Since Kepler launched in 2009, 21 planets less than twice the size of Earth have been discovered in their star’s habitable zone–the region in a planetary system where liquid water might pool on the surface of an orbiting planet. These planets are plotted relative to the temperature of their star and with respect to the amount of energy received from their star in their orbit in Earth units. The sizes of the exoplanets indicate the sizes relative to one another. The images of Earth, Venus and Mars are placed on this diagram for reference. The light and dark green shaded regions indicate the conservative and optimistic habitable zone.
Credit: NASA Ames / N. Batalha and W. Stenzel