  Planetary atmospheres are subject to mass loss through a variety of
mechanisms including irradiation by XUV photons from their host star. Here we
explore the consequences of XUV irradiation by supermassive black holes as they
grow by the accretion of gas in galactic nuclei. Based on the mass distribution
of stars in galactic bulges and disks and the luminosity history of individual
black holes, we estimate the probability distribution function of XUV fluences
as a function of galaxy halo mass, redshift, and stellar component. We find
that about 50% of all planets in the universe may lose the equivalent of a
Martian atmosphere, 10% may lose an Earth's atmosphere, and 0.2% may lose the
mass of Earth's oceans. The fractions are appreciably higher in the spheroidal
components of galaxies, and depend strongly on galaxy mass, but only weakly on
redshift.
