Exoplanets beyond the Conservative Habitable Zone. II. Occurrence

Abstract We show that the extension of the habitable zone (HZ) due to the presence of liquid water on the nightside of tidally locked planets, calculated in earlier works, significantly increases the number of potentially habitable planets. We calculate the occurrence of habitable planets orbiting M-, K-, and G-dwarf stars within the conservative and extended HZ, beyond the inner and outer boundaries of the conservative HZ. Integrating over the phase space in the HZ diagram and normalizing our calculation to a relatively recent analysis of the Kepler data, we show that potentially habitable planets may be as much as 50 times more abundant than in the most conservative estimate, limited to the conservative HZ of G-type stars only. We find that the extended HZ could imply more than one habitable planet per star, and hundreds of habitable planets within 10 pc from Earth.