Pluto and Charon. Artwork of the planet Pluto and its moon Charon (right), shown to scale. Pluto is the most distant planet from the Sun. Its diameter is less than a fifth of Earth's and its surface area is about the same as that of Russia. 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. It is thought they are a mixture of rock and ices, which include frozen gases at the extremely cold (below -200 degrees Celsius) surface temperatures. Impact craters are seen on the surfaces.