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arXiv · 2007.13731

EUV irradiation of exoplanet atmospheres occurs on Gyr timescales

George W. King·Peter J. Wheatley
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Abstract

Exoplanet atmospheres are known to be vulnerable to mass loss through irradiation by stellar X-ray and extreme-ultraviolet emission. We investigate how this high-energy irradiation varies with time by combining an empirical relation describing stellar X-ray emission with a second relation describing the ratio of Solar X-ray to extreme-ultraviolet emission. In contrast to assumptions commonly made when modelling atmospheric escape, we find that the decline in stellar extreme-ultraviolet emission is much slower than in X-rays, and that the total extreme-ultraviolet irradiation of planetary atmospheres is dominated by emission after the saturated phase of high energy emission (which lasts around 100 Myr after the formation of the star). The extreme-ultraviolet spectrum also becomes much softer during this slow decline. Furthermore, we find that the total combined X-ray and extreme-ultraviolet emission of stars also occurs mostly after this saturated phase. Our results suggest that models of atmospheric escape that focus on the saturated phase of high-energy emission are over-simplified, and when considering the evolution of planetary atmospheres it is necessary to follow EUV-driven escape on Gyr timescales. This may make it more difficult to use stellar age to separate the effects of photoevaporation and core-powered mass-loss when considering the origin the planet radius valley.

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George W. King, Peter J. Wheatley. 2020-07-27. EUV irradiation of exoplanet atmospheres occurs on Gyr timescales. https://doi.org/10.1093/mnrasl%2Fslaa186

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