Kepler-78b, which is slightly larger than Earth, is the most similar in size to the Earth of any exoplanet yet directly discovered. However, the similarities end there. 

Kepler-78b circles a Sun-like star at an extremely close orbit - about 40 times closer, in fact, than planet Mercury. That means that, Kepler-78b, according to planet modeling predictions, should not exist. It certainly cannot support life. It is so hot on Kepler-78b that even rock is liquid. Eventually the planet is doomed. It will eventually decay and merge with its parent star.

Illustration Credit: David A. Aguilar (CfA