NASA today announced the discovery of what may be a tenth planet in our solar system. They're naming it Sedna, after the Inuit goddess of the sea. It is slightly larger than Quaoar, another planet-like body discovered two years ago, and slightly smaller than Pluto, discovered in 1930.
All three are at the fringes of the solar system and may or may not be planets in the proper sense of the term. As the above illustration indicates, all are smaller than the moon, which is classed as a satellite rather than a planet.