The evaporation of planetary atmospheres

In recent years the focus of exoplanet research has shifted from the mere detection to detailed characterization. Precise measurements of the masses and radii of transiting planets have shown that some low-mass planets have extended atmospheres while others are bare rocks. Hybrid atmospheres consisting of a mixture of Hydrogen and large amount of heavy elements have also been detected. A key factor in explaining this diversity of planetary atmospheres is the erosion by the X-ray and EUV-radiation (XUV) from the host-star. The evaporation through XUV-radiation has already been measured for a few exoplanets.The apparent weakness of the Ca II HK and the Mg II h&k emission cores has been interpreted as evidence for the evaporation of planetary atmospheres. The interpretation is that the evaporating material from the planet forms a thick torus which absorbs the Ca II HK and the Mg II h&k lines from the host star. In this contribution a new way how to prove, or disprove this hypothesis by observations is proposed. It is furthermore shown that there are enough bright targets already known that can be observed, and more will be found with the PLATO mission.