Atmospheric Escape Processes and Planetary Atmospheric Evolution: from misconceptions to challenges

The recent discoveries of roughly earth-mass exoplanets in the temperate zones of different stars have led to questioning the nature of their atmosphere, which is required to determine their habitability. Atmospheric escape is one of the challenging problems to be solved: simply adapting what is currently observed in the solar system is doomed to fail due to the large variations in the conditions encountered around other stars. A better strategy is to review the different processes that shape planetary atmospheres and to evaluate their importance depending upon the stellar conditions. This approach has shown that processes like ion-pickup were a more important way to lose atmosphere at Mars in the past and that hydrodynamic escape was a major process in the Early Solar System.

We have reviewed the different escape mechanisms and their magnitude in function of the different conditions. This led us to discover discrepancies in the current literature concerning problems such as the Xenon paradox or the importance of a magnetic field in protecting an atmosphere. In addition, our work shows that one should be very careful before claiming the presence of an atmosphere on planets in the temperate zone of M-dwarfs: a new criterion such as the Alfven surface location with respect to the planet should be taken into account when considering habitability.