The Mass Distribution of Extrasolar Planet Candidates and Spectroscopic Binary Low-Mass Companions

We consider the mass distribution of the nine unseen companions orbitingsolar-type stars with minimum possible masses in the planetary mass rangereported as of March 1998. We compare the mass distribution of these nineextrasolar planet candidates with the distribution of low-mass secondaries inspectroscopic binaries. We choose to use a logarithmic scale to study thecombined mass distribution, because of the large range of masses, 0.5-300Jupiter masses, involved. Although the results are based on a very small number of systems, thecombined distribution looks different at the high- and low-ends. At thehigh-mass end the distribution drops steeply from 200 to 20 Jupiter masses. Atthe planetary range of masses the distribution is flat, and might even risemildly from, say, 20 to 0.6 Jupiter masses, depending on the assumed detectionthreshold for the precise surveys. The transition region between the two slopesis at about 10-30 Jupiter masses. One possible interpretation of this result is that we have here two differentpopulations. Maybe the lower-mass population was formed like planets, out of anaccretion disc, while the high-mass population was formed like binary stars. Ifthe shape of the combined distribution can be verified by many more detections,and if the planetary-mass objects prove to be extrasolar planets, this can giveus the long-sought clue for how to distinguish planets from low-mass stellarcompanions.Comment: 8 pages, 2 figures, latex, to appear in ApJ Letter