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Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System (after Eris) and the tenth-most-massive body observed directly orbiting the Sun. Originally classified as the ninth planet from the Sun, Pluto was recategorized as a dwarf planet and plutoid due to the discovery that it is one of several large bodies within the newly charted Kuiper belt.
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. As of 2011, it is 32.1 AU from the Sun.
From its discovery in 1930 until 2006, Pluto was classified as a planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's relatively low mass, its status as a major planet began to be questioned. 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. On August 24, 2006, the International Astronomical Union (IAU) defined what it means to be a "planet" within the Solar System. This definition excluded Pluto as a planet and added it as a member of the new category "dwarf planet" along with Eris and Ceres. After the reclassification, Pluto was added to the list of minor planets and given the number 134340. A number of scientists continue to hold that Pluto should be classified as a planet.
Pluto has four known moons, the largest being Charon discovered in 1978, along with Nix and Hydra, discovered in 2005, and the provisionally named S/2011 P 1, discovered in 2011. Pluto and Charon are sometimes described as a binary system because the barycenter of their orbits does not lie within either body. However, the IAU has yet to formalise a definition for binary dwarf planets, and as such Charon is officially classified as a moon of Pluto.