First rocky, Earth-sized exoplanet discovered

Astronomers have found the first Earth-sized exoplanet with a rocky composition similar to that of Earth's.  This planet, called Kepler-78b, is 1.2 times as big as Earth, and weighs 1.7 times as much.  Despite these striking similarities, Kepler-78b and Earth differ in a major way.  While Earth takes 365 days to make one trip around the sun, Kepler-78b is so close to its star that it only takes it 8.5 hours  to orbit once.  This close orbit makes Kepler-78b extremely hot, and therefore incapable of supporting life.

Kepler-78b was identified as a planet candidate by the Kepler Space Telescope, which looks for dips in the light from a star as a planet passes in front of it.  It was confirmed to be a planet with observations made at the Keck Observatory in Hawaii.  

Even though we haven't found our planetary twin in Kepler-78b, it's still an incredibly interesting object to study.  Kepler-78b is a member of a class of newly discovered "ultrashort period" planets.  These are small planets that orbit their stars in a matter of hours, and astronomers are still working on figuring out how they form and wind up so close to their hosts.