Kepler-186f is seen in a NASA artist's concept released April 17, 2014. The planet is the first validated Earth-size planet to orbit a distant star in the habitable zone which is a range of distance from a star where liquid water might pool on the planet's surface. The discovery is the closest scientists have come so far to finding a true Earth twin. The star, known as Kepler-186 and located about 500 light years away in the constellation Cygnus, is smaller and redder than the sun. REUTERS/NASA/JPL-Caltech/Handout