Pluto and Charon. Artwork of the planet Pluto and its moon Charon (right) seen from space with some other Kuiper Belt objects. Pluto, the most distant planet from the Sun, has a diameter of less than a fifth of Earth's. Charon, at only half its size, makes this a double planet system. Little is known about these worlds, which are yet to be visited by spacecraft from Earth. Surface temperatures are below -200 degrees Celsius. Some astronomers consider Pluto as the largest object in the Kuiper Belt, rather than a true planet. The Kuiper Belt is a cloud of icy rocks orbiting beyond Pluto. The Milky Way is seen in the background.