Pluto (IPA: /ˈplu.toʊ/), also designated 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun. Originally considered a planet, Pluto has since been recognised as the largest member of a distinct region called the Kuiper belt. Like other members of the belt, it is primarily composed 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 orbit that takes it from 29 to 49 AU (4.3–7.3 billion km / 2.7–4.5 billion mi) from the Sun, and is highly inclined with respect to the planets. As a result, Pluto occasionally comes closer to the Sun than the planet Neptune.
Pluto and its largest satellite, Charon, are often considered a binary system because the barycentre of their orbits does not lie within either body. However, the International Astronomical Union (IAU) has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, Charon remains a moon of Pluto. Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.
From the time of 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, most notably the scattered disc object Eris, which is 27% more massive than Pluto. On August 24, 2006 the IAU defined the term "planet" for the first time. This definition excluded Pluto from planethood, and reclassified it under the new category of dwarf planet along with Eris and Ceres. After the reclassification, Pluto was added to the list of minor planets and given the number 134340.