Artist's impression of the exoplanet Kepler-186f, which is the first known Earth-sized planet orbiting a star in the "habitable zone" – the range of distances from a star where liquid water might exist on the surface of an orbiting planet. Credit: NASA/Ames/SETI Institute/JPL–Caltech

Artist's impression of the exoplanet Kepler-186f, which is the first known Earth-sized planet orbiting a star in the "habitable zone" – the range of distances from a star where liquid water might exist on the surface of an orbiting planet. Credit: NASA/Ames/SETI Institute/JPL–Caltech

Artist's impression of the exoplanet Kepler-186f, which is the first known Earth-sized planet orbiting a star in the "habitable zone" – the range of distances from a star where liquid water might exist on the surface of an orbiting planet. Credit: NASA/Ames/SETI Institute/JPL–Caltech