pluto

Pluto (pronounced en-us-Pluto.ogg /ˈpluːtoʊ/ (help·info), from Latin: Plūto), formal designation (134340) Pluto, is the second-largest known dwarf planet in the Solar System (after Eris) and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto is now considered the largest member of a distinct population called the Kuiper belt.[7]

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 a highly eccentric and highly inclined orbit.

Pluto's eccentricity takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun, causing Pluto to occasionally come closer to the Sun than Neptune. Pluto and its largest moon, Charon, are often treated together as a binary system because the barycentre of their orbits does not lie within either body.[8] The International Astronomical Union (IAU) has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, Charon is classified as a moon of Pluto.[9] Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.[10]

From its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 20th and early 21st centuries, however, many objects similar to Pluto were discovered in the outer solar system, notably the scattered disc object Eris, which is 27% more massive than Pluto.[11] On August 24, 2006, the IAU defined the term "planet" for the first time. This definition excluded Pluto, which the IAU reclassified as a member of the new category of dwarf planets along with Eris and Ceres: Just as Ceres is part of the asteroid belt without being dominant within it, so Pluto is part of the Kupier belt without being dominant within it.[12] After the reclassification, Pluto was added to the list of minor planets and given the number 134340.[13][14] A number of scientists continue to hold that Pluto should be classified as a planet