Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest known trans-Neptunian object by volume, by a small margin, but is less massive than Eris. Like other Kuiper belt objects, Pluto is made primarily of ice and rock and is much smaller than the inner planets. Pluto has roughly one-sixth the mass of Earth's moon, and one-third its volume.
This article is about the dwarf planet. For the deity, see Pluto (mythology). For other uses, see Pluto (disambiguation).Discovery
February 18, 1930
(134340) Pluto
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(historically astronomical, now mostly astrological) or .svg){kind=small}
(mostly astrological)
August 20, 1909
- 49.305 AU
- (7.37593 billion km)
- February 2114
- 39.482 AU
- (5.90638 billion km)
0.2488
366.73 days[3]
4.743 km/s[3]
14.53 deg
- 17.16°
- (11.88° to Sun's equator)
110.299°
113.834°
2,376.6±1.6 km (observations consistent with a sphere, predicted deviations too small to be observed)[5]
<1%[7]
- 1.774443×107 km2[c]
- 0.035 Earths
- (7.057±0.004)×109 km3[d]
- 0.00651 Earths
1.853±0.004 g/cm3[8]
1.212 km/s[f]
- −6.387230 d
- −6 d, 9 h, 17 m, 36 s
47.18 km/h
122.53° (to orbit)[3]
132.993°[10]
−6.163°[10]
−0.44[12]
Pluto has a moderately eccentric and inclined orbit, ranging from 30 to 49 astronomical units (4.5 to 7.3 billion kilometers; 2.8 to 4.6 billion miles) from the Sun. Light from the Sun takes 5.5 hours to reach Pluto at its orbital distance of 39.5 AU (5.91 billion km; 3.67 billion mi). Pluto's eccentric orbit periodically brings it closer to the Sun than Neptune, but a stable orbital resonance prevents them from colliding.
Pluto has five known moons: Charon, the largest, whose diameter is 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, and they are tidally locked. The New Horizons mission was the first spacecraft to visit Pluto and its moons, making a flyby on July 14, 2015, and taking detailed measurements and observations.
Pluto was discovered in 1930 by Clyde W. Tombaugh, making it by far the first known object in the Kuiper belt. It was immediately hailed as the ninth planet, but it was always the odd object out,[15]: 27 and its planetary status was questioned when it was found to be much smaller than expected. These doubts increased following the discovery of additional objects in the Kuiper belt starting in the 1990s, and particularly the more massive scattered disk object Eris in 2005. In 2006, the International Astronomical Union (IAU) formally redefined the term planet to exclude dwarf planets such as Pluto. Many planetary astronomers, however, continue to consider Pluto and other dwarf planets to be planets.
Rotation
Pluto's rotation period, its day, is equal to 6.387 Earth days.[3][98] Like Uranus and 2 Pallas, 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.[99] The reason for this unusual orientation has been debated. Research from the 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.[100] 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 polar wandering effect seen on Pluto would be observed on Earth were the Antarctic ice sheet several times larger.[101]
Multispectral Visual Imaging Camera image of Pluto in enhanced color to bring out differences in surface composition.
Distribution of numerous impact craters and basins on both Pluto and Charon. The variation in density (with none found in Sputnik Planitia) indicates a long history of varying geological activity. Precisely for this reason, the confidence of numerous craters on Pluto remain uncertain.[117] The lack of craters on the left and right of each map is due to low-resolution coverage of those anti-encounter regions.
![Distribution of numerous impact craters and basins on both Pluto and Charon. The variation in density (with none found in Sputnik Planitia) indicates a long history of varying geological activity. Precisely for this reason, the confidence of numerous craters on Pluto remain uncertain.[117] The lack of craters on the left and right of each map is due to low-resolution coverage of those anti-encounter regions.](http://upload.wikimedia.org/wikipedia/commons/thumb/1/13/Pluto_Charon_crater_map_Robbins_Dones_2023.jpg/246px-Pluto_Charon_crater_map_Robbins_Dones_2023.jpg)
Geologic map of Sputnik Planitia and surroundings (context), with convection cell margins outlined in black
Regions where water ice has been detected (blue regions)
