Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It was the first object discovered in the Kuiper belt and remains the largest known body in that area. After Pluto was discovered in 1930, it was declared the ninth planet from the Sun. However, beginning in the 1990s, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt and the scattered disc, including the dwarf planet Eris, leading the International Astronomical Union (IAU) in 2006 to define the term planet formally—excluding Pluto and reclassifying it as a dwarf planet.
|Discovered by||Clyde W. Tombaugh|
|Discovery site||Lowell Observatory|
|Discovery date||February 18, 1930|
|Earliest precovery date||August 20, 1909|
Average orbital speed
|Dimensions||2,376.6±1.6 km (observations consistent with a sphere, predicted deviations too small to be observed)|
Equatorial rotation velocity
|122.53° (to orbit)|
North pole right ascension
North pole declination
|13.65 to 16.3 |
(mean is 15.1)
|0.06″ to 0.11″|
|1.0 Pa (2015)|
|Composition by volume||Nitrogen, methane, carbon monoxide|
Pluto is 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—one-sixth the mass of the Moon and one-third its volume. It 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. Therefore, Pluto periodically comes closer to the Sun than Neptune. Still, a stable orbital resonance with Neptune prevents them from colliding. As a result, light from the Sun takes 5.5 hours to reach Pluto at its average distance (39.5 AU [5.91 billion km; 3.67 billion mi]).
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. Pluto and Charon are tidally locked.
The New Horizons spacecraft performed a flyby of Pluto on July 14, 2015, becoming the first and, to date, only 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 19,000 km (12,000 mi) to the orbiting moon.