The impact of AGN outflows on the surface habitability of terrestrial planets in the Milky Way

Abstract

It is well-known that active galactic nuclei (AGN) are accompanied by winds and outflows, some of which may reach weakly relativistic speeds of about 10 percent the speed of light. Yet, in spite of their ubiquity, the impact of AGN outflows in modulating surface habitability of terrestrial planets on galactic scales, using the Milky Way as the basis for comparison, is poorly investigated and inadequately understood. In this work, we address this issue by focusing on two key mechanisms: AGN winds can heat atmospheres and drive atmospheric escape, as well as stimulate the formation of nitrogen oxides and thence cause ozone depletion. By developing simple models, we estimate the maximal distance up to which these deleterious effects are rendered significant for Earth-like planets in the Milky Way, and thereby demonstrate that this value may extend to 1 kpc. In the case of quasars hosting larger supermassive black holes, such effects could actually influence the AGN host galaxy as a whole.

On the relative importance of AGN winds for the evolution of exoplanet atmospheres

“…This is, in fact, comparable with the galacto-centric radius derived in Ambrifi et al (2022) inside which they conclude heating from an energy-drive AGN wind could evaporate the planet's atmosphere.…”

“…In such a case, it would indeed be possible to unbind a substantial fraction of an Earthlike planet's atmosphere over a Salpeter time. Ambrifi et al (2022) conclude that, for exoplanets located in galaxies harboring more massive or powerful AGN, the critical radius could encompass the entire galaxy.…”

“…To investigate the effect of outflows from AGN on planetary atmospheres, Ambrifi et al (2022) present a straight forward energy flux calculation, arguing that, under certain conditions, a sub-Eddington outflow from a black hole like Sgr A* can deposit a sufficient amount of energy to thermally unbind a planet's atmosphere over a Salpeter time (the characteristic timescale for AGN accretion), for exoplanets located closer than about 1 kpc from the center of the Galaxy.…”

“…Taking the solar wind as a benchmark, we can estimate the stand-off distance from the star outside of which the AGN wind dominates. Using the more relevant case discussed in (Ambrifi et al 2022) and following their notation, the case of a thermally/energy drive AGN wind, and adopting their notation, the thrust ṗ of the AGN wind is…”

“…Given that we know that accretion in AGN is almost always accompanied by energetic outflows, either in the form or winds or collimated jets, it is important to ask what their effect may be on an exoplanet's atmosphere. Ambrifi et al (2022) recently presented an initial investigation of how AGN outflows from the center of the host galaxy may impact the atmospheres of exoplanets through heating and chemical processing, thus possibly affecting the extent of the Galactic habitable zone.…”