The erosion of planetary atmospheres

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In our Solar System there are basically two species of planets: gas, or ice giants which have masses larger than 15 MEarth and densities between 0.7 to 1.6 gcm-3, and low-mass, rocky planets with densities between 3.7 to 5.5 gcm-3. Thanks to the research on extrasolar planets of the past years, we have learned that there is another species of planets which have masses in between the gas/ice-giants and the rocky planets. What is very surprising, is that these planets have a huge spread in densities. Even planets of the same mass can have very different densities. How can this diversity be explained? In the first 100 million years of their evolution, planets are exposed to strong EUV and X-ray (XUV) radiation from the host star. Observations, and theoretical models have shown that such radiation can lead to the erosion of planetary atmospheres. This process naturally explains the diversity of planets: Planets with atmospheres have low densities, planets without atmospheres have high densities. If this idea of atmospheric erosion is correct, it would have important consequences for the habitability of planets, since planets without atmospheres cannot be habitable. The erosion of planetary atmospheres is particularly important for potentially habitable planets of M-dwarfs, because they orbit at very short distances from the host stars were the erosion is particularly strong. In recent years it became clear that the quiescent XUV-radiation alone is usually not strong enough to erode the atmospheres of planets completely. However, very little is known about the impact of flares and coronal-mass ejections (CMEs) on planetary atmospheres within the first 100 million years of their evolution. The situation is particularly difficult for CMEs, as only very few such events in young M-dwarfs have been observed. Thanks to the new instrumentation, it is now possible to determine the flare-rate for young M-dwarfs, and to study also the CMEs. The aim of this project thus is to determine the statistics of flares on M-dwarfs at different ages, and for the first time, obtain information about the mass-loss caused by CMEs. In collaboration with the research teams in Graz and Ondrejov we will use these result in order to determine the mass-loss rate for planets of different masses, and distances from the host stars. In this way we will find out whether this process can explain the diversity of exoplanets. This project also sheds new light on to the question which planets can potentially be habitable and which not.