Pluto

Until 2006, Pluto was considered to be the ninth planet of the solar system. The mass of Pluto is less than one-fifth that of the Moon.

Image of Pluto from NASA, ESA, and M. Buie (Southwest Research Institute)

However it is now known that Pluto is just one of many dwarf planets, and it is now classified as such.

Pluto's orbit lies in a different plane from all the other planets.

It probably consists of a core of icy rock covered by a thick layer of water ice. The surface is reddish in color and contains frozen methane, nitrogen and carbon monoxide. There is a thin atmosphere.