Habitability of Terrestrial-Mass Planets in the HZ of M Dwarfs. I. H/He-Dominated Atmospheres
Owen et al
The ubiquity of M dwarfs, combined with the relative ease of detecting terrestrial-mass planets around them, has made them prime targets for finding and characterising planets in the "Habitable Zone" (HZ). However, Kepler has revealed that terrestrial-mass exoplanets are often born with voluminous H/He envelopes, comprising mass-fractions (Menv/Mcore) ≳1\%. If these planets retain such envelopes over Gyr timescales, they will not be "habitable" even within the HZ. Given the strong X-ray/UV fluxes of M dwarfs, we study whether these planets can lose sufficient envelope-mass through photoevaporation to become habitable. We improve upon previous work by using hydrodynamic models that account for radiative cooling as well as the transition from hydrodynamic to ballistic escape. Adopting the XUV spectrum of the active M dwarf AD Leo as a template, including stellar evolution, and considering both evaporation and thermal evolution, we show that: (1) the envelope-mass lost is significantly lower than previous estimates that use an "energy-limited" formalism and ignore the transition to Jeans escape, (2) at the inner edge of the HZ, planets with a core mass ≲0.9 M⊕, can lose enough H/He to be habitable at late times if their initial H/He envelope mass-fraction is ∼1\%, (3) at the outer edge of the HZ, evaporation is ineffective at removing a ∼1\% H/He envelope even from cores down to 0.8 M⊕. Thus, if planets form with bulky H/He envelopes, only those with low core masses may eventually become habitable. Cores ≳ 1 M⊕ with ≳1\% natal H/He envelopes, will not be habitable in the HZ of M dwarfs.