The first Earth-sized planet found in another star’s habitable zone was Kepler-186f.
NASA announced Kepler-186f on April 17, 2014. The Kepler space telescope found it through the transit method, detecting the slight dimming that occurs when the planet crosses its star. It orbits Kepler-186, a cool red dwarf about 580 light-years away in the constellation Cygnus.
Kepler-186f has a radius about 1.1 to 1.2 times Earth’s and completes an orbit in roughly 130 days. Its distance from its faint star places it within that system’s habitable zone, the region where surface liquid water could be possible under suitable atmospheric conditions.
“Habitable zone” does not mean “inhabited” or even definitively habitable. Kepler-186f’s atmosphere, surface composition and mass remain uncertain, so scientists cannot say whether it has liquid water or life. It is also not an Earth twin: its host is an M-type red dwarf, unlike our Sun. Kepler-22b and Kepler-62e were earlier habitable-zone discoveries, but they are larger than Earth.