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.