  The questions of how planets form and how common Earth-like planets are can
be addressed by measuring the distribution of exoplanet masses and orbital
periods. We report the occurrence rate of close-in planets (with orbital
periods less than 50 days) based on precise Doppler measurements of 166
Sun-like stars. We measured increasing planet occurrence with decreasing planet
mass (M). Extrapolation of a power law mass distribution fitted to our
measurements, df/dlogM = 0.39M^-0.48, predicts that 23% of stars harbor a
close-in Earth-mass planet (ranging from 0.5 to 2.0 Earth masses). Theoretical
models of planet formation predict a deficit of planets in the domain from 5 to
30 Earth masses and with orbital periods less than 50 days. This region of
parameter space is in fact well populated, implying that such models need
substantial revision.
