Pluto no longer a planet, but few people know about former planet Orpheus.

If We Had No Moon is narrated by Patrick Stewart. It was on TV not too long ago, but you can find an archived version at this link (requires BitTorrent and the xvidmovies Codec).

In it, it explains the theory that there used to be a planet between Earth and Mars named Orpheus. Orpheus appears to have collided with the Earth millions of years ago because its orbit was too close to Earth, which resulted in the formation of Earth's moon. There is alot of scientific evidence to support the theory, including the composition of the moon rocks brought back.