Claim: Pluto is less massive than Eris.

Pluto -LRB- minor-planet designation : 134340 Pluto -RRB- is a dwarf planet in the Kuiper belt , a ring of bodies beyond Neptune . It was the first Kuiper belt object to be discovered .   Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun . After 1992 , its planethood was questioned following the discovery of several objects of similar size in the Kuiper belt . In 2005 , Eris , a dwarf planet in the scattered disc which is 27 % more massive than Pluto , was discovered . This led the International Astronomical Union -LRB- IAU -RRB- to define the term `` planet '' formally in 2006 , during their 26th General Assembly . That definition excluded Pluto and reclassified it as a dwarf planet .   Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun . It is the largest known trans-Neptunian object by volume but is less massive than Eris . Like other Kuiper belt objects , Pluto is primarily made of ice and rock and is relatively small -- about one-sixth the mass of the Moon and one-third its volume . It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU -LRB- 4.4 -- 7.4 billion km -RRB- from the Sun . This means that Pluto periodically comes closer to the Sun than Neptune , but a stable orbital resonance with Neptune prevents them from colliding . Light from the Sun takes about 5.5 hours to reach Pluto at its average distance -LRB- 39.5 AU -RRB- .   Pluto has five known moons : Charon -LRB- the largest , with a diameter just over half that of Pluto -RRB- , Styx , Nix , Kerberos , and Hydra . Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body .   On July 14 , 2015 , the New Horizons spacecraft became the first spacecraft to fly by Pluto . During its brief flyby , New Horizons made detailed measurements and observations of Pluto and its moons . In September 2016 , astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins , organic macromolecules that may be ingredients for the emergence of life , and produced from methane , nitrogen and other gases released from the atmosphere of Pluto and transferred about 19,000 km to the orbiting moon .