Artwork of the dwarf planet Pluto. Pluto, some 6 billion kilometres from the Sun, is a small, rocky, icy world with a thin atmosphere and a large moon called Charon (right). Astronomers have also discovered that in addition to Charon, Pluto plays host to at least four other moons. In this illustration the innermost satellite (after Charon) is shown in front of Pluto. Known as Styx, this moon is much smaller than Charon, with a diameter no larger than 25 km. Charon, by contrast, measures 1205 km across. The other moons, in order of distance from Pluto, are Nix, Kerberos and Hydra.