Pluto. Cutaway illustration showing the interior structure of the planet Pluto. Behind it is its moon, Charon. Pluto's core consists of rock (here coloured gold). On top of this is a thick layer of ice and a thin top layer of frozen methane. Rela- tively little is known about Pluto, which is the farthest planet from the Sun and the only one never to have been visited by a spacecraft. At only 2320km across it is also the smallest planet in the Solar System. Its moon is unusual in being very close to the planet and about half its size, which suggests that Pluto and Charon were once part of a single body that somehow broke apart.