On orbital elements of extrasolar planetary candidates and spectroscopic binaries

Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, USA

Corresponding author: T. Stepinski, tom@lpi.usra.edu

Received: 29 November 2000
Accepted: 9 March 2001

Abstract

We estimate probability densities of orbital elements, periods and eccentricities, for the population of extrasolar planetary candidates (EPC) and, separately, for the population of spectroscopic binaries (SB) with solar-type primaries. We construct empirical cumulative distribution functions (CDFs) in order to infer probability distribution functions (PDFs) for orbital periods and eccentricities. We also derive a joint probability density for period-eccentricity pairs in each population. Comparison of respective distributions reveals that in all cases EPC and SB populations are, in the context of orbital elements, indistinguishable from each other to a high degree of statistical significance. Probability densities of orbital periods in both populations have ~P^-1 functional form, whereas the PDFs of eccentricities can be best characterized as a Gaussian with a mean of about 0.35 and standard deviation of about 0.2 turning into a flat distribution at small values of eccentricity. These remarkable similarities between EPC and SB must be taken into account by theories aimed at explaining the origin of extrasolar planetary candidates, and constitute an important clue as to their ultimate nature.