A planet having protective ozone within the collimated beam of a Gamma Ray Burst ( GRB ) may suffer ozone depletion , potentially causing a mass extinction event to existing life on a planet ’ s surface and oceans .
We model the dangers of long GRBs to planets in the Milky Way and utilize a static statistical model of the Galaxy that matches major observable properties , such as the inside-out star formation history , metallicity evolution , and 3-dimensional stellar number density distribution .
The GRB formation rate is a function of both the star formation history and metallicity ; however , the extent to which chemical evolution reduces the GRB rate over time in the Milky Way is still an open question .
Therefore , we compare the damaging effects of GRBs to biospheres in the Milky Way using two models .
One model generates GRBs as a function of the inside-out star formation history .
The other model follows the star formation history , but generates GRB progenitors as a function of metallicity , thereby favoring metal-poor host regions of the Galaxy over time .
If the GRB rate only follows the star formation history , the majority of the GRBs occur in the inner Galaxy .
However , if GRB progenitors are constrained to low metallicity environments , then GRBs only form in the metal-poor outskirts at recent epochs .
Interestingly , over the past 1 Gyr , the surface density of stars ( and their corresponding planets ) that survive a GRB is still greatest in the inner galaxy in both models .
The present day danger of long GRBs to life at the solar radius ( R _ { \odot } = 8 kpc ) is low .
We find that at least \sim 65 % of stars survive a GRB over the past 1 Gyr .
Furthermore , when the GRB rate was expected to have been enhanced at higher redshifts , such as z \gtrsim 0.5 , our results suggest that a large fraction of planets would have survived these lethal GRB events .