Earth-like exoplanet, illustration. In astronomy and astrobiology, the circumstellar habitable zone, or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the CHZ are based on Earth's position in the Solar System and the amount of radiant energy. Among exoplanets, a review in 2015 came to the conclusion that Kepler-621, Kepler-186f and Kepler-442b were likely the best candidates for being potentially habitable. Kepler-186f is similar in size to Earth with a 1.2 Earth radius measure, and is located to the outer-edge of the habitable zone around its red dwarf star.