  The distribution of orbital period ratios of adjacent planets in extra-solar
planetary systems discovered by the {\it Kepler} space telescope exhibits a
peak near $\sim1.5$--$2$, a long tail of larger period ratios, and a steep
drop-off in the number of systems with period ratios below $\sim1.5$. We find
from this data that the dimensionless orbital separations have an approximately
log-normal distribution. Using Hill's criterion for the dynamical stability of
two planets, we find an upper bound on planet masses such that the most common
planet mass does not exceed $10^{-3.2}m_*$, or about two-thirds Jupiter mass
for solar mass stars. Assuming that the mass ratio and the dynamical separation
(orbital spacings in units of mutual Hill radius) of adjacent planets are
independent random variates, and adopting empirical distributions for these, we
use Hill's criterion in a statistical way to estimate the planet mass
distribution function from the observed distribution of orbital separations. We
find that the planet mass function is peaked in logarithm of mass, with a peak
value and standard deviation of $\log m/M_\oplus$ of $\sim(0.6-1.0)$ and
$\sim(1.1-1.2)$, respectively.
