  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.
