  Radial-velocity planet search campaigns are now beginning to detect low-mass
"Super-Earth" planets, with minimum masses M sin i < 10 M_earth. Using two
independently-developed methods, we have derived detection limits from nearly
four years of the highest-precision data on 24 bright, stable stars from the
Anglo-Australian Planet Search. Both methods are more conservative than a human
analysing an individual observed data set, as is demonstrated by the fact that
both techniques would detect the radial velocity signals announced as
exoplanets for the 61 Vir system in 50% of trials. There are modest differences
between the methods which can be recognised as arising from particular criteria
that they adopt. What both processes deliver is a quantitative selection
process such that one can use them to draw quantitative conclusions about
planetary frequency and orbital parameter distribution from a given data set.
Averaging over all 24 stars, in the period range P<300 days and the
eccentricity range 0.0<e<0.6, we could have detected 99% of planets with
velocity amplitudes K>7.1 m/s. For the best stars in the sample, we are able to
detect or exclude planets with K>3 m/s, corresponding to minimum masses of 8
M_earth (P=5 days) or 17 M_earth (P=50 days). Our results indicate that the
observed "period valley," a lack of giant planets (M>100 M_earth) with periods
between 10-100 days, is indeed real. However, for planets in the mass range
10-100 M_earth, our results suggest that the deficit of such planets may be a
result of selection effects.
