How Habitable is an Earth-Like Exoplanet?

Looking for planets outside the solar system is cool and all, but the obvious subtext behind much of the searching is the possibility of finding another Earth out there -- a relatively small, mostly rocky planet with liquid water and, thus, habitable conditions. Today scientists announce they've found a planet that meets all but one of those criteria. The planet named Kepler-78b, some 400 light-years away, was already known to be just 1.2 times bigger than Earth, but two papers in Nature t