<Tr> <Th> Composition by volume </Th> <Td> Nitrogen, methane, carbon monoxide </Td> </Tr> <P> Pluto (minor - planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune . It was the first Kuiper belt object to be discovered . </P> <P> Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun . After 1992, its status as a planet 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 (IAU) to define the term "planet" formally in 2006, during their 26th General Assembly . That definition excluded Pluto and reclassified it as a dwarf planet . </P> <P> 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 (4.4--7.4 billion km) 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 (39.5 AU). </P>

What year did pluto become a dwarf planet