Origins of Eccentric Extrasolar Planets: Testing the Planet–planet Scattering Model

Any planetary system with two or more giant planets may become dynamically unstable, leading to collisions or ejections through strong planet–planet scattering. Following an ejection, the other planet is left in a highly eccentric orbit. Previous studies for simple initial configurations with two equalmass planets revealed two discrepancies between the results of numerical simulations and the observed orbital elements of extrasolar planets: the potential for frequent collisions between giant planets and a narrow distribution of final eccentricities following ejections. Here, we show that simulations for two planets with unequal masses predict a reduced frequency of collisions and a broader range of final eccentricities. We show that the two-planet scattering model can easily reproduce the observed eccentricities with a plausible distribution of planet mass ratios. Further, the two-planet scattering model predicts a maximum eccentricity of about 0.8, independent of the distribution of planet mass ratios. This compares favorably with current observations and will be tested by future planet discoveries. Moreover, we show that the combination of planet–planet scattering and tidal circularization may be able to explain the existence of some giant planets with very short period orbits. However, the presence of giant planets in circular orbits at slightly larger orbital periods (small enough to require significant migration, but large enough that tidal circularization is ineffective) is more difficult to explain. As part of this work, we also re-examine and discuss various possible correlations between eccentricities and other properties of observed extrasolar planets. We demonstrate that the observed distribution of planet masses, orbital periods, and eccentricities can provide constraints for models of planet formation and evolution. Subject headings: planetary systems — planetary systems: formation — planets and satellites: general — celestial mechanics