The discovery of thousands of highly irradiated, low-mass, exoplanets has led to the idea that atmospheric escape is an important process that can drive their evolution. Of particular interest is the inference from recent exoplanet detections that there is a large population of low mass planets possessing significant, hydrogen dominated atmospheres, even at masses as low as $\sim 2~\mbox{M}_{\oplus }$ . The size of these hydrogen dominated atmospheres indicates the envelopes must have been accreted from the natal protoplanetary disc. This inference is in contradiction with the Solar System terrestrial planets, that did not reach their final masses before disc dispersal, and only accreted thin hydrogen dominated atmospheres. In this review, we discuss the evidence for hydrogen dominated atmospheres on terrestrial mass ( $\lesssim 2~\mbox{M}_{\oplus }$ ) planets. We then discuss the possible origins and evolution of these atmospheres with a focus on the role played by hydrodynamic atmospheric escape driven by the stellar high-energy emission (X-ray and EUV; XUV).

中文翻译：

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类地行星上以氢为主的大气：证据、起源和演化

数以千计的高辐射、低质量系外行星的发现使人们认为大气逃逸是一个重要的过程，可以推动它们的进化。特别令人感兴趣的是最近的系外行星探测推断，即使质量低至 $\sim 2~\mbox{M​​}_{\oplus }$ ，也有大量低质量行星拥有重要的、以氢为主的大气层。这些以氢为主的大气层的大小表明这些包层一定是从出生的原行星盘中吸积而来的。这个推论与太阳系类地行星相矛盾，类地行星在圆盘扩散之前没有达到最终质量，并且只吸积了以氢为主的稀薄大气。在这次审查中，我们讨论了地球质量（ $\lesssim 2~\mbox{M​​}_{\oplus }$ ）行星上以氢为主的大气的证据。然后，我们讨论这些大气的可能起源和演化，重点是由恒星高能发射（X 射线和 EUV；XUV）驱动的流体动力学大气逃逸所发挥的作用。