Resonance capture and the formation of the outer planets

In this paper we study a possible process for the formation of the outer planets, in which resonance capture induces the accretion of the planetary cores near exterior mean-motion commensurabilities of existing bodies. A two-dimensional N-body simulation is performed to test this hypothesis in the case of Saturn. Initial conditions consist of a swarm of 1000 equal-mass planetesimals distributed over a planar ring with extrema at 6.5 and 15 au. All bodies are originally in circular orbits. For the dynamical evolution of the population, the following interactions are considered: mutual gravitation between the bodies, physical collisions, gravitational perturbations from an existing Jupiter (present mass and orbit) and gas drag. The simulation is followed until a single body remains. The results show a single planetary core in a stable orbit with elements a = 9.78 au and e = 0.086, well in accord with the presentday Saturn. We also discuss the possible extension of these results to the formation of the other major planets.