Abstract

Exoplanets close to their parent stars could lose some fraction of their atmospheres because of the extreme level of irradiation they receive. Atmospheric escape has been observed during the past 12 years for hot gas giants, as large ultraviolet absorption signals during transits. Whereas these giants are massive enough to resist evaporation, lower-mass planets could be dramatically impacted by the mass loss. In particular, hot rocky planets observed by Corot and Kepler might have lost all of their atmosphere, having begun as Neptune-like. After recalling the state-of-the-art knowledge about upper atmospheres of exoplanets, I will present recent Hubble Space Telescope observations of the Neptune-mass exoplanet GJ 436b. These new data are allowing substantial developments in the field of star-planet interaction and open exciting observational perspectives, which I will discuss in the context of forthcoming exoplanet surveys.