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

Abstract

We consider the mass distribution of the nine unseen companions orbiting solar-type stars with minimum possible masses in the planetary mass range reported as of March 1998. We compare the mass distribution of these nine extrasolar planet candidates with the distribution of low-mass secondaries in spectroscopic binaries. We choose to use a logarithmic scale to study the combined mass distribution, because of the large range of masses, 0.5-300 Jupiter masses, involved. Although the results are based on a very small number of systems, the combined distribution looks different at the high- and low-ends. At the high-mass end the distribution drops steeply from 200 to 20 Jupiter masses. At the planetary range of masses the distribution is flat, and might even rise mildly from, say, 20 to 0.6 Jupiter masses, depending on the assumed detection threshold for the precise surveys. The transition region between the two slopes is at about 10-30 Jupiter masses. One possible interpretation of this result is that we have here two different populations. Maybe the lower-mass population was formed like planets, out of an accretion disc, while the high-mass population was formed like binary stars. If the 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 give us the long-sought clue for how to distinguish planets from low-mass stellar companions.