Pluto ( minor-planet designation : 134340 Pluto ) is a dwarf planet in the Kuiper belt , a ring of bodies beyond the orbit of Neptune . It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun . It is the largest known trans-Neptunian object by volume by a small margin, but is less massive than Eris . Like other Kuiper belt objects, Pluto is made primarily of ice and rock and is much smaller than the inner planets . Pluto has roughly one-sixth the mass of the Moon and one-third its volume. Originally considered a planet, its classification was changed when astronomers adopted a new definition of planet .
