This rocky world — if it does indeed exist — is just 20 light-years away from our solar system. It's likely two to three times as massive as Earth and zips around its parent star, the red dwarf Gliese 581, every 30 days or so. This orbit places the planet squarely in the star's "habitable zone" — that just-right range of distances where liquid water, and perhaps life as we know it, could exist.

For more, check out this Scientific American story.