Pluto
From Wikipedia, the free encyclopedia
This article is about the dwarf planet. For other uses, see Pluto (disambiguation).
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| Discovery | |||||||
|---|---|---|---|---|---|---|---|
| Discovered by | Clyde W. Tombaugh | ||||||
| Discovery date | February 18, 1930 | ||||||
| Designations | |||||||
| MPC designation | 134340 Pluto | ||||||
| Pronunciation |  i /ˈpluːtoʊ/,[note 2] | ||||||
| Named after | Pluto | ||||||
| Minor planet category | dwarf planet, TNO, plutoid, KBO, plutino | ||||||
| Adjective | Plutonian | ||||||
| Epoch J2000 | |||||||
| Aphelion | 7,375,927,931 km 49.305 032 87 AU | ||||||
| Perihelion | 4,436,824,613 km 29.658 340 67 AU | ||||||
| Semi-major axis | 5,906,376,272 km 39.481 686 77 AU | ||||||
| Eccentricity | 0.248 807 66 | ||||||
| Orbital period | 90,613.305 days 248.09 years 14,164.4 Pluto solar days[1] | ||||||
| Synodic period | 366.73 days | ||||||
| Average orbital speed | 4.666 km/s | ||||||
| Inclination | 17.141 75° 11.88° to Sun's equator | ||||||
| Longitude of ascending node | 110.303 47° | ||||||
| Argument of perihelion | 113.763 29° | ||||||
| Satellites | 3 | ||||||
| Physical characteristics | |||||||
| Mean radius | 1,153 ± 10 km[2] 0.18 Earths | ||||||
| Surface area | 1.665 × 107 km²[note 3] 0.033 Earths | ||||||
| Volume | 6,39 × 109 km³[note 4] 0.0059 Earths | ||||||
| Mass | (1.305 ± 0.007) × 1022 kg[2] 0.002 1 Earths 0.178 moon | ||||||
| Mean density | 2.03 ± 0.06 g/cm³[2] | ||||||
| Equatorial surface gravity | 0.658 m/s²[note 5] 0.067 g | ||||||
| Escape velocity | 1.229 km/s[note 6] | ||||||
| Sidereal rotation period | −6.387 230 day 6 d 9 h 17 m 36 s | ||||||
| Equatorial rotation velocity | 47.18 km/h | ||||||
| Axial tilt | 119.591 ± 0.014° (to orbit)[2][note 7] | ||||||
| North pole right ascension | 133.046 ± 0.014°[2] | ||||||
| North pole declination | −6.145 ± 0.014°[2] | ||||||
| Albedo | 0.49–0.66 (varies by 35%)[3][4] | ||||||
| Surface temp. Kelvin |
| ||||||
| Apparent magnitude | 13.65[4] to 16.3[5] (mean is 15.1)[4] | ||||||
| Absolute magnitude (H) | −0.7[6] | ||||||
| Angular diameter | 0.065" to 0.115"[4][note 8] | ||||||
| Atmosphere | |||||||
| Surface pressure | 0.30 Pa (summer maximum) | ||||||
| Composition | nitrogen, methane | ||||||
Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth's Moon and a third its volume. It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun than Neptune.
From its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's very low mass, its status as a major planet began to be questioned.[7] In the late 20th and early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris in 2005, which is 27% more massive than Pluto.[8] On August 24, 2006, the International Astronomical Union (IAU) defined the term "planet" for the first time. This definition excluded Pluto as a planet, and added it as a member of the new category "dwarf planet" along with Eris and Ceres.[9] After the reclassification, Pluto was added to the list of minor planets and given the number 134340.[10][11] A number of scientists continue to hold that Pluto should be classified as a planet.[12]
Pluto and its largest moon, Charon, are sometimes treated together as a binary system because the barycentre of their orbits does not lie within either body.[13] 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.[14] Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.[15]
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