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Pluto Astronomical symbol of Pluto
Computer-generated map of Pluto from Hubble images, synthesised true colour[note 1] and among the highest resolutions possible with current technology
Discovered byClyde W. Tombaugh
Discovery timeFebruary 18, 1930
Name134340 Pluto
How to say ithow to say: /ˈpluːtoʊ/,[note 2]
Groupdwarf planet,
Reference date J2000
Longest distance from the Sun7,375,927,931 km
49.305 032 87 AU
Shortest distance from the Sun4,436,824,613 km
29.658 340 67 AU
Longest distance from the center of its orbital path
("semi-major axis")
5,906,376,272 km
39.481 686 77 AU
How egg-shaped its orbit is
0.248 807 66
How long it takes to complete an orbit90,613.305 days
248.09 years
14,164.4 Pluto solar days[1]
How long an orbit seems to take
(from the central body)
366.73 days
Average speed4.666 km/s
Angle above the reference plane
17.141 75°
11.88° to Sun's equator
Longitude of where it comes up through the reference plane110.303 47°
Angle between its shortest distance from what it orbits around and where it comes up through the reference plane
("argument of periapsis")
113.763 29°
Natural things which orbit around it5
Size and Other Qualities
Average distance from its center to its surface1,153 ± 10 km[2]
0.18 Earths
Area of its surface1.665×107 km²[note 3]
0.033 Earths
Volume inside it6,39×109 km³[note 4]
0.0059  Earths
Mass(1.305 ± 0.007)×1022 kg[2]
0.002 1 Earths
0.178 moon
Average density2.03 ± 0.06 g/cm³[2]
Gravity at its surface0.658 m/s²[note 5]
0.067 g
Slowest speed able to escape into space
("escape velocity")
1.229 km/s[note 6]
How long it takes to turn around one time
(in relation to the stars)
−6.387 230 day
6 d 9 h 17 m 36 s
Turning speed47.18 km/h
Angle at which it turns
(in relation to its orbit)
119.591 ± 0.014° (to orbit)[2][note 7]
Long. around the celestial equator
("right ascension")
133.046 ± 0.014°[2]
Angle above the celestial equator
−6.145 ± 0.014°[2]
How much light it reflects0.49–0.66 (varies by 35%)[3][4]
Surface temp. Min. Avg. Max.
Kelvin 33 K 44 K 55 K
Seeming brightness
("apparent magnitude")
up to 13.65 (mean is 15.1)[4]
True brightness
("absolute magnitude")
Seeming size
("angular diameter")
0.065" to 0.115"[4][note 8]
Pressure0.30 Pa (summer maximum)
Make upnitrogen, methane

Pluto is the second-largest dwarf planet in the Solar System. It is smaller than the largest known dwarf planet, Eris. Its formal name is 134340 Pluto. The dwarf planet is the tenth-largest body that moves around the Sun. At first, Pluto was called a planet. Now, it is considered the second largest of the bodies in the Kuiper belt.[note 9]

Like other members of the Kuiper belt, Pluto is mainly made of rock and ice. It is quite small. It is about a fifth (⅕) of the weight of the Earth's Moon. It is only a third (⅓) its volume. It has an odd orbit and this orbit is very sloped. It takes Pluto to 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to sometimes go closer to the Sun than Neptune.

Since it was discovered in 1930, Pluto was thought to be the Solar System's ninth planet. In the late 1970s, the minor planet 2060 Chiron was found and people learned that Pluto had a small size. They asked why it was a major planet from then on because it was really small.[6] Later, in the early 21st century, the scattered disc object Eris and other objects like Pluto were discovered. Eris is 27% larger than Pluto.[7] On August 24, 2006, the International Astronomical Union (IAU) gave a definition to the word "planet" for the first time. By this definition, Pluto was not a planet anymore. It became a "dwarf planet" along with Eris and Ceres.[8] After this, Pluto was put on the list of minor planets. It was given the number 134340.[9][10] A number of scientists continue to hold that Pluto should be classified as a planet.[11]

Pluto and its largest moon, Charon, are sometimes called a "binary system". This is because the barycentre of their orbits does not lie within them.[12] The IAU has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, they classify Charon as a moon of Pluto.[13] Pluto has four known smaller moons, Nix and Hydra, discovered in 2005,[14] Kerberos, discovered in 2011, and Styx, discovered in 2012.


In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the planet Neptune existed. He discovered this after studying the orbit of Uranus.[15] Neptune was seen later in the late 19th century. Astronomers then felt that Uranus's orbit was being disturbed by another planet. In 1906, Percival Lowell, a person from Boston began an extensive to search the ninth planet. He called it "Planet X".[16] By 1909, Lowell and William H. Pickering had suggested many possible places in the Solar System where the planet could be.[17] Lowell continued the research till 1916. However, it fetched no good result. On March 19, 1915, his observatory had captured two images of Pluto. Lowell did not know this. The pictures was not recognized then for what it was.[17][18]

Constance Lowell, Percival's widow had a ten-year long legal battle for Percival's estate. For this reason, the search for Planet X again started in 1929.[19] The director of the mission, Vesto Melvin Slipher, gave the job to Clyde Tombaugh, a 23-year-old Kansas man. Tombaugh had just come at the Lowell Observatory. Slipher had been impressed by his astronomical drawings.[19]

Tombaugh's had to systematically get pictures of the night sky in pairs every two weeks. Then, he had to look at each pair. This was to know whether any object had shifted. He used a machine called a blink comparator. He quickly shifted between the different views of each of the plates. This helped him to see whether any object had changed their position. On February 18, 1930, Tombaugh discovered an object which seemed to move from the photographic plates taken on January 23 and January 29 of the year. Another picture taken on January 21 confirmed this.[20] After the observatory did more research about it, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930.[17]


The discovery made headlines across the Earth. The Lowell Observatory had the right to name the new object. They received over 1000 suggestions from all over the world. Some proposed Atlas as the name. Others wanted to name it Zymal.[21] Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did.[21] Constance Lowell proposed Zeus, then Lowell, and finally Constance. These suggestions were not used.[22]

The name Pluto was proposed by Venetia Burney (later Venetia Phair). She was an 11-year-old schoolgirl in Oxford, England then.[23] Venetia was interested in classical mythology and astronomy. The name was of the Roman god of the underworld. She thought it was a good name for a dark and cold world. She suggested it when she was talking with her grandfather Falconer Madan. He was a former librarian at the University of Oxford's Bodleian Library. Madan passed the name to Professor Herbert Hall Turner. Turner told this proposed name to the astronomers in the United States.[24]

The object was officially named on March 24, 1930.[25][26] Each member of the Lowell Observatory was allowed to vote on a short-list of three names. The names were Minerva (which was already the name for an asteroid), Cronus, and Pluto. Pluto received all votes.[27] The name was announced on May 1, 1930.[23] Upon the announcement, Madan gave Venetia five pounds as a reward.[23]

The name became popular in culture. The Disney character, Pluto, introduced in the same year. He was named in the object's honour.[28] In 1941, Glenn T. Seaborg named the newly created element plutonium after Pluto. This was to keep the tradition of naming new elements after newly discovered planets. For example, uranium had been named after Uranus, and neptunium after Neptune.[29]


  1. The HST observations were made in two wavelengths, which is insufficient to directly make a true colour image. However, the surface maps at each wavelength do limit the shape of the spectrum that could be produced by the materials that are potentially on Pluto's surface. These spectra, generated for each resolved point on the surface, are then converted to the RGB colour values seen here. See Buie et al., 2010.
  2. In US dictionary transcription, US dict: plōō′·tō. From the Latin: Plūto
  3. Surface area derived from the radius r: [math]4\pi r^2[/math].
  4. Volume v derived from the radius r: [math]4\pi r^3/3[/math].
  5. Surface gravity derived from the mass m, the gravitational constant G and the radius r: [math]Gm/r^2[/math].
  6. Escape velocity derived from the mass m, the gravitational constant G and the radius r: 2Gm/r.
  7. Based on the orientation of Charon's orbit, which is assumed the same as Pluto's spin axis due to the mutual tidal locking.
  8. Based on geometry of minimum and maximum distance from Earth and Pluto radius in the factsheet
  9. Although Eris is larger than Pluto, it is in the scattered disc. Wikipedia convention says that is in the Kuiper belt, so it is the biggest thing in it.


  1. Seligman, Courtney. "Rotation Period and Day Length". Retrieved 2009-08-13.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 M. W. Buie, W. M. Grundy, E. F. Young, L. A. Young, S. A. Stern (2006). "Orbits and photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2". Astronomical Journal 132: 290. doi:10.1086/504422 . arXiv:astro-ph/0512491.
  3. Calvin J. Hamilton (2006-02-12). "Dwarf Planet Pluto". Views of the Solar System. Retrieved 2007-01-10.
  4. 4.0 4.1 4.2 David R. Williams. "Planetary Fact Sheet". NASA. Retrieved 2007-03-31.
  5. "JPL Small-Body Database Browser: 134340 Pluto". Retrieved 2008-06-12.
  6. Ian Ridpath (December 1978). "Pluto—Planet or Impostor?". Astronomy: 6–11.
  7. "Astronomers Measure Mass of Largest Dwarf Planet". hubblesite. 2007. Retrieved 2007-11-03.
  8. A. Akwagyiram (2005-08-02). "Farewell Pluto?". BBC News. Retrieved 2006-03-05.
  9. T. B. Spahr (2006-09-07). "MPEC 2006-R19 : Editorial Notice". Minor Planet Center. Retrieved 2006-09-07.
  10. D. Shiga (2006-09-07). "Pluto added to official "minor planet" list". NewScientist. Retrieved 2006-09-08.
  11. Richard Gray (2008-08-10). "Pluto should get back planet status, say astronomers". The Telegraph. Retrieved 2008-08-09.
  12. C.B. Olkin, L.H. Wasserman, O.G. Franz (2003). "The mass ratio of Charon to Pluto from Hubble Space Telescope astrometry with the fine guidance sensors" (PDF). Icarus 164: 254–259. doi:10.1016/S0019-1035(03)00136-2 . Retrieved 2007-03-13.
  13. O. Gingerich (2006). "The Path to Defining Planets" (PDF). Harvard-Smithsonian Center for Astrophysics and IAU EC Planet Definition Committee chair. Retrieved 2007-03-13.
  14. B. Sicardy, W. Beisker et al. (2006). "Observing Two Pluto Stellar Approaches In 2006: Results On Pluto's Atmosphere And Detection Of Hydra". Retrieved 2007-03-13.
  15. K. Croswell (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. The Free Press. pp. 43. ISBN 978-0684832524 .
  16. Tombaugh, C. W. (1946). "The Search for the Ninth Planet, Pluto". Astronomical Society of the Pacific Leaflets 5: 73–80.
  17. 17.0 17.1 17.2 W. G. Hoyt (1976). "W. H. Pickering's Planetary Predictions and the Discovery of Pluto". Isis 67 (4): 551–564.. doi:10.1086/351668 . Retrieved 2007-06-27.
  18. Mark Littman (1990). Planets Beyond: Discovering the Outer Solar System. Wiley. pp. 70. ISBN 047151053X .
  19. 19.0 19.1 Croswell, p. 50
  20. Croswell p. 52
  21. 21.0 21.1 J. Rao (March 11, 2005). "Finding Pluto: Tough Task, Even 75 Years Later". Retrieved 2006-09-08.
  22. B. Mager. "The Search Continues". Pluto: The Discovery of Planet X. Retrieved 2007-03-27.
  23. 23.0 23.1 23.2 P. Rincon (2006-01-13). "The girl who named a planet". Pluto: The Discovery of Planet X. BBC News. Retrieved 2007-04-12.
  24. K. M. Claxton. "The Planet 'Pluto'". Parents' Union School Diamond Jubilee Magazine, 1891–1951 (Ambleside: PUS, 1951), p. 30–32. Retrieved 2007-10-15.
  25. "The Trans-Neptunian Body: Decision to call it Pluto". The Times: pp. 15. May 27, 1930.
  26. "Name Pluto Given to Body Believed to Be Planet X". The New York Times. The Associated Press (New York City): p. 1. May 25, 1930. ISSN 1556067 . "Pluto, the title of the Roman gods of the region of darkness, was announced tonight at Lowell Observatory here as the name chosen for the recently discovered trans-Neptunian body, which is believed to be the long-sought Planet X."
  27. Croswell pp. 54–55
  28. Allison M. Heinrichs (2006). "Dwarfed by comparison". Pittsburgh Tribune-Review. Retrieved 2007-03-26.
  29. David L. Clark and David E. Hobart (2000). "Reflections on the Legacy of a Legend" (PDF). Retrieved 2007-08-09.

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