''New Horizons'', the surface pressure is about 1 Pascal (unit), Pa (10 Bar (unit), 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 (phase transition), sublimation of Pluto's atmosphere cools its surface. The presence of atmospheric gases was traced up to 1670 kilometers high; the atmosphere does not have a sharp upper boundary. 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 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, in Pluto's atmosphere creates a Inversion (meteorology), temperature inversion, 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 satellites. The closest to Pluto is
Charon In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. M ...

Charon
. First identified in 1978 by astronomer James W. Christy, James Christy, Charon is the only moon of Pluto that may be in hydrostatic equilibrium. 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 and were both discovered in 2005, was discovered in 2011, and
Styx In Greek mythology, Styx (; grc, Στύξ ) is a deity and a river that forms the boundary between Gaia, Earth (Gaia) and the Greek underworld, Underworld. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the centre of the ...

Styx
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 orbits would be stable. The orbital periods of all Pluto's moons are linked in a system of
orbital resonance . Conjunctions ''Conjunctions'' is a biannual American literature, American literary journal based at Bard College. It was founded in 1981 and is currently edited by Bradford Morrow. Morrow received the PEN/Nora Magid Award for Magazine Editing ...
s and orbital resonance#Coincidental 'near' ratios of mean motion, near resonances. When apsidal precession, precession 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 617 Patroclus, Patroclus–Menoetius system is a smaller example, and the Jupiter#Mass and size, 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 planet, double dwarf planet. The system is also unusual among planetary systems in that each is Tidal locking, tidally locked to the other, which means that Pluto and Charon always have the same hemisphere facing each other. 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 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.


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 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 comets. Pluto is now known to be the largest member of the
Kuiper belt The Kuiper belt () is a circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments in orbit ar ...
, 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 comets; for example, the solar wind 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'', ...

Triton
, which is slightly larger than Pluto, is similar to it both geologically and atmospherically, and is thought to be a captured Kuiper belt object. Eris (#Classification, 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 A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or colli ...
. A large number of Kuiper belt objects, like Pluto, are in a 2:3 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutinos", after Pluto. Like other members of the Kuiper belt, Pluto is thought to be a residual planetesimal; a component of the original protoplanetary disc around the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto owes its current position to a planetary migration, 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 A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or colli ...
, 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 Côte d'Azur Observatory, Observatoire de la Côte d'Azur in Nice 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 600 million years after the Solar System's formation and the origin of the Jupiter trojans. It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune's migration Perturbation (astronomy), perturbed it into a resonant capture. The Nice model 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 Space exploration, exploration 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 ...

New Horizons
'' space probe flew through the Pluto system, providing much information about it.


Observation

Pluto's visual apparent magnitude 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 is only 0.11". The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observations of eclipses by its largest moon, Charon. Observations were made of the change in the total average brightness of the Pluto–Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time. Better maps were produced from images taken by the Hubble Space Telescope (HST), which offered higher angular resolution, resolution, 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 planetary flyby, 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 from Jupiter. ''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 package that included digital imaging, imaging instruments and a radio science investigation tool, as well as spectroscopy, spectroscopic 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–Huygens, ''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 (NASA Institute for Advanced Concepts, NIAC) program describes a fusion-enabled Pluto orbiter and lander based on the Princeton field-reversed configuration experiment, Princeton field-reversed configuration reactor.

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, but didn't reach quite as far south. file:Pluto southern hemisphere psjac2743f10 hr.jpg, upright=1.28, (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'' * List of geological features on Pluto * Planets in astrology#Pluto, Pluto in astrology * Pluto in fiction * List of gravitationally rounded objects of the Solar System#Planets, 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 * 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
Interactive 3D gravity simulation of the Plutonian system
{{Featured article Pluto, Articles containing video clips Astronomical objects discovered in 1930, 19300218 Discoveries by Clyde Tombaugh Discoveries by the Lowell Observatory, 134340 Minor planets named from Roman mythology Minor planets visited by spacecraft Named minor planets, Pluto Objects observed by stellar occultation, 134340 Plutinos Slow rotating minor planets, 134340 Dwarf planets