How black hole activity may influence exoplanetary evolution in our Galaxy

Abstract

ABSTRACT An increasing number of exoplanets have been discovered in the Milky Way galaxy, which is also known to harbour a super-massive black hole (Sagittarius A*) at its centre. Here, we investigate how the central black hole (BH) activity may affect the evolution of exoplanets in our Galaxy. Accreting BHs emit high-energy radiation – extreme ultraviolet and X-rays – which can lead to XUV photoevaporation of the planetary atmospheres. We evaluate the atmospheric mass-loss using both theoretical estimates of the BH radiative output and observational constraints on the past activity history of Sgr A*. The resulting mass-loss is analysed as a function of the galactocentric distance. For the first time, we compute the exoplanet atmospheric evolution under BH irradiation by explicitly including the temporal evolution of the central luminosity output (i.e. the BH activity history). We obtain that Sgr A* could have a major impact on exoplanets located in the inner region of the Galaxy (e.g. Galactic bulge); a significant fraction of the atmospheric mass can be removed by BH irradiation; and in extreme cases, the initial atmosphere may be completely stripped away. Such mass-loss can have important consequences on the atmospheric chemistry and potential biological evolution. We discuss the physical implications for planetary habitability, and we also briefly consider the case of stellar-mass BHs. Overall, accreting black holes may play a significant role in the evolution of exoplanets in our Galaxy across cosmic time.