The erosion of planetary atmospheres

行星大气层的侵蚀

• 批准号: 361764429
• 负责人: Dr. Eike W. Guenther
• 金额: --
• 依托单位: Thüringer Landessternwarte Tautenburg (TLS) - Karl-Schwarzschild-Observatorium
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2017
• 资助国家: 德国
• 起止时间: 2016-12-31 至 2020-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/361764429?language=en
• 关键词: erosion; planetary; atmospheres

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.

在我们的太阳系中，基本上有两种行星：质量大于15 MEarth，密度在0.7到1.6 gcm-3之间的气体或冰巨星，以及密度在3.7到5.5 gcm-3之间的低质量岩石行星。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另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，这些行星的密度有很大的差异。即使是质量相同的行星，密度也可能大不相同。如何解释这种多样性？在其演化的前1亿年，行星暴露在来自宿主星星的强烈EUV和X射线（XUV）辐射下。观测和理论模型表明，这种辐射会导致行星大气层的侵蚀。这个过程自然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。如果这种大气侵蚀的想法是正确的，它将对行星的可居住性产生重要影响，因为没有大气层的行星不可能适合居住。行星大气层的侵蚀对于可能适合居住的M-矮星行星特别重要，因为它们的轨道距离宿主恒星非常近，侵蚀特别强烈。近年来，人们逐渐清楚地认识到，仅仅是静止的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）在其演化的前1亿年内对行星大气的影响知之甚少。这种情况对于CME来说尤其困难，因为在年轻的M矮星中只有很少的事件被观察到。得益于新的仪器，现在可以确定年轻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造成的质量损失的信息。我们将与格拉兹和翁德雷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，以及与宿主恒星的距离。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。该项目还为哪些行星可能适合居住以及哪些行星不适合居住的问题提供了新的线索。