EUV irradiation of exoplanet atmospheres occurs on Gyr timescales.

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). 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.