Statistical properties of exoplanets IV. The period--eccentricity relations of exoplanets and of binary stars

TL;DR

This study compares the eccentricity-period relations of exoplanets and binary stars, revealing differences in tidal circularization processes and suggesting distinct formation mechanisms for the two classes of objects.

Contribution

It introduces a method to analyze period-eccentricity relations unaffected by tidal effects and provides new insights into the formation and evolution of exoplanets and binary stars.

Findings

Tidal circularization is complete for planetary orbits with periods shorter than 5 days.

Exoplanets have significantly smaller eccentricities than binary stars beyond the circularization limit.

Binary and planetary systems likely have different formation processes.

Abstract

A sample of spectroscopic binaries and a sample of single planetary systems, both having main-sequence solar-type primary components, are selected in order to compare their eccentricities. The positions of the objects in the (P.(1-e^2)^(3/2), e) plane is used to determine parts in the period-eccentricity diagram that are not affected by tidal circularization. The original eccentricities of binaries and planets are derived and compared. They seem to be weakly or not at all correlated with period in both samples, but two major differences are found :(1) The tidal circularization of planetary orbits is almost complete for periods shorter than 5 days, but it is not visible when P.(1-e^2)^{3/2} is longer than this limit. This suggests that the circularization occurs rapidly after the end of the migration process and is probably simultaneous with the end of the formation of the planet. By contrast, we confirm that the circularization of the binary orbits is a process still progressing a long time after the formation of the systems.(2) Beyond the circularization limit, the eccentricities of the orbits of the planets are significantly smaller than those of binary orbits, and this discrepancy cannot be due to a selection effect. Moreover, the eccentricities of binaries with small mass ratios are quite similar to those of all binaries with q<0.8. This suggests that the low eccentricities of exoplanet orbits are not a consequence of low-mass secondaries in a universal process. These remarks are in favor of the idea that binaries and exoplanets are two different classes of object from the point of view of their formation.