Pluto is the second-most massive known dwarf planet, after Eris. It is the largest object in the Kuiper belt and possibly the largest known trans-Neptunian object by volume. It is the tenth-most-massive known body directly orbiting the Sun. Like other Kuiper belt objects, Pluto is primarily made of rock and ice, and is relatively small- about 1Ú6 the mass of the Moon and 1Ú3 its volume.
It has a moderately eccentric and inclined orbit that takes it from 30 to 49 AU (4.4Ð7.4 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune. However, an orbital resonance with Neptune prevents them from colliding.
In 2014, Pluto was 32.6 AU from the Sun. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance (39.4 AU). Pluto was discovered in 1930 and was originally considered the ninth planet from the Sun. Its status as a planet fell into question following further study of it and the outer Solar System over the following 75 years. Starting in 1977 with the discovery of the minor planet Chiron, numerous icy objects with eccentric orbits were found.
The scattered disc object Eris, discovered in 2005, is 27% more massive than Pluto. The knowledge that Pluto is only one of several large icy bodies in the outer Solar System prompted the International Astronomical Union (IAU) to formally define the term “planet” in 2006, which excluded Pluto and reclassified it as a member of the new “dwarf planet” category (and specifically as a plutoid). Astronomers who oppose the exclusion assert that Pluto should remain classified as a planet and other dwarf planets, and even moons, should be added to the list of planets.
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 IAU has not formalized a definition for binary dwarf planets, and Charon is officially classified as a moon of Pluto.
