Dynamical Instabilities in Extrasolar Planetary Systems
  Instabilities and strong dynamical interactions between multiple giant planets have been proposed as a possible explanation for the surprising orbital properties of extrasolar planetary systems. In particular, dynamical instabilities seem to provide a natural mechanism for producing the highly eccentric orbits seen in many systems. Previously, we performed numerical integrations for the dynamical evolution of planetary systems containing two giant planets of equal masses initially in nearly circular orbits very close to the dynamical stability limit. We found the ratio of collisions to ejections in these simulations was greater than the ratio of circular orbits to eccentric orbits among the known extrasolar planets. Further, the mean eccentricity of the planets remaining after an ejection was larger than the mean eccentricity of the known extrasolar planets. Recently, we have performed additional integrations, generalizing to consider two planets of unequal masses. Our new simulations reveal that the two-planet scattering model can produce a distribution of eccentricities consistent with the observed eccentricity distribution for plausible mass distributions. Additionally, this model predicts a maximum eccentricity of about 0.8, in agreement with observations. Early results from simulations of three equal-mass planets also reveal a reduced frequency of collisions and a broad range of final eccentricities for the retained inner planet. 
Eric B. Ford (Princeton), Frederic A. Rasio (Northwestern) and Kenneth   Yu (Northwestern)