Title: Exoplanets atmospheres: evolution and habitability
Abstract: Most of the small exoplanets detected have a size that is unusual in our Solar System (SS). They also seem to have an evolution that in some cases might differ from that of the terrestrial planets in our SS. Evidence of that is seen in the observational detection of a localized reduction in the small planet occurrence rate, sometimes termed a 'gap'. This gap appears to define a transition region in which sub-Neptune planets are believed to have lost their primordial H/He envelope. These small planets could have developed a secondary atmosphere and should attract attention in the next years. Here we investigate the evolution of the atmosphere of observed small close-in planets by looking into the relationships between their radius, insolation, and density, and by tracking the evolution of their envelope due to photoevaporation. We also show that secondary atmospheres in small exoplanets can be detected using haze and we show that haze can be widely characterized using observations taken with HST/STIS. Although the presence of a secondary atmosphere can be one of the key factors for the habitability of terrestrial planets, other factors can also have a major influence, such as the activity of the host star. Here we determine the habitability of terrestrial planets under the environment of a flaring star M dwarf and G-type stars.