  Determining the occurrence rate of terrestrial-mass planets (m_p < 10M_earth)
is a critically important step on the path towards determining the frequency of
Earth-like planets (eta-Earth), and hence the uniqueness of our Solar system.
Current radial-velocity surveys, achieving precisions of 1 m/s, are able to
detect terrestrial-mass planets and provide meaningful estimates of their
occurrence rate. We present an analysis of 67 solar-type stars from the
Anglo-Australian Planet Search specifically targeted for very high-precision
observations. When corrected for incompleteness, we find that the planet
occurrence rate increases sharply with decreasing planetary mass. Our results
are consistent with those from other surveys: in periods shorter than 50 days,
we find that 1.5% of stars host a giant (m_p > 100M_earth) planet, and that
18.5% of stars host a planet with m_p < 10M_earth. The preponderance of
low-mass planets in short-period orbits is in conflict with formation
simulations in which the majority of terrestrial-mass planets reside at larger
orbital distances. This work gives a hint as to the size of eta-Earth, but to
make meaningful predictions on the frequency of terrestrial planets in longer,
potentially habitable orbits, low-mass terrestrial planet searches at periods
of 100-200 days must be made an urgent priority for ground-based Doppler planet
searches in the years ahead.
