EUV irradiation of exoplanet atmospheres occurs on Gyr timescales

TL;DR

This study shows that EUV irradiation of exoplanet atmospheres persists over Gyr timescales, challenging previous models that focused only on early stellar activity, and highlights the importance of long-term EUV-driven atmospheric escape.

Contribution

It introduces a combined empirical model of stellar X-ray and EUV emission evolution, emphasizing the prolonged EUV impact on planetary atmospheres beyond the saturated phase.

Findings

EUV emission declines more slowly than X-ray emission.

Most EUV irradiation occurs after the saturated phase (~100 Myr).

Long-term EUV-driven escape influences planetary atmosphere evolution.

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.