Evolution of galaxy habitability
  We combine a semi-analytic model of galaxy evolution with constraints on
circumstellar habitable zones and the distribution of terrestrial planets to
probe the suitability of galaxies of different mass and type to host habitable
planets, and how it evolves with time. We find that the fraction of stars with
terrestrial planets in their habitable zone (known as habitability) depends
only weakly on galaxy mass, with a maximum around 4e10 Msun. We estimate that
0.7% of all stars in Milky Way type galaxies to host a terrestrial planet
within their habitable zone, consistent with the value derived from Kepler
observations. On the other hand, the habitability of passive galaxies is
slightly but systematically higher, unless we assume an unrealistically high
sensitivity of planets to supernovae. We find that the overall habitability of
galaxies has not changed significantly in the last ~8 Gyr, with most of the
habitable planets in local disk galaxies having formed ~1.5 Gyr before our own
solar system. Finally, we expect that ~1.4e9 planets similar to present-day
Earth have existed so far in our galaxy.
