Smallest Exoplanet So Far

NASA's Kepler spacecraft has found the smallest exoplanet yet discovered. The world is 1.4 times the size of Earth, 4.6 times as massive, and orbits a star similar to the Sun. That star is 500 light years away.

The size and mass of the exoplanet mean it's clearly a rocky world, like Earth-- another breakthrough for planet hunters. Unfortunately, its orbit around its star is 20 times closer than Mercury's circuit around the Sun, so this is a hot, inhospitable place. Its density, by the way, is something like that of Mercury, which may or may not hint at a basic, close relationship between dense, rocky worlds and Sun-like stars