Characterizing Evaporating Atmospheres of Exoplanets

Exoplanets orbiting close to their parent stars have been observed to lose a fraction of their atmosphere into space: they "evaporate. " The escape of atmospheric gas is due to the large X-ray to ultraviolet (XUV) energy input from the nearby host-star into the upper atmosphere of the planet. Evaporation is characterized through the transit observations of escaping atoms and ions, like hydrogen atoms at Lyman-', ionized or neutral carbon, oxygen, and magnesium in the UV. Detailed modeling of this phenomenon has to take into account a large number of important physical processes, like the interactions of the upper atmosphere with the stellar environment, radiation pressure, photoionization, self-shielding, charge exchanges, and magnetic interactions with the stellar wind. Massive evaporation can lead some gaseous exoplanets to lose a large fraction of their atmosphere and undergo a significant change in their nature. As a result, some short period, small exoplanets are potentially the remnant of former more massive planets. Evaporation is thus an important process that sculpts the structure of planetary systems with a significant and measurable effect on exoplanets at short orbital distances.