  Panspermia is the hypothesis that life originated on Earth from the
bombardment of foreign interstellar ejecta harboring polyextremophile
microorganisms. Since the 2017 discovery of the interstellar body 'Oumuamua
(1I/2017 U1) by the Pans-STARRS telescope, various studies have re-examined
panspermia based on updated number density models that accommodate for
'Oumuamua's properties. By utilizing 'Oumuamua's properties as an anchor, we
estimate the mass and number density of ejecta in the ISM (rho_m [kg au^-3] and
rho_n [au^-3]). We build upon prior work by first accounting for the minimum
ejecta size to shield microbes from supernova radiation. Second, we estimate
the total number of impact events C_n on Earth after its formation and prior to
the emergence of life (~0.8Gyr). We derive a conditional probability relation
for the likelihood of panspermia for Earth specifically of <10^-5, given a
number of factors including f_B, the fraction of ejecta harboring extremophiles
and other factors that are poorly constrained. However, we find that panspermia
is a plausible potential life-seeding mechanism for (optimistically)
potentially up to ~10^5 of the ~10^9 Earth-sized habitable zone worlds in our
Galaxy.
