So far, The Earth is the most well-documented planet - and the only one known to be capable of sustaining life - that humans are aware of, what with us living there and all that, but there's bound to be other similar habitable planets orbiting different stars, in other Solar Systems. But we don't know much about them yet.
- (the Earth)
- Planet X
They're the main ones, anyway. There's another sizeable-ish thing in between Uranus and Planet X, which was called Pluto but then some guys decided it wasn't really a planet after all so it doesn't count. Apparently.