  We investigated the habitability of the Milky Way, making use of recent
observational analysis on the prevalence of Earth-sized planets, in order to
estimate where and when potentially habitable star systems may have formed over
the course of the Galaxy's history. We were then able to estimate the age
distribution of potential intelligent life in our Galaxy using our own
evolution and the age of the Sun as a proxy. To do this we created a galactic
chemical evolution model and applied the following habitability constraints to
the Sun-like (G-type) stars formed in our model: an environment free from
life-extinguishing supernovae, a high enough metallicity for Earth-sized planet
formation and sufficient time for the evolution of complex life. We determined
a galactic habitable zone as the region containing all the potentially
habitable star systems in our model. Our galactic habitable zone contains stars
formed between 11 and 3.8 billion years ago at radial distances of between 7
and 14 kiloparsecs. We found that most potentially habitable star systems are
much older than the Sun and located farther from the galactic centre. By
comparing the ages of these systems we estimated that 77% of potentially
habitable star systems are on average 3.13 billion years older than the Sun.
This suggests that any intelligent life in the Galaxy is likely to be
incredibly more advanced than we are assuming that they have evolved under
similar timescales than we have. Implications and limitations of our study are
discussed.
