Pluto (pronounce /ˈpluːtoʊ/ (help·info), from Latin: Plūto, Greek: Πλούτων), also designated 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 region called the Kuiper belt. Like other members of the belt, it 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. The 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. 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. Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.