Kepler-186f exoplanet, illustration. Discovered on 17 April 2014, this was the first Earth-sized planet in a habitable zone, meaning liquid water may exist on its surface. Kepler-186f, shown here as a rocky planet, is around ten percent larger than Earth, but its precise composition is not known. It orbits its star once every 130 days and receives one-third the energy that Earth does from the Sun. The Kepler-186 system is about 500 light-years from Earth in the constellation of Cygnus. The four inner planets are also shown in this artwork near the parent star (lower left). The parent star (a red dwarf) is half the size and mass of the Sun. Image published in 2014.