This is an image of Pluto with its moon Charon. Click on image for full size version (36K GIF) Image courtesy of NASA

Of all the planets and moons in the solar system, Pluto and Charon are the two which resemble each other the most closely. They are almost the same size, and they are very close together. They are so close together that they orbit the center of mass between them, and the center of mass goes around the sun. Pluto and Charon are so close that they may even share an atmosphere. Molecules may be drawn off Pluto and turn to solids on the surface of Charon.

The only other planet and moon in the solar system which could be a double planet are the Earth and the Moon. As in the case of the Earth and the Moon, the origin of such a system is hard to explain. The explanation which seems best is that some large object struck Pluto (and the Earth) and tore out a blob which became Charon (and the Moon) from the crust of the planet. In the case of the Moon, the Moon seems to be made of material very much like the Earth's crust and not like the Earth's more metallic interior. Likewise Charon may be made of more icy material which resembles Pluto's surface, rather than the more rocky interior of Pluto.