Pluto 2nd Astronomical symbol for Pluto
Pluto in True Color - High-Res.jpg
Northern hemisphere of Pluto in true color[a]
Discovered byClyde W. Tombaugh
Discovery dateFebruary 18, 1930
MPC designation(134340) Pluto
Pronunciation/ (About this soundlisten)
Named after
Orbital characteristics[3][b]
Epoch J2000
Earliest precovery dateAugust 20, 1909
  • 49.305 AU
  • (7.37593 billion km)
  • February 2114
  • 29.658 AU
  • (4.43682 billion km)[1]
  • (September 5, 1989)[2]
  • 39.48 AU
  • (5.90638 billion km)
366.73 days[1]
4.67 km/s[1]
14.53 deg
  • 17.16°
  • (11.88° to Sun's equator)
Known satellites5
Physical characteristics
Mean radius
  • 1.779×107 km2[c]
  • 0.035 Earths
  • (7.057±0.004)×109 km3[d]
  • 0.00651 Earths
Mean density
1.854±0.006 g/cm3[4][6]
1.212 km/s[f]
  • 6.387230 d
  • 6 d, 9 h, 17 m, 36 s
Equatorial rotation velocity
47.18 km/h
122.53° (to orbit)[1]
North pole right ascension
North pole declination
Albedo0.49 to 0.66 (geometric, varies by 35%)[1][8]
Surface temp.minmeanmax
Kelvin33 K44 K (−229 °C)55 K
13.65[1] to 16.3[9]
(mean is 15.1)[1]
0.06″ to 0.11″[1][g]
Surface pressure
1.0 Pa (2015)[6][12]
Composition by volumeNitrogen, methane, carbon monoxide[11]
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 and is the largest known plutoid (or ice dwarf).

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 (after Eris) 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.

The New Horizons spacecraft performed a flyby of Pluto on July 14, 2015, becoming the first ever spacecraft to do so. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of 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.



The same area of night sky with stars, shown twice, side by side. One of the bright points, located with an arrow, changes position between the two images.
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.[13] 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".[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 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 earliest made by the Yerkes Observatory on August 20, 1909.[17]

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.[18] 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.[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 on March 13, 1930.[15] Pluto has yet to complete a full orbit of the Sun since its discovery because one Plutonian year is 247.68 years long.[20]


The discovery made headlines around the globe.[21] 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.[22] Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did.[22] Constance Lowell proposed Zeus, then Percival and finally Constance. These suggestions were disregarded.[23]

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.[24] 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.[24]

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.[25] The name was announced on May 1, 1930.[24][26] Upon the announcement, Madan gave Venetia £5 (equivalent to 300 GBP, or 450 USD in 2014)[27] as a reward.[24]

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 (Pluto symbol.svg, Unicode U+2647, ♇) was then created as a monogram constructed from the letters "PL".[28] Pluto's astrological symbol resembles that of Neptune (Neptune symbol.svg), but has a circle in place of the middle prong of the trident (Pluto's astrological symbol.svg).

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.[29] 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.[30]

Most languages use the name "Pluto" in various transliterations.[h] 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)).[31][32][33] 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.[32] Polynesian languages also tend to use the indigenous god of the underworld, as in Māori Whiro.[32]

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.[14] Estimates of Pluto's mass were revised downward throughout the 20th century.[34]

Mass estimates for Pluto
Year Mass Estimate by
1915 7 Earth Lowell (prediction for Planet X)[14]
1931 1 Earth Nicholson & Mayall[35][36][37]
1948 0.1 (1/10) Earth Kuiper[38]
1976 0.01 (1/100) Earth Cruikshank, Pilcher, & Morrison[39]
1978 0.0015 (1/650) Earth Christy & Harrington[40]
2006 0.00218 (1/459) Earth Buie et al.[41]

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.[36][38] 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.[39] (Pluto's albedo is 1.4–1.9 times that of Earth.[1])

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,[42] 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.[43] Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist.[44] 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,[45] a view still held today.[43]


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.[46]

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.[47] Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.[48]

IAU classification

The debate came to a head in August 2006, with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three conditions for an object in the Solar System to be considered a planet:

  1. The object must be in orbit around the Sun.
  2. 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.
  3. It must have cleared the neighborhood around its orbit.[49][50]

Pluto fails to meet the third condition.[51] Its mass is substantially less than the combined mass of the other objects in its orbit: 0.07 times, in contrast to Earth, which is 1.7 million times the remaining mass in its orbit (excluding the moon).[52][50] 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".[53] 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.[54]

There has been some resistance within the astronomical community toward the reclassification.[55][56][57] Alan Stern, principal investigator with NASA's New Horizons mission to Pluto, derided the IAU resolution, stating that "the definition stinks, for technical reasons".[58] 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.[59] He argued that all big spherical moons, including the Moon, should likewise be considered planets.[60] He also stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.[59] Marc W. Buie, then at the Lowell Observatory petitioned against the definition.[61] 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."[62]

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".[63] 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.[64][65] 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."[66] 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.[67]

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".[68]

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.[69] Entitled "The Great Planet Debate",[70] the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of planet.[71] 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.[72][73][74]

Other Languages
Afrikaans: Pluto
Alemannisch: Pluto
አማርኛ: ፕሉቶ
Ænglisc: Pluto
العربية: بلوتو
অসমীয়া: প্লুটো
Avañe'ẽ: Plutõ
azərbaycanca: Pluton
تۆرکجه: پلوتو
বাংলা: প্লুটো
Bân-lâm-gú: Mê-ông-chheⁿ
Basa Banyumasan: Pluto
башҡортса: Плутон
беларуская (тарашкевіца)‎: Плютон
भोजपुरी: प्लूटो
bosanski: Pluton
Чӑвашла: Плутон
Deutsch: Pluto
Diné bizaad: Tłóotoo
eesti: Pluuto
emiliàn e rumagnòl: Plutån
Esperanto: Plutono
فارسی: پلوتون
føroyskt: Pluto
Gaelg: Pluto
Gàidhlig: Plùto
galego: Plutón
ગુજરાતી: પ્લૂટો
客家語/Hak-kâ-ngî: Mèn-vòng-sên
한국어: 명왕성
Hawaiʻi: ‘Ilioki
հայերեն: Պլուտոն
hrvatski: Pluton
Ido: Plutono
Ilokano: Pluto
Bahasa Indonesia: Pluto
interlingua: Pluton
ᐃᓄᒃᑎᑐᑦ/inuktitut: ᑉᓘᑐ
Ирон: Плутон
isiZulu: UPlutho
עברית: פלוטו
Jawa: Pluto
Kabɩyɛ: Plʊtɔɔ
ಕನ್ನಡ: ಪ್ಲುಟೊ
Kapampangan: Pluto
ქართული: პლუტონი
kaszëbsczi: Pluton
Kiswahili: Pluto
коми: Плутон
Kreyòl ayisyen: Pliton (planèt)
kurdî: Pluto
Кыргызча: Плутон
кырык мары: Плутон
Lëtzebuergesch: (134340) Pluto
лезги: Плутон
Lingua Franca Nova: Pluto
la .lojban.: pluton
Malagasy: Ploto
മലയാളം: പ്ലൂട്ടോ
მარგალური: პლუტონი
مازِرونی: پلوتو
Bahasa Melayu: Pluto
Mìng-dĕ̤ng-ngṳ̄: Mìng-uòng-sĭng
Mirandés: Pluton
монгол: Дэлхийн ван
မြန်မာဘာသာ: ပလူတို
नेपाल भाषा: प्लुटो
日本語: 冥王星
Napulitano: Plutone
norsk: Pluto
norsk nynorsk: Dvergplaneten Pluto
олык марий: Плутон
ଓଡ଼ିଆ: ପ୍ଲୁଟୋ
oʻzbekcha/ўзбекча: Pluton
ਪੰਜਾਬੀ: ਪਲੂਟੋ
پنجابی: پلوٹو
Patois: Pluuto
ភាសាខ្មែរ: ភ្លុយតុង
Plattdüütsch: Pluto (Dwargplanet)
português: Plutão
Qaraqalpaqsha: Pluton (planeta)
română: Pluto
romani čhib: Yama
русский: Плутон
संस्कृतम्: प्लूटो-ग्रहः
Scots: Pluto
Seeltersk: Pluto
sicilianu: Plutoni
සිංහල: ප්ලූටෝ
Simple English: Pluto
سنڌي: پلوٽو
slovenčina: 134340 Pluto
slovenščina: Pluton
Soomaaliga: Buluuto
کوردی: پلووتۆ
српски / srpski: Плутон
srpskohrvatski / српскохрватски: Pluton
Basa Sunda: Pluto
suomi: Pluto
svenska: Pluto
Tagalog: Pluto
தமிழ்: புளூட்டோ
తెలుగు: ప్లూటో
тоҷикӣ: Плутон
Türkçe: Plüton
тыва дыл: Плутон
اردو: پلوٹو
ئۇيغۇرچە / Uyghurche: پلۇتون
Tiếng Việt: Sao Diêm Vương
Võro: Pluuto
文言: 冥王星
Winaray: Pluton
Wolof: Pluton
吴语: 冥神星
ייִדיש: פלוטאן
Yorùbá: Plùtò
粵語: 冥王星
Zazaki: Pluton
žemaitėška: Plutuons (planeta)
中文: 冥王星