The erosion of planetary atmospheres

在我们的太阳系中，基本上有两种行星：气体或冰巨星，质量大于15个地球，密度在0.7到1.6 gcm-3之间；低质量的岩石行星，密度在3.7到5.5 gcm-3之间。由于过去几年对太阳系外行星的研究，我们已经了解到还有一种行星的质量介于气体/冰巨星和岩石行星之间。令人惊讶的是，这些行星的密度分布非常大。即使是质量相同的行星，其密度也可能相差很大。如何解释这种多样性呢？在它们进化的最初1亿年里，行星暴露在来自宿主恒星的强烈的EUV和x射线（XUV）辐射中。观测和理论模型表明，这种辐射会导致行星大气的侵蚀。这个过程自然地解释了行星的多样性：有大气层的行星密度低，没有大气层的行星密度高。如果这种关于大气侵蚀的观点是正确的，那么它将对行星的可居住性产生重要影响，因为没有大气的行星不可能适合居住。行星大气的侵蚀对于可能适合居住的m型矮星行星来说尤其重要，因为它们的轨道距离主星非常近，而且侵蚀特别强烈。近年来，人们逐渐清楚地认识到，仅靠静止的xuv辐射通常不足以完全侵蚀行星的大气层。然而，人们对耀斑和日冕物质抛射（cme）在其演化的最初1亿年里对行星大气的影响知之甚少。日冕物质抛射的情况尤其困难，因为在年轻的m矮星中只有很少的此类事件被观察到。由于有了新的仪器，现在有可能确定年轻m矮星的耀斑速率，并研究cme。因此，该项目的目的是确定不同年龄m矮星上耀斑的统计数据，并首次获得有关cme引起的质量损失的信息。我们将与格拉茨和翁德列约夫的研究小组合作，利用这些结果来确定不同质量的行星的质量损失率，以及与主星的距离。通过这种方式，我们将发现这个过程是否可以解释系外行星的多样性。这个项目还揭示了哪些行星可能适合居住，哪些不适合居住的问题。

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.