  To assess the number of life-bearing worlds in astrophysical environments, it
is necessary to take the intertwined processes of abiogenesis (birth),
extinction (death), and transfer of life (migration) into account. We construct
a mathematical model that incorporates this trio of mechanisms and accordingly
derive the probability distribution function and other statistical properties
(e.g., mean) for the number of worlds with biospheres. We show that a given
astrophysical setting may become eventually saturated with life if the rate of
successful transfers of organisms is higher than the extinction rate of
biospheres. Based on the available data, we suggest that this criterion might
be fulfilled for star-forming clusters (and perhaps the Galactic bulge under
optimal circumstances), thereby indicating that such regions could constitute
promising abodes for hosting and detecting life.
