Among the 4000 exoplanets discovered so far, about 10-20 are considered potentially habitable. Most these potentially habitable exoplanets are tidally-locked because they are all within the habitable zone around M dwarfs.
For tidally-locked exoplanets, it has been of a great concern that such kind of exoplanets may not be habitable because of several reasons. First, exoplanets within such short distance to M dwarfs can be exposed to high levels of X-ray and extreme ultraviolet radiation (X-EUV) and strong particle fluxes from stellar winds or coronal mass ejections. Second, tidal-locking exoplanets receive very uneven stellar heating. While the dayside can be warm enough to sustain liquid water, the nightside could be so cold that any gases condense out there.
We have focused on the second problem in the past few years, with 3-dimensional climate-model simulations. First, we show that oceanic heat transports are efficient to warm the nightside of tidally-locked exoplanets, so that atmosphere collapse and water freezing on the nightside do not happen. If greenhouse gas concentration is high enough, the nighside can be ice free. Second, we that ice-sheet depth on the nightside depends on geothermal and gravity, and that ice-sheets cannot grow too deep to freeze all water. Third, we show how orbit, obliquity, and eccentricity affect on climates of tidally-locked exoplanets. Fourth, we show that ice-planets/satellites would directly jump to runaway greenhouse, bypassing the habitable climate state.
