EFFECTS OF OBLIQUITY ON THE HABITABILITY OF EXOPLANETS AROUND M DWARFS

Most previous studies on how obliquity affects planetary habitability focused on planets around Sun-like stars. Their conclusions may not be applicable to habitable planets around M dwarfs due to the tidal-locking feature and associated insolation pattern of these planets. Here we use a comprehensive three-dimensional atmospheric general circulation model to investigate this issue. We find that the climates of planets with higher obliquities are generally warmer, consistent with previous studies. The mechanism of warming is, however, completely different. Significant reduction of low clouds, instead of sea-ice cover, within the substeller region (which moves if the obliquity is non-zero) is the key in warming M-dwarf planets with high obliquities. For a total insolation of 1237 W m(-2), the climate warms by 21 K when the obliquity increases from 0 degrees to 90 degrees. Correspondingly, the runaway greenhouse inner edge of the habitable zone shifts outward from 2500 to 2100 W m(-2). The moist greenhouse inner edge, based on our crude estimation, shifts less, from 2180 to 2075 W m(-2). Near the outer edge, in contrast, the climates of planets with higher obliquities are colder due to their reduced ability to maintain a hotspot at the surface. Therefore, the outer edge moves inward when obliquity is increased, opposite to the finding of previous studies on planets around Sun-like stars. Our results thus indicate that the habitable zone for M dwarfs narrows if the obliquity of their planets increases.