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Pluto
Pluto
(minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt
Kuiper belt
object to be discovered. Pluto
Pluto
was discovered by Clyde Tombaugh
Clyde Tombaugh
in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, a dwarf planet in the scattered disc which is 27% more massive than Pluto, was discovered. This led the International Astronomical Union
International Astronomical Union
(IAU) to define the term "planet" formally in 2006, during their 26th General Assembly. That definition excluded Pluto
Pluto
and reclassified it as a dwarf planet. Pluto
Pluto
is the largest and second-most-massive known dwarf planet in the Solar System, and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris. Like other Kuiper belt
Kuiper belt
objects, Pluto
Pluto
is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon
Moon
and one-third its volume. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU (4.4–7.4 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun
Sun
than Neptune, but a stable orbital resonance with Neptune
Neptune
prevents them from colliding. Light from the Sun
Sun
takes about 5.5 hours to reach Pluto
Pluto
at its average distance (39.5 AU). Pluto
Pluto
has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Pluto
Pluto
and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body. On July 14, 2015, the New Horizons
New Horizons
spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons
New Horizons
made detailed measurements and observations of Pluto
Pluto
and its moons. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, and produced from methane, nitrogen and other gases released from the atmosphere of Pluto
Pluto
and transferred about 19,000 km (12,000 mi) to the orbiting moon.

Contents

1 History

1.1 Discovery 1.2 Name 1.3 Planet
Planet
X disproved 1.4 Classification

1.4.1 IAU classification

2 Orbit

2.1 Relationship with Neptune

2.1.1 Other factors

2.2 Quasi-satellite

3 Rotation 4 Geology

4.1 Surface 4.2 Internal structure

5 Mass
Mass
and size 6 Atmosphere 7 Satellites 8 Origin 9 Observation and exploration

9.1 Observation 9.2 Exploration 9.3 Videos

10 See also 11 Notes 12 References 13 Further reading 14 External links

History Discovery Further information: Planets beyond Neptune

Discovery photographs of Pluto

Clyde Tombaugh, in Kansas

In the 1840s, Urbain Le Verrier
Urbain Le Verrier
used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune
Neptune
after analyzing perturbations in the orbit of Uranus.[13] Subsequent observations of Neptune
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—a wealthy Bostonian who had founded Lowell Observatory in Flagstaff, Arizona, in 1894—started an extensive project in search of a possible ninth planet, which he termed "Planet X".[14] By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet.[15] 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
Pluto
on March 19 and April 7, 1915, but they were not recognized for what they were.[15][16] There are fourteen other known precovery observations, with the oldest made by the Yerkes Observatory
Yerkes Observatory
on August 20, 1909.[17] Percival's widow, Constance Lowell, entered into a ten-year legal battle with the Lowell Observatory
Lowell Observatory
over her husband's legacy, and the search for Planet
Planet
X did not resume until 1929.[18] Vesto Melvin Slipher, the observatory director, gave the job of locating Planet
Planet
X to 23-year-old Clyde Tombaugh, who had just arrived at the observatory after Slipher had been impressed by a sample of his astronomical drawings.[18] 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, 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.[19] After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory
Harvard College Observatory
on March 13, 1930.[15] Name The discovery made headlines around the globe.[20] 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.[21] Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did.[21] Constance Lowell proposed Zeus, then Percival and finally Constance. These suggestions were disregarded.[22] The name Pluto, after the god of the underworld, was proposed by Venetia Burney
Venetia Burney
(1918–2009), an eleven-year-old schoolgirl in Oxford, England, who was interested in classical mythology.[23] She suggested it in a conversation with her grandfather Falconer Madan, a former librarian at the University of Oxford's Bodleian Library, who passed the name to astronomy professor Herbert Hall Turner, who cabled it to colleagues in the United States.[23] Each member of the Lowell Observatory
Lowell Observatory
was allowed to vote on a short-list of three potential names: Minerva
Minerva
(which was already the name for an asteroid), Cronus
Cronus
(which had lost reputation through being proposed by the unpopular astronomer Thomas Jefferson Jackson See), and Pluto. Pluto
Pluto
received every vote.[24] The name was announced on May 1, 1930.[23][25] Upon the announcement, Madan gave Venetia £5 (equivalent to 300 GBP, or 450 USD
USD
in 2014)[26] as a reward.[23] The final choice of name was helped in part by the fact that the first two letters of Pluto
Pluto
are the initials of Percival Lowell. Pluto's astronomical symbol (, Unicode
Unicode
U+2647, ♇) was then created as a monogram constructed from the letters "PL".[27] Pluto's astrological symbol resembles that of Neptune
Neptune
(), but has a circle in place of the middle prong of the trident (). The name was soon embraced by wider culture. In 1930, Walt Disney
Walt Disney
was apparently inspired by it when he introduced for Mickey Mouse
Mickey Mouse
a canine companion named Pluto, although Disney animator Ben Sharpsteen could not confirm why the name was given.[28] In 1941, Glenn T. Seaborg named the newly created element plutonium after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following uranium, which was named after Uranus, and neptunium, which was named after Neptune.[29] Most languages use the name "Pluto" in various transliterations.[h] In Japanese, Houei Nojiri
Houei Nojiri
suggested the translation Meiōsei (冥王星, "Star of the King (God) of the Underworld"), and this was borrowed into Chinese, Korean, and Vietnamese (which instead uses "Sao Diêm Vương", which was derived from the Chinese term 閻王 (Yánwáng), as "minh" is a homophone for the Sino-Vietnamese words for "dark" (冥) and "bright" (明)).[30][31][32] Some Indian languages use the name Pluto, but others, such as Hindi, use the name of Yama, the God of Death in Hindu
Hindu
and Buddhist mythology.[31] Polynesian languages also tend to use the indigenous god of the underworld, as in Maori Whiro.[31] Planet
Planet
X disproved Once Pluto
Pluto
was found, its faintness and lack of a resolvable disc cast doubt on the idea that it was Lowell's Planet
Planet
X.[14] Estimates of Pluto's mass were revised downward throughout the 20th century.[33]

Mass
Mass
estimates for Pluto

Year Mass Estimate by

1915 7 Earth Lowell (prediction for Planet
Planet
X)[14]

1931 1 Earth Nicholson & Mayall[34][35][36]

1948 0.1 (1/10) Earth Kuiper[37]

1976 0.01 (1/100) Earth Cruikshank, Pilcher, & Morrison[38]

1978 0.0015 (1/650) Earth Christy & Harrington[39]

2006 0.00218 (1/459) Earth Buie et al.[40]

Astronomers initially calculated its mass based on its presumed effect on Neptune
Neptune
and Uranus. In 1931, Pluto
Pluto
was calculated to be roughly the mass of Earth, with further calculations in 1948 bringing the mass down to roughly that of Mars.[35][37] In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii
University of Hawaii
calculated Pluto's albedo for the first time, finding that it matched that for methane ice; this meant Pluto
Pluto
had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of Earth.[38] (Pluto's albedo is 1.4–1.9 times that of Earth.[2]) In 1978, the discovery of Pluto's moon 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
Planet
X, notably by Robert Sutton Harrington,[41] failed. In 1992, Myles Standish used data from Voyager 2's flyby of Neptune
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
Planet
X, vanished.[42] Today, the majority of scientists agree that Planet
Planet
X, as Lowell defined it, does not exist.[43] Lowell had made a prediction of Planet
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,[44] a view still held today.[42] Classification Further information: Definition of planet

Artistic comparison of Pluto, Eris, Makemake, Haumea, Sedna, 2002 MS4, 2007 OR10, Quaoar, Salacia, Orcus, and Earth
Earth
along with the Moon. [

v t e

]

From 1992 onward, many bodies were discovered orbiting in the same volume as Pluto, showing that Pluto
Pluto
is part of a population of objects called the Kuiper belt. This made its official status as a planet controversial, with many questioning whether Pluto
Pluto
should be considered together with or separately from its surrounding population. Museum and planetarium directors occasionally created controversy by omitting Pluto
Pluto
from planetary models of the Solar System. The Hayden Planetarium
Hayden Planetarium
reopened—in February 2000, after renovation—with a model of only eight planets, which made headlines almost a year later.[45] As objects increasingly closer in size to Pluto
Pluto
were discovered in the region, it was argued that Pluto
Pluto
should be reclassified as one of the Kuiper belt
Kuiper belt
objects, just as Ceres, Pallas, Juno and Vesta lost their planet status after the discovery of many other asteroids. On July 29, 2005, astronomers at Caltech
Caltech
announced the discovery of a new trans-Neptunian object, Eris, which was substantially more massive than Pluto
Pluto
and the most massive object discovered in the Solar System since Triton in 1846. Its discoverers and the press initially called it the tenth planet, although there was no official consensus at the time on whether to call it a planet.[46] Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto
Pluto
as a minor planet.[47] IAU classification Main article: IAU definition of planet 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 to be considered a planet:

The object must be in orbit around the 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 cleared the neighborhood around its orbit.[48][49]

Pluto
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.[47][49] The IAU further decided that bodies that, like Pluto, meet criteria 1 and 2, but do not meet criterion 3 would be called dwarf planets. In September 2006, the IAU included Pluto, and Eris and its moon Dysnomia, in their Minor Planet
Planet
Catalogue, giving them the official minor planet designations "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia".[50] Had Pluto
Pluto
been included upon its discovery in 1930, it would have likely been designated 1164, following 1163 Saga, which was discovered a month earlier.[51] There has been some resistance within the astronomical community toward the reclassification.[52][53][54] Alan Stern, principal investigator with NASA's New Horizons
New Horizons
mission to Pluto, derided the IAU resolution, stating that "the definition stinks, for technical reasons".[55] 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.[56] He argued that all big spherical moons, including the Moon, should likewise be considered planets.[57] He also stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.[56] Marc W. Buie, then at the Lowell Observatory petitioned against the definition.[58] 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."[59] Public reception to the IAU decision was mixed. Many accepted the reclassification, but some sought to overturn the decision with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California State Assembly facetiously called the IAU decision a "scientific heresy".[60] 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
Pluto
Planet
Planet
Day.[61][62] 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
Pluto
was "unfairly downgraded to a 'dwarf' planet" by the IAU."[63] 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
Pluto
as a planet and will continue to do so regardless of the IAU decision.[64] 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".[65] 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.[66] Entitled "The Great Planet
Planet
Debate",[67] the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of planet.[68] In June 2008, the IAU had announced in a press release that the term "plutoid" would henceforth be used to refer to Pluto
Pluto
and other objects that have an orbital semi-major axis greater than that of Neptune
Neptune
and enough mass to be of near-spherical shape.[69][70][71] Orbit

Pluto's motion 1900-2048

Pluto
Pluto
was discovered in 1930 near the star δ Geminorum, and merely coincidentally crossing the ecliptic at this time of discovery. Pluto moves about 7 degrees east per decade with small apparent retrograde motion as seen from Earth. Pluto
Pluto
was closer to the Sun
Sun
than Neptune between 1979 and 1999.

Pluto's orbital period is presently about 248 years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun
Sun
close to a flat reference plane called the ecliptic. In contrast, Pluto's orbit is moderately 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
Sun
than Neptune's. The Pluto–Charon barycenter came to perihelion on September 5, 1989,[3][i] and was last closer to the Sun
Sun
than Neptune
Neptune
between February 7, 1979, and February 11, 1999.[72] In the long term, Pluto's orbit is chaotic. Computer simulations can be used to predict its position for several million years (both forward and backward in time), but after intervals longer than the Lyapunov time of 10–20 million years, calculations become speculative: Pluto
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.[73][74] The semi-major axis 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.[75]

Orbit of Pluto—ecliptic view. This "side view" of Pluto's orbit (in red) shows its large inclination to the ecliptic.

Orbit of Pluto—polar view. This "view from above" shows how Pluto's orbit (in red) is less circular than Neptune's (in blue), and how Pluto
Pluto
is sometimes closer to the Sun
Sun
than Neptune. The darker halves of both orbits show where they pass below the plane of the ecliptic.

Relationship with Neptune Despite Pluto's orbit appearing to cross that of Neptune
Neptune
when viewed from directly above, the two objects' orbits are aligned so that they can never collide or even approach closely. The two orbits do not intersect. When Pluto
Pluto
is closest to the Sun, and hence closest to Neptune's orbit as viewed from above, it is also the farthest above Neptune's path. Pluto's orbit passes about 8 AU above that of Neptune, preventing a collision.[76][77][78] 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
Pluto
is also protected by its 2:3 orbital resonance with Neptune: for every two orbits that Pluto
Pluto
makes around the Sun, Neptune
Neptune
makes three. Each cycle lasts about 495 years. This pattern is such that, in each 495-year cycle, the first time Pluto
Pluto
is near perihelion, Neptune
Neptune
is over 50° behind Pluto. By Pluto's second perihelion, Neptune
Neptune
will have completed a further one and a half of its own orbits, and so will be nearly 130° ahead of Pluto. Pluto
Pluto
and Neptune's minimum separation is over 17 AU, which is greater than Pluto's minimum separation from Uranus
Uranus
(11 AU).[78] The minimum separation between Pluto
Pluto
and Neptune
Neptune
actually occurs near the time of Pluto's aphelion.[75] The 2:3 resonance between the two bodies is highly stable, and has been preserved over millions of years.[79] 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.[78] The long term stability of the mean-motion resonance is due to phase protection. If Pluto's period is slightly shorter than 3/2 of Neptune
Neptune
its orbit relative to Neptune will drift, causing it to make closer approaches behind Neptune's orbit. The strong gravitational pull between the two causes angular momentum to be transferred to Pluto, at Neptune's expense. This moves Pluto
Pluto
into a slightly larger orbit, where it travels slightly more slowly, according to Kepler's third law. After many such repetitions, Pluto
Pluto
is sufficiently slowed, and Neptune
Neptune
sufficiently sped up, that Pluto
Pluto
orbit relative to Neptune
Neptune
drifts in the opposite direction until the process is reversed. The whole process takes about 20,000 years to complete.[78][79][80] Other factors Numerical studies have shown that over millions of years, the general nature of the alignment between the orbits of Pluto
Pluto
and Neptune
Neptune
does not change.[76][75] 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, librates around 90°.[75] This means that when Pluto
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,[76] 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
Neptune
is always greater than 52° (90°–38°). The closest such angular separation occurs every 10,000 years.[79] 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
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.[76] Quasi-satellite In 2012, it was hypothesized that 15810 Arawn
15810 Arawn
could be a quasi-satellite of Pluto, a specific type of co-orbital configuration.[81] According to the hypothesis, the object would be a quasi-satellite of Pluto
Pluto
for about 350,000 years out of every two-million-year period.[81][82] This hypothesis was disproven in 2016, when more-accurate observations of the position of Arawn were made by New Horizons.[83] Rotation Pluto's rotation period, its day, is equal to 6.39 Earth
Earth
days.[84] Like Uranus, Pluto
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.[85] The reason for this unusual orientation has been debated. Research from 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.[86] 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 −240 °C (−400.0 °F; 33.1 K), causing nitrogen to freeze as water would freeze on Earth. The same effect seen on Pluto
Pluto
would be observed on Earth
Earth
if the Antarctic ice sheet
Antarctic ice sheet
was several times larger.[87] Geology

High-resolution MVIC image of Pluto
Pluto
in enhanced color to bring out differences in surface composition

Regions where water ice has been detected (blue regions)

Main articles: Geology of Pluto
Pluto
and Geography of Pluto Surface The plains on Pluto's surface are composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.[88] 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.[89] The mountains are made of water ice.[90] Pluto's surface is quite varied, with large differences in both brightness and color.[91] Pluto
Pluto
is one of the most contrastive bodies in the Solar System, with as much contrast as Saturn's moon Iapetus.[92] The color varies from charcoal black, to dark orange and white.[93] Pluto's color is more similar to that of Io with slightly more orange and significantly less red than Mars.[94] Notable geographical features include Tombaugh Regio, or the "Heart" (a large bright area on the side opposite Charon), Cthulhu Macula,[1] or the "Whale" (a large dark area on the trailing hemisphere), and the "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 pits towards their margins;[95][96][97] there are obvious signs of glacial flows both into and out of the basin.[98][99] It has no craters that were visible to New Horizons, indicating that its surface is less than 10 million years old.[100] Latest studies have shown that the surface has an age of 7005180000000000000♠180000+90000 −40000 years.[101] The New Horizons
New Horizons
science team summarized initial findings as " Pluto
Pluto
displays a surprisingly wide variety of geological landforms, including those resulting from glaciological and surface–atmosphere interactions as well as impact, tectonic, possible cryovolcanic, and mass-wasting processes."[6]

Distribution of over 1000 craters of all ages on Pluto. The variation in density (with none found in Sputnik Planitia) indicates a long history of varying geological activity.

Geologic map of Sputnik Planitia
Sputnik Planitia
and surroundings (context), with convection cell margins outlined in black

Sputnik Planitia
Sputnik Planitia
is covered with churning nitrogen ice "cells" that are geologically young and turning over due to convection.

Internal structure

Internal structure of Pluto[102]

1. Frozen nitrogen[88] 2. Water ice 3. Rock

Pluto's density is 7003186000000000000♠1.860±0.013 g/cm3.[6] 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 surrounded by a mantle of water ice. The diameter of the core is hypothesized to be approximately 7006170000000000000♠1700 km, 70% of Pluto's diameter.[102] It is possible that such heating continues today, creating a subsurface ocean of liquid water 100 to 180 km thick at the core–mantle boundary.[102][103][104] 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.[105] Pluto
Pluto
has no magnetic field.[106] Mass
Mass
and size

Selected size estimates for Pluto

Year Radius Notes

1993 1195 km Millis, et al.[107] (if no haze)[108]

1993 1180 km Millis, et al. (surface & haze)[108]

1994 1164 km Young & Binzel[109]

2006 1153 km Buie, et al.[40]

2007 1161 km Young, Young, & Buie[110]

2011 1180 km Zalucha, et al.[111]

2014 1184 km Lellouch, et al.[112]

2015 1187 km New Horizons
New Horizons
measurement (from optical data)[113]

2017 1188.3 km New Horizons
New Horizons
measurement (from radio occultation data)[5][1]

Pluto's diameter is 7006237660000000000♠2376.6±3.2 km[5] and its mass is 7022130299999999999♠(1.303±0.003)×1022 kg, 17.7% that of the Moon
Moon
(0.22% that of Earth).[114] Its surface area is 7013177900000000000♠1.779×107 km2, or roughly the same surface area as Russia. Its surface gravity is 0.063 g (compared to 1 g for Earth). The discovery of Pluto's satellite Charon 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 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.[115]

Size comparisons: Earth, the Moon, and Pluto

With less than 0.2 lunar masses, Pluto
Pluto
is much less massive than the terrestrial planets, and also less massive than seven moons: Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton. The mass is much less than thought before Charon was discovered. Pluto
Pluto
is more than twice the diameter and a dozen times the mass of the dwarf planet Ceres, the largest object in the asteroid belt. It is less massive than the dwarf planet Eris, a trans-Neptunian object discovered in 2005, though Pluto
Pluto
has a larger diameter of 2376.6 km[5] compared to Eris's approximate diameter of 2326 km.[116] Determinations of Pluto's size had been complicated by its atmosphere,[110] and hydrocarbon haze.[108] 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.[112] On July 13, 2015, images from NASA's New Horizons
New Horizons
mission Long Range Reconnaissance Imager (LORRI), along with data from the other instruments, determined Pluto's diameter to be 2,370 km (1,470 mi),[116][117] which was later revised to be 2,372 km (1,474 mi) on July 24,[113] and later to 7006237400000000000♠2374±8 km.[6] Using radio occultation data from the New Horizons
New Horizons
Radio Science Experiment (REX), the diameter was found to be 7006237660000000000♠2376.6±3.2 km.[5]

Atmosphere Main article: Atmosphere
Atmosphere
of Pluto

A near-true-color image taken by New Horizons
New Horizons
after its flyby. Numerous layers of blue haze float in Pluto's atmosphere. Along and near the limb, mountains and their shadows are visible.

Image of Pluto
Pluto
in X-rays by Chandra X-ray Observatory
Chandra X-ray Observatory
(blue spot). The X-rays are probably created by interaction of the gases surrounding Pluto
Pluto
with solar wind, although details of their origin are not clear.

Pluto
Pluto
has a tenuous atmosphere consisting of nitrogen (N2), methane (CH4), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto's surface.[118][119] According to the measurements by New Horizons, the surface pressure is about 1 Pa (10 μbar),[6] 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
New Horizons
data and ground-based occultations show that Pluto's atmospheric density increases, and that it likely remains gaseous throughout Pluto's orbit.[120][121] New Horizons observations showed that atmospheric escape of nitrogen to be 10,000 times less than expected.[121] Alan Stern
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
Mars
to a quarter that of the Earth). At such densities, nitrogen could flow across the surface as liquid.[121] Just like sweat cools the body as it evaporates from the skin, the sublimation of Pluto's atmosphere cools its surface.[122] The presence of atmospheric gases was traced up to 1670 kilometers high; the atmosphere does not have a sharp upper boundary. The presence of methane, a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion, with the average temperature of its atmosphere tens of degrees warmer than its surface,[123] though observations by New Horizons
New Horizons
have revealed Pluto's upper atmosphere to be far colder than expected (70 K, as opposed to about 100 K).[121] Pluto's atmosphere is divided into roughly 20 regularly spaced haze layers up to 150 km high,[6] thought to be the result of pressure waves created by airflow across Pluto's mountains.[121] Satellites Main article: Moons of Pluto Pluto
Pluto
has five known natural satellites: Charon, first identified in 1978 by astronomer James Christy; Nix and Hydra, both discovered in 2005;[124] Kerberos, discovered in 2011;[125] and Styx, discovered in 2012.[126] The satellites' orbits are circular (eccentricity < 0.006) and coplanar with Pluto's equator (inclination < 1°),[127][128] 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.[129] Closest to Pluto
Pluto
is Charon, which is large enough to be in hydrostatic equilibrium and to cause the barycenter of the Pluto–Charon system to be outside Pluto. Beyond Charon there are four much smaller circumbinary moons, Styx, Nix, Kerberos, and Hydra. The orbital periods of all Pluto's moons are linked in a system of orbital resonances and near resonances.[128][130] When precession is accounted for, the orbital periods of Styx, Nix, and Hydra are in an exact 18:22:33 ratio.[128] 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.[128][131]

An oblique view of the Pluto–Charon system showing that Pluto
Pluto
orbits a point outside itself. The two bodies are mutually tidally locked.

The Pluto–Charon system is one of the few in the Solar System
Solar System
whose barycenter lies outside the primary body; the Patroclus–Menoetius system is a smaller example, and the Sun– Jupiter
Jupiter
system is the only larger one.[132] The similarity in size of Charon and Pluto
Pluto
has prompted some astronomers to call it a double dwarf planet.[133] The system is also unusual among planetary systems in that each is tidally locked to the other, which means that Pluto
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.[134] This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its barycenter.[84] In 2007, observations by the Gemini Observatory
Gemini Observatory
of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers.[135] Pluto's moons are hypothesized to have been formed by a collision between Pluto
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.[136]

1. The Pluto
Pluto
system: Pluto, Charon, Styx, Nix, Kerberos, and Hydra, imaged by the Hubble Space Telescope
Hubble Space Telescope
in July 2012. 2. Pluto
Pluto
and Charon, to scale. Image acquired by New Horizons
New Horizons
on July 8, 2015. 3. Family portrait of the five moons of Pluto, to scale.[137] 4. Pluto's moon Charon as viewed by New Horizons
New Horizons
on July 13, 2015

Origin Further information: Kuiper belt
Kuiper belt
and Nice
Nice
model

Plot of the known Kuiper belt
Kuiper belt
objects, set against the four giant planets

Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto
Pluto
was an escaped moon of Neptune,[138] knocked out of orbit by its largest current moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto
Pluto
never approaches Neptune
Neptune
in its orbit.[139] Pluto's true place in the Solar System
Solar System
began to reveal itself only in 1992, when astronomers began to find small icy objects beyond Neptune that were similar to Pluto
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
Pluto
is now known to be the largest member of the Kuiper belt,[j] 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.[140] Like other Kuiper-belt objects (KBOs), Pluto
Pluto
shares features with comets; for example, the solar wind is gradually blowing Pluto's surface into space.[141] It has been claimed that if Pluto
Pluto
were placed as near to the Sun
Sun
as Earth, it would develop a tail, as comets do.[142] This claim has been disputed with the argument that Pluto's escape velocity is too high for this to happen.[143] Though Pluto
Pluto
is the largest Kuiper belt
Kuiper belt
object discovered,[108] Neptune's moon Triton, which is slightly larger than Pluto, is similar to it both geologically and atmospherically, and is thought to be a captured Kuiper belt
Kuiper belt
object.[144] Eris (see above) is about the same size as Pluto
Pluto
(though more massive) but is not strictly considered a member of the Kuiper belt
Kuiper belt
population. Rather, it is considered a member of a linked population called the scattered disc. A large number of Kuiper belt
Kuiper belt
objects, like Pluto, are in a 2:3 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutinos", after Pluto.[145] Like other members of the Kuiper belt, Pluto
Pluto
is thought to be a residual planetesimal; a component of the original protoplanetary disc around the Sun
Sun
that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto
Pluto
owes its current position to a sudden migration undergone by Neptune
Neptune
early in the Solar System's formation. As Neptune
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, 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.[146] A computer model created in 2004 by Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice
Nice
suggested that the migration of Neptune
Neptune
into the Kuiper belt
Kuiper belt
may have been triggered by the formation of a 1:2 resonance between Jupiter
Jupiter
and Saturn, which created a gravitational push that propelled both Uranus and Neptune
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
Kuiper belt
could also explain the Late Heavy Bombardment
Late Heavy Bombardment
600 million years after the Solar System's formation and the origin of the Jupiter
Jupiter
trojans.[147] It is possible that Pluto
Pluto
had a near-circular orbit about 33 AU from the Sun
Sun
before Neptune's migration perturbed it into a resonant capture.[148] The Nice model
Nice model
requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk, which included Triton and Eris.[147] Observation and exploration Pluto's distance from Earth
Earth
makes its in-depth study and exploration difficult. On July 14, 2015, NASA's New Horizons
New Horizons
space probe flew through the Pluto
Pluto
system, providing much information about it.[149] Observation

Computer-generated rotating image of Pluto
Pluto
based on observations by the Hubble Space Telescope
Hubble Space Telescope
in 2002–2003

Pluto's visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion.[2] To see it, a telescope is required; around 30 cm (12 in) aperture being desirable.[150] 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
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.[151][152] Better maps were produced from images taken by the Hubble Space Telescope (HST), which offered higher resolution, and showed considerably more detail,[92] resolving variations several hundred kilometers across, including polar regions and large bright spots.[94] These maps were produced by complex computer processing, which finds the best-fit projected maps for the few pixels of the Hubble images.[153] These remained the most detailed maps of Pluto
Pluto
until the flyby of New Horizons
New Horizons
in July 2015, because the two cameras on the HST used for these maps were no longer in service.[153] Exploration Main articles: Exploration of Pluto
Pluto
and New Horizons

The portions of Pluto's surface mapped by New Horizons
New Horizons
(annotated)

The New Horizons
New Horizons
spacecraft, which flew by Pluto
Pluto
in July 2015, is the first and so far only attempt to explore Pluto
Pluto
directly. Launched in 2006, it captured its first (distant) images of Pluto
Pluto
in late September 2006 during a test of the Long Range Reconnaissance Imager.[154] 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
Pluto
and other Kuiper belt
Kuiper belt
objects. In early 2007 the craft made use of a gravity assist from Jupiter. New Horizons
New Horizons
made its closest approach to Pluto
Pluto
on July 14, 2015, after a 3,462-day journey across the Solar System. Scientific observations of Pluto
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 imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments. The scientific goals of New Horizons were to characterize the global geology and morphology of Pluto
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
New Horizons
from its close encounter with Pluto.[155][156][157][158]

Videos

Pluto
Pluto
flyover animated (July 14, 2015)

Play media

(00:30; released September 18, 2015)

Play media

(00:50; released December 5, 2015)

Play media

This mosaic strip – extending across the hemisphere that faced the New Horizons
New Horizons
spacecraft as it flew past Pluto. (No Audio – 1080p 60fps)

See also

Solar System
Solar System
portal

Book: Dwarf Planets of the Solar System
Solar System
& Their Satellites Book: Solar System

How I Killed Pluto
Pluto
and Why It Had It Coming Pluto
Pluto
in astrology Pluto
Pluto
in fiction

Notes

^ This is a composite of four near-true color photographs taken by the New Horizons
New Horizons
spacecraft on July 14, 2015 from a distance of 720,000 km (450,000 mi). The most prominent feature in the image, the bright, youthful plains of Tombaugh Regio
Tombaugh Regio
and Sputnik Planitia, can be seen at lower right. It contrasts the darker, more cratered terrain of Cthulhu Macula[1] at lower left. Because of Pluto's 119.591° tilt at its axis, the southern hemisphere is barely visible in this image; the equator runs through Cthulhu and the southern parts of Sputnik Planitia. ^ The mean elements here are from the Theory of the Outer Planets (TOP2013) solution by the Institut de mécanique céleste et de calcul des éphémérides (IMCCE). They refer to the standard equinox J2000, the barycenter of the Solar System, and the epoch J2000. ^ Surface area
Surface area
derived from the radius r:

4 π

r

2

displaystyle 4pi r^ 2

. ^ Volume
Volume
v derived from the radius r:

4 π

r

3

/

3

displaystyle 4pi r^ 3 /3

. ^ Surface gravity derived from the mass M, the gravitational constant G and the radius r:

G M

/

r

2

displaystyle GM/r^ 2

. ^ Escape velocity
Escape velocity
derived from the mass M, the gravitational constant G and the radius r:

2 G M

/

r

displaystyle sqrt 2GM/r

. ^ Based on geometry of minimum and maximum distance from Earth
Earth
and Pluto
Pluto
radius in the factsheet ^ The equivalence is less close in languages whose phonology differs widely from Greek's, such as Somali Buluuto and Navajo Tłóotoo. ^ The discovery of Charon in 1978 allowed astronomers to accurately calculate the mass of the Plutonian system. But it did not indicate the two bodies' individual masses, which could only be estimated after other moons of Pluto
Pluto
were discovered in late 2005. As a result, because Pluto
Pluto
came to perihelion in 1989, most Pluto
Pluto
perihelion date estimates are based on the Pluto–Charon barycenter. Charon came to perihelion 4 September 1989. The Pluto–Charon barycenter came to perihelion 5 September 1989. Pluto
Pluto
came to perihelion 8 September 1989. ^ The dwarf planet Eris is roughly the same size as Pluto, about 2330 km; Eris is 28% more massive than Pluto. Eris is a scattered-disc object, often considered a distinct population from Kuiper-belt objects like Pluto; Pluto
Pluto
is the largest body in the Kuiper belt
Kuiper belt
proper, which excludes the scattered-disc objects.

References

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Pluto
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Adaptive optics
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Pluto
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New Horizons
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Nitrogen
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Pluto
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Moon
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Pluto
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Moon
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Pluto
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Pluto
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University of Arizona
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NASA
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New Horizons
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Further reading

Stern, S A and Tholen, D J (1997), Pluto
Pluto
and Charon, University of Arizona Press ISBN 978-0816518401

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Other moons

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Astronomy

Definition

Alan Stern Definition of planet Double planet Dwarf planet IAU definition of planet International Astronomical Union Michael E. Brown Neil deGrasse Tyson Planet Plutino Plutoid Trans-Neptunian object

Discovery

Clyde Tombaugh James W. Christy Lowell Observatory Percival Lowell Planet
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Asteroid
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IAU: Ceres No longer rounded and therefore not dwarf planets (Former candidates: Pallas Vesta Hygiea)

Centaurs

Possibly: Chariklo Chiron

Centaurs (extended)

Possibly: 1995 SN55
1995 SN55
(lost) Ceto 2010 TY53

Plutinos

IAU: Pluto Additional objects proposed by Brown and Tancredi: Orcus Ixion 2003 AZ84 2004 UX10 Huya Possibly: 2003 UZ413 2003 VS2 2002 VR128 2004 PF115 2002 XV93 2007 JH43 2001 QF298

Twotinos

Possibly: 2002 WC19

Other resonances/ unknown resonances:

IAU: Haumea Possibly: 2002 XW93 1999 DE9 1999 KR16 2001 YH140 2011 GM27 2010 VK201 2013 FZ27 2014 UM33 1999 CD158

Cubewanos:

IAU: Makemake Additional objects proposed by Brown and Tancredi: 2002 MS4 Salacia Quaoar Varuna Varda 2004 GV9 2002 AW197 2002 TX300 2005 RR43 2003 OP32 2005 RN43 Possibly: Chaos Altjira Sila-Nunam 2002 UX25 2007 JJ43 2005 UQ513 2004 TY364 2003 QW90 1998 SN165 2002 KX14 Praamzius 2004 NT33 2010 FX86 2002 CY248 2010 VR11 Rhadamanthus

Scattered disc

IAU: Eris Additional objects proposed by Brown and Tancredi: 2007 OR10 1996 TL66 2001 UR163 2005 RM43 Possibly: 2002 TC302 2004 XA192 2005 QU182 2006 QH181 2007 UK126 2010 EK139 2010 KZ39 2010 RE64 2010 RF43 2010 TJ 2013 FY27 2008 OG19 1996 GQ21 2012 HH2 1999 CC158 2010 VZ98

Area uncertain

Captured satellites that were once dwarf planets: Triton (captured by Neptune) Phoebe (captured by Saturn, and no longer rounded) Possibly: 2011 FW62
2011 FW62
(lost) 2000 YW134 V774104

Detached objects

Possibly: 2004 XR190 2010 GB174 2008 ST291 1995 TL8 2005 TB190 2003 QX113 2003 FY128 2004 VN112 2000 CR105

Sednoids

Objects proposed by Brown and Tancredi: Sedna Possibly: 2012 VP113

See also: Charon Vanth Dysnomia Hiʻiaka Mesoplanet Planemo List of trans-Neptunian objects List of possible dwarf planets List of Solar System
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Neptunian

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first discovered: Ceres Pallas Juno Vesta

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Hypothetical objects

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Outline of the Solar System Portals Solar System Astronomy Earth
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