Pluto - Wikipedia Pluto This article is about the dwarf planet. For the deity, see Pluto ( mythology ). For other uses, see Pluto ( disambiguation ). Pluto Full - disc view of Pluto in near - true color, imaged by New Horizons Discovery Discovered by Clyde W. Tombaugh Discovery date February 18, 1930 Designations MPC designation ( 134340 ) Pluto Pronunciation / ˈpluːtoʊ / ( listen ) Named after Pluto Minor planet category Dwarf planet Trans - Neptunian object Plutoid Kuiper belt object Plutino Adjectives Plutonian Orbital characteristics Epoch J2000 Earliest precovery date August 20, 1909 Aphelion 7012737592301486350 ♠ 49.305 AU ( 7012737593000000000 ♠ 7,375.93 Gm ) February 2114 Perihelion 7012443677364922060 ♠ 29.658 AU ( 7012443682000000000 ♠ 4,436.82 Gm ) ( September 5, 1989 ) Semi-major axis 7012590612393523600 ♠ 39.48 AU ( 7012590638000000000 ♠ 5,906.38 Gm ) Eccentricity 6999248800000000000 ♠ 0.2488 Orbital period 7002248000000000000 ♠ 248.00 years 7009782438400000000 ♠ 90,560 d Synodic period 366.73 days Average orbital speed 4.67 km / s Mean anomaly 7001145300000000000 ♠ 14.53 deg Inclination 6999299498499642227 ♠ 17.16 ° ( 11.88 ° to Sun 's equator ) Longitude of ascending node 7000192508071165723 ♠ 110.299 ° Argument of perihelion 7000198677810071522 ♠ 113.834 ° Known satellites 5 Physical characteristics Mean radius 7006118830000000000 ♠ 1,188.3 ± 0.8 km 0.1868 Earths Flattening < 1 % Surface area 7013177900000000000 ♠ 1.779 × 10 km 0.035 Earths Volume 7018705700000000000 ♠ ( 7.057 ± 0.004 ) × 10 km 6997651000000000000 ♠ 0.006 51 Earths Mass 7022130299999999999 ♠ ( 1.303 ± 0.003 ) × 10 kg 6997218000000000000 ♠ 0.00218 Earths 0.177 Moons Mean density 7003185400000000000 ♠ 1.854 ± 0.006 g / cm Surface gravity 6999620000000000000 ♠ 0.620 m / s 0.063 g Escape velocity 7000121200000000000 ♠ 1.212 km / s Sidereal rotation period 7005551856672000000 ♠ 6.387 230 d 6 d, 9 h, 17 m, 36 s Equatorial rotation velocity 47.18 km / h Axial tilt 7000213855193246865 ♠ 122.53 ° ( to orbit ) North pole right ascension 132.993 ° North pole declination − 6.163 ° Albedo 0.49 to 0.66 ( geometric, varies by 35 % ) Surface temp. min mean max Kelvin 33 K 44 K ( − 229 ° C ) 55 K Apparent magnitude 13.65 to 16.3 ( mean is 15.1 ) Absolute magnitude ( H ) − 0.7 Angular diameter 0.06 '' to 0.11 '' Atmosphere Surface pressure 1.0 Pa ( 2015 ) Composition by volume Nitrogen, methane, carbon monoxide Mosaic of best - resolution images of Pluto from different angles 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 object to be discovered. Pluto was discovered by 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 ( IAU ) to define the term `` planet '' formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet. 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 objects, Pluto is primarily made of ice and rock and is relatively small -- about one - sixth the mass of the 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 than Neptune, but a stable orbital resonance with Neptune prevents them from colliding. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance ( 39.5 AU ). Pluto has five known moons : Charon ( the largest, with a diameter just over half that of Pluto ), Styx, Nix, Kerberos, and Hydra. 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 spacecraft became the first spacecraft to fly by Pluto. 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 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 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 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 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 used Newtonian mechanics to predict the position of the then - undiscovered planet Neptune after analyzing perturbations in the orbit of 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 -- 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 ''. By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916, but to no avail. Unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7, 1915, but they were not recognized for what they were. There are fourteen other known precovery observations, with the oldest made by the 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, 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, 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 on March 13, 1930. Name 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, then Percival and finally Constance. These suggestions were disregarded. The name Pluto, after the god of the underworld, was proposed by Venetia Burney ( 1918 -- 2009 ), an eleven - year - old schoolgirl in Oxford, England, who was interested in classical mythology. 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. Each member of the Lowell Observatory was allowed to vote on a short - list of three potential names : Minerva ( which was already the name for an asteroid ), Cronus ( which had lost reputation through being proposed by the unpopular astronomer Thomas Jefferson Jackson See ), and Pluto. Pluto received every vote. The name was announced on May 1, 1930. Upon the announcement, Madan gave Venetia £ 5 ( equivalent to 300 GBP, or 450 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 (, Unicode U + 2647, ♇ ) was then created as a monogram constructed from the letters `` PL ''. Pluto 's astrological symbol resembles that of 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 was apparently inspired by it when he introduced for Mickey Mouse a canine companion named Pluto, although Disney animator Ben Sharpsteen could not confirm why the name was given. 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. Most languages use the name `` Pluto '' in various transliterations. In Japanese, 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 ) ). Some Indian languages use the name Pluto, but others, such as Hindi, use the name of Yama, the God of Death in Hindu and Buddhist mythology. Polynesian languages also tend to use the indigenous god of the underworld, as in Maori Whiro. Planet X disproved Once Pluto was found, its faintness and lack of a resolvable disc cast doubt on the idea that it was Lowell 's Planet X. Estimates of Pluto 's mass were revised downward throughout the 20th century. Mass estimates for Pluto Year Mass Estimate by 1915 7 Earth Lowell ( prediction for Planet X ) 1931 1 Earth Nicholson & Mayall 1948 0.1 ( 1 / 10 ) Earth Kuiper 1976 0.01 ( 1 / 100 ) Earth Cruikshank, Pilcher, & Morrison 1978 0.0015 ( 1 / 650 ) Earth Christy & Harrington 2006 0.00218 ( 1 / 459 ) Earth Buie et al. 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, with further calculations in 1948 bringing the mass down to roughly that of Mars. In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii calculated Pluto 's albedo 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 1.4 -- 1.9 times that of Earth. ) 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 X, notably by Robert Sutton Harrington, failed. In 1992, Myles Standish used data from Voyager 2 '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, a view still held today. Classification Further information : Definition of planet Artistic comparison of Pluto, Eris, Makemake, Haumea, Sedna, 2002 MS, 2007 OR, Quaoar, Salacia, Orcus, and Earth along with the Moon. 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. 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. The Hayden Planetarium reopened -- in February 2000, after renovation -- with a model of only eight planets, which made headlines almost a year later. As objects increasingly closer in size to Pluto were discovered in the region, it was argued that Pluto should be reclassified as one of the 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 announced the discovery of a new trans - Neptunian object, Eris, which was substantially more massive than 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. Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet. 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. 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. 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 Catalogue, giving them the official minor planet designations `` ( 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, should likewise be considered planets. He also stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community. Marc W. Buie, then at the Lowell Observatory petitioned against the definition. Others have supported the IAU. Mike Brown, the astronomer who discovered Eris, said `` through this whole crazy circus - like procedure, somehow the right answer was stumbled on. It 's been a long time coming. Science is self - correcting eventually, even when strong emotions are involved. '' Public reception to the IAU decision was mixed. 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 ''. 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 objects that have an orbital semi-major axis greater than that of Neptune and enough mass to be of near - spherical shape. Orbit Pluto 's motion 1900 - 2048 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 was closer to the 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 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 than Neptune 's. The Pluto -- Charon barycenter came to perihelion on September 5, 1989, and was last closer to the Sun than Neptune between February 7, 1979, and February 11, 1999. 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 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 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. 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 is sometimes closer to the 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 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 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. 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. Pluto is also protected by its 2 : 3 orbital resonance with Neptune : for every two orbits that Pluto makes around the Sun, Neptune makes three. Each cycle lasts about 495 years. 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. If 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 strong gravitational pull between the two 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, and Neptune sufficiently sped up, that Pluto 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, 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 ° ( 90 ° -- 38 ° ). The closest such angular separation occurs every 10,000 years. Second, the longitudes of ascending nodes of the two bodies -- the points where they cross the ecliptic -- are in near - resonance with the above libration. When the two longitudes are the same -- that is, when one could draw a straight line through both nodes and the Sun -- Pluto 's perihelion lies exactly at 90 °, and 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. This hypothesis was disproven in 2016, when more - accurate observations of the position of Arawn were made by New Horizons. Rotation Pluto 's rotation period, its day, is equal to 6.39 Earth days. Like 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 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 − 240 ° C ( − 400.0 ° F ; 33.1 K ), causing nitrogen to freeze as water would freeze on Earth. The same effect seen on Pluto would be observed on Earth if the Antarctic ice sheet was several times larger. Geology High - resolution MVIC image of 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 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. 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. The color varies from charcoal black, to dark orange and white. Pluto 's color is more similar to that of Io with slightly more orange and significantly less red than Mars. Notable geographical features include Tombaugh Regio, or the `` Heart '' ( a large bright area on the side opposite Charon ), Cthulhu Macula, 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 ; 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 7005180000000000000 ♠ 180 000 + 90 000 − 40 000 years. The New Horizons science team summarized initial findings as `` Pluto displays a surprisingly wide variety of geological landforms, including those resulting from glaciological and surface -- atmosphere interactions as well as impact, tectonic, possible cryovolcanic, and mass - wasting processes. '' 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 and surroundings ( context ), with convection cell margins outlined in black 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 1. Frozen nitrogen 2. Water ice 3. Rock Pluto 's density is 7003186000000000000 ♠ 1.860 ± 0.013 g / cm. 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. 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. 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. Pluto has no magnetic field. Mass and size Selected size estimates for Pluto Year Radius Notes 1993 1195 km Millis, et al. ( if no haze ) 1993 1180 km Millis, et al. ( surface & haze ) 1164 km Young & Binzel 2006 1153 km Buie, et al. 2007 1161 km Young, Young, & Buie 2011 1180 km Zalucha, et al. 2014 1184 km Lellouch, et al. 2015 1187 km New Horizons measurement ( from optical data ) 2017 1188.3 km New Horizons measurement ( from radio occultation data ) Pluto 's diameter is 7006237660000000000 ♠ 2376. 6 ± 3.2 km and its mass is 7022130299999999999 ♠ ( 1.303 ± 0.003 ) × 10 kg, 17.7 % that of the Moon ( 0.22 % that of Earth ). Its surface area is 7013177900000000000 ♠ 1.779 × 10 km, 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. Size comparisons : Earth, the Moon, and Pluto With less than 0.2 lunar masses, 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 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 has a larger diameter of 2376.6 km compared to Eris 's approximate diameter of 2326 km. Determinations of Pluto 's size had 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 2,370 km ( 1,470 mi ), which was later revised to be 2,372 km ( 1,474 mi ) on July 24, and later to 7006237400000000000 ♠ 2374 ± 8 km. Using radio occultation data from the New Horizons Radio Science Experiment ( REX ), the diameter was found to be 7006237660000000000 ♠ 2376. 6 ± 3.2 km. Atmosphere Main article : Atmosphere of Pluto A near - true - color image taken by 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 in X-rays by Chandra X-ray Observatory ( blue spot ). The X-rays are probably created by interaction of the gases surrounding Pluto with solar wind, although details of their origin are not clear. Pluto has a tenuous atmosphere consisting of nitrogen ( N ), methane ( CH ), and carbon monoxide ( CO ), which are in equilibrium with their ices on Pluto 's surface. According to the measurements by New Horizons, the surface pressure is about 1 Pa ( 10 μbar ), roughly one million to 100,000 times less than Earth 's atmospheric pressure. It was initially thought that, as Pluto moves away from the Sun, its atmosphere should gradually freeze onto the surface ; studies of New Horizons data and ground - based occultations show that Pluto 's atmospheric density increases, and that it likely remains gaseous throughout Pluto 's orbit. New Horizons observations showed that atmospheric escape of nitrogen to be 10,000 times less than expected. Alan Stern has contended that even a small increase in Pluto 's surface temperature can lead to exponential increases in Pluto 's atmospheric density ; from 18 hPa to as much as 280 hPa ( three times that of Mars to a quarter that of the Earth ). At such densities, nitrogen could flow across the surface as liquid. Just like sweat cools the body as it evaporates from the skin, the sublimation of Pluto 's atmosphere cools its surface. 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, 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 Main article : Moons of Pluto Pluto has five known natural satellites : Charon, first identified in 1978 by astronomer James Christy ; Nix and Hydra, both discovered in 2005 ; Kerberos, discovered in 2011 ; and Styx, 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. Closest to 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. When 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. An oblique view of the Pluto -- Charon system showing that 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 whose barycenter lies outside the primary body ; the Patroclus -- Menoetius system is a smaller example, and the Sun -- Jupiter system is the only larger one. The similarity in size of Charon and Pluto has prompted some astronomers to call it a double dwarf planet. The system is also unusual among planetary systems in that each is tidally locked 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. 1. The Pluto system : Pluto, Charon, Styx, Nix, Kerberos, and Hydra, imaged by the Hubble Space Telescope in July 2012. 2. Pluto and Charon, to scale. Image acquired by New Horizons on July 8, 2015. 3. Family portrait of the five moons of Pluto, to scale. 4. Pluto 's moon Charon as viewed by New Horizons on July 13, 2015 Origin Further information : Kuiper belt and Nice model Plot of the known Kuiper belt objects, set against the four giant planets Pluto 's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton. 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, 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. Though Pluto is the largest Kuiper belt object discovered, 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 object. Eris ( see above ) is about the same size as Pluto ( though more massive ) but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called the scattered disc. 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 that failed to fully coalesce into a full - fledged planet. Most astronomers agree that Pluto owes its current position to a sudden migration undergone by Neptune early in the Solar System 's formation. As Neptune migrated outward, it approached the objects in the proto - Kuiper belt, setting one in orbit around itself ( Triton ), locking others into resonances, and knocking others into chaotic orbits. The objects in the scattered disc, a dynamically unstable region overlapping the Kuiper belt, are thought to have been placed in their current positions by interactions with Neptune 's migrating resonances. A computer model created in 2004 by Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice 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 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 exploration difficult. On July 14, 2015, NASA 's New Horizons space probe flew through the Pluto system, providing much information about it. Observation Computer - generated rotating image of Pluto based on observations by the Hubble Space Telescope in 2002 -- 2003 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 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 Main articles : Exploration of Pluto and New Horizons The portions of Pluto 's surface mapped by New Horizons ( annotated ) The New Horizons spacecraft, which flew by Pluto in July 2015, is the first and so far only attempt to explore Pluto directly. Launched in 2006, it captured its first ( distant ) images of Pluto in late September 2006 during a test of the Long Range Reconnaissance Imager. The images, taken from a distance of approximately 4.2 billion kilometers, confirmed the spacecraft 's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper belt objects. In early 2007 the craft made use of a gravity assist 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 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 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. Videos 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 spacecraft as it flew past Pluto. ( No Audio -- 1080p 60fps ) See also Solar System portal Book : Dwarf Planets of the Solar System & Their Satellites Book : Solar System How I Killed Pluto and Why It Had It Coming Pluto in astrology Pluto in fiction Notes Jump up ^ This is a composite of four near - true color photographs taken by the 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 and Sputnik Planitia, can be seen at lower right. It contrasts the darker, more cratered terrain of Cthulhu Macula 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. Jump up ^ 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. Jump up ^ Surface area derived from the radius r : 4 π r 2 ( displaystyle 4 pi r ^ ( 2 ) ). Jump up ^ Volume v derived from the radius r : 4 π r 3 / 3 ( displaystyle 4 pi r ^ ( 3 ) / 3 ). Jump up ^ Surface gravity derived from the mass M, the gravitational constant G and the radius r : G M / r 2 ( displaystyle GM / r ^ ( 2 ) ). Jump up ^ Escape velocity derived from the mass M, the gravitational constant G and the radius r : 2 G M / r ( displaystyle ( sqrt ( 2GM / r ) ) ). Jump up ^ Based on geometry of minimum and maximum distance from Earth and Pluto radius in the factsheet Jump up ^ The equivalence is less close in languages whose phonology differs widely from Greek 's, such as Somali Buluuto and Navajo Tłóotoo. Jump up ^ 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 were discovered in late 2005. As a result, because Pluto came to perihelion in 1989, most 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 came to perihelion 8 September 1989. Jump up ^ 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 is the largest body in the Kuiper belt proper, which excludes the scattered - disc objects. References ^ Jump up to : S. Alan Stern. William Grundy, William B. McKinnon, Harold A. Weaver, Leslie A. Young ( 18 December 2017 ). `` The Pluto System After New Horizons '' ( PDF ). 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Jump up ^ Ley, Willy ( August 1956 ). `` The Demotion of Pluto ''. For Your Information. Galaxy Science Fiction. pp. 79 -- 91. Jump up ^ Stern, S. Alan ; Tholen, David J. ( 1997 ). Pluto and Charon. University of Arizona Press. p. 623. ISBN 978 - 0 - 8165 - 1840 - 1. Jump up ^ Sheppard, Scott S. ; Trujillo, Chadwick A. ; Udalski, Andrzej ; et al. ( 2011 ). `` A Southern Sky and Galactic Plane Survey for Bright Kuiper Belt Objects ''. Astronomical Journal. 142 ( 4 ) : 98. arXiv : 1107.5309. Bibcode : 2011AJ... 142... 98S. doi : 10.1088 / 0004 - 6256 / 142 / 4 / 98. Jump up ^ `` Colossal Cousin to a Comet ? ''. pluto.jhuapl.edu -- NASA New Horizons mission site. Johns Hopkins University Applied Physics Laboratory. Archived from the original on November 13, 2014. Retrieved February 15, 2014. Jump up ^ Tyson, Neil deGrasse ( 1999 ). `` Pluto Is Not a Planet ''. The Planetary Society. Archived from the original on September 27, 2011. Retrieved November 30, 2011. 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Jump up ^ Brown, Dwayne ; Buckley, Michael ; Stothoff, Maria ( January 15, 2015 ). `` January 15, 2015 Release 15 - 011 -- NASA 's New Horizons Spacecraft Begins First Stages of Pluto Encounter ''. NASA. Retrieved January 15, 2015. Jump up ^ `` New Horizons ''. pluto.jhuapl.edu. Retrieved May 15, 2016. Further reading Stern, SA and Tholen, DJ ( 1997 ), Pluto and Charon, University of Arizona Press ISBN 978 - 0816518401 External links Find more aboutPlutoat Wikipedia 's sister projects Definitions from Wiktionary Media from Wikimedia Commons News from Wikinews Quotations from Wikiquote Texts from Wikisource Textbooks from Wikibooks Learning resources from Wikiversity New Horizons homepage Pluto Profile at NASA 's Solar System Exploration site NASA Pluto factsheet Website of the observatory that discovered Pluto Earth telescope image of Pluto system Keck infrared with AO of Pluto system Gray, Meghan ( 2009 ). `` Pluto ''. Sixty Symbols. Brady Haran for the University of Nottingham. 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 CGI video of Pluto flyover ( July 14, 2017 ) CGI video simulation of rotating Pluto by Seán Doran ( see album for more ) Google Pluto 3D, interactive map of the dwarf planet Articles related to Pluto Pluto Geography ( features ) Regions `` Brass Knuckles '' Krun Ala Balrog Vucub - Came Hun - Came Meng - p'o Cadejo Cthulhu Hayabusa Lowell Morgoth Pioneer Tombaugh Venera Viking Voyager Mountains Challenger Colles Coleta de Dados Colles Hillary Montes Tenzing Montes Plains Sputnik Planitia Valleys and depressions Quidlivun Cavus Lineae Luna Linea General Atmosphere Geology Moons Charon ( features ) Regions Gallifrey Mordor Vulcan Mountains Kubrick Mons Chasms Nostromo Chasma Craters Ripley Crater Vader Crater Geology Geology Other moons Styx Nix Kerberos Hydra 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 X Roger Putnam Eclipses Solar eclipses on Pluto General Kuiper belt Exploration Current Hubble Space Telescope Former New Horizons Cancelled Pluto Fast Flyby / Pluto Kuiper Express Mariner Mark II Old proposals Voyager 1 TAU Related Fiction Mythology Dwarf planets Asteroid belt IAU : Ceres No longer rounded and therefore not dwarf planets ( Former candidates : Pallas Vesta Hygiea ) Centaurs Possibly : Chariklo Chiron Centaurs ( extended ) Possibly : 1995 SN 55 ( lost ) Ceto 2010 TY 53 Plutinos IAU : Pluto Additional objects proposed by Brown and Tancredi : Orcus Ixion 2003 AZ 2004 UX Huya Possibly : 2003 UZ 2003 VS 2002 VR 2004 PF 2002 XV 93 2007 JH 43 2001 QF 298 Twotinos Possibly : 2002 WC Other resonances / unknown resonances : IAU : Haumea Possibly : 2002 XW 1999 DE 1999 KR 2001 YH 2011 GM 27 2010 VK 201 2013 FZ 27 2014 UM 33 1999 CD 158 Cubewanos : IAU : Makemake Additional objects proposed by Brown and Tancredi : 2002 MS Salacia Quaoar Varuna Varda 2004 GV 2002 AW 2002 TX 2005 RR 2003 OP 2005 RN Possibly : Chaos Altjira Sila - Nunam 2002 UX 2007 JJ 2005 UQ 2004 TY 2003 QW 1998 SN 2002 KX Praamzius 2004 NT 2010 FX 86 2002 CY 248 2010 VR 11 Rhadamanthus Scattered disc IAU : Eris Additional objects proposed by Brown and Tancredi : 2007 OR 1996 TL 2001 UR 2005 RM Possibly : 2002 TC 2004 XA 2005 QU 2006 QH 181 2007 UK 2010 EK 139 2010 KZ 39 2010 RE 64 2010 RF 43 2010 TJ 2013 FY 27 2008 OG 19 1996 GQ 2012 HH 1999 CC 2010 VZ 98 Area uncertain Captured satellites that were once dwarf planets : Triton ( captured by Neptune ) Phoebe ( captured by Saturn, and no longer rounded ) Possibly : 2011 FW 62 ( lost ) 2000 YW V774104 Detached objects Possibly : 2004 XR 190 2010 GB 174 2008 ST 291 1995 TL 2005 TB 2003 QX 113 2003 FY 2004 VN 112 2000 CR Sednoids Objects proposed by Brown and Tancredi : Sedna Possibly : 2012 VP 113 See also : Charon Vanth Dysnomia Hiʻiaka Mesoplanet Planemo List of trans - Neptunian objects List of possible dwarf planets List of Solar System objects List of Solar System objects by size Trans - Neptunian objects TNO classes Cubewanos Scattered - disc objects Detached objects Resonant objects Neptune trojans Plutinos Twotinos TNO moons Dwarf planets ( moons ) Pluto Charon Styx Nix Kerberos Hydra Haumea Namaka Hi'iaka Makemake S / 2015 ( 136472 ) 1 Eris Dysnomia Sednoids 90377 Sedna 2012 VP 113 Minor planets navigator ( 134339 ) 5628 T - 3 134340 Pluto ( 134341 ) 1979 MA The Solar System The Sun Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Pluto Haumea Makemake Eris Planets Terrestrial planets Mercury Venus Earth Mars Giant planets Jupiter Saturn Uranus Neptune Dwarf planets Ceres Pluto Haumea Makemake Eris Rings Jovian Saturnian ( Rhean ) Charikloan Chironean Uranian Neptunian Haumean Moons Terrestrial Moon Other near - Earth objects Martian Phobos Deimos Jovian Ganymede Callisto Io Europa all 69 Saturnian Titan Rhea Iapetus Dione Tethys Enceladus Mimas Hyperion Phoebe all 62 Uranian Titania Oberon Umbriel Ariel Miranda all 27 Neptunian Triton Proteus Nereid all 14 Plutonian Charon Nix Hydra Kerberos Styx Haumean Hiʻiaka Namaka Makemakean S / 2015 ( 136472 ) 1 Eridian Dysnomia Lists Solar System objects By size By discovery date Minor planets Gravitationally rounded objects Possible dwarf planets Natural satellites Comets Small Solar System bodies Meteoroids Minor planets moons Comets Damocloids Mercury - crossers Venus - crossers Venus trojans Near - Earth objects Earth - crossers Earth trojans Mars - crossers Mars trojans Asteroid belt Asteroids first discovered : Ceres Pallas Juno Vesta Families Notable asteroids Kirkwood gap Main - belt comets Jupiter trojans Jupiter - crossers Centaurs Saturn - crossers Uranus trojans Uranus - crossers Neptune trojans Cis - Neptunian objects Trans - Neptunian objects Neptune - crossers Plutoids Kuiper belt Plutinos Cubewanos Scattered disc Detached objects Sednoids Hills cloud Oort cloud Hypothetical objects Vulcan Vulcanoids Phaeton Planet V Theia Fifth giant Planets beyond Neptune Tyche Nemesis Planet Nine Exploration ( outline ) Discovery Astronomy Timeline Spaceflight Robotic spacecraft Human spaceflight Colonization List of probes Timeline Mercury Venus Moon Mars Ceres Asteroids Mining Comets Jupiter Saturn Uranus Neptune Pluto Deep space Outline of the Solar System Portals Solar System Astronomy Earth sciences Mars Jupiter Uranus Cosmology Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Virgo Supercluster → Laniakea Supercluster → Observable universe → Universe Each arrow ( → ) may be read as `` within '' or `` part of ''. 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Question: what year did pluto become a dwarf planet
Answer: 2006