The Best Place and Time to Live in the Milky Way

Context.Counted among the most powerful cosmic events, supernovae (SNe) andγ-ray bursts (GRBs) can be highly disruptive for life: Their radiation can be harmful for biota or induce extinction by removing most of the protective atmospheric ozone layer from terrestrial planets (TPs). Nearby high-energy transient astrophysical events have been proposed as possible triggers of mass extinctions on Earth.Aims.We assess the habitability of the Milky Way (MW) throughout its cosmic history against potentially disruptive astrophysical transients with the aim of identifying the safest places and epochs within our Galaxy. We also test the hypothesis that one long GRB played a leading role in the late Ordovician mass-extinction event (∼445 Myr ago).Methods.We characterised the habitability of the MW throughout its cosmic history as a function of galactocentric distance of TPs. We estimated the dangerous effects of transient astrophysical events (long and short GRBs and SNe) with a model that connects their rate to the specific star formation and metallicity evolution within the Galaxy throughout its cosmic history. Our model also accounts for the probability that TPs form around FGK and M stars.Results.Until about six billion years ago, the outskirts of the Galaxy were the safest places to live, despite the relatively low density of TPs. In the last about four billion years, regions between 2 and 8 kpc from the center, which had a higher density of TPs, became the best places for a relatively safer biotic life growth. We confirm the hypothesis that one long GRB played a leading role in the late Ordovician mass-extinction event. In the last 500 Myr, the safest neighborhood in the Galaxy was a region at a distance of 2 to 8 kpc from the Galactic center, whereas the MW outskirts were sterilized by two to five long GRBs.