Migration and dynamical relaxation in crowded systems of giant planets

This paper explores the intermediate-time dynamics of newly formed solar systems with a focus on possible mechanisms for planetary migration. We consider two limiting corners of the available parameter space-crowded systems containing N = 10 giant planets in the outer solar system and solar systems with N = 2 planets that are tidally interacting with a circumstellar disk. Crowded planetary systems can be formed in accumulation scenarios-if the disk is metal rich and has large mass-and through gravitational instabilities. The planetary system adjusts itself toward stability by spreading out, ejecting planets, and sending bodies into the central star. For a given set of initial conditions, dynamical relaxation leads to a well-defined distribution of possible solar systems. For each class of initial conditions, we perform large numbers (hundreds to thousands) of N-body simulations to obtain a statistical description of the possible outcomes. For N = 10 planet systems, we consider several different planetary mass distributions; we also perform secondary sets of simulations to explore chaotic behavior and longer term dynamical evolution. For systems with 10 planets initially populating the radial range 5 AU less than or equal to a less than or equal to 30 AU, these scattering processes naturally produce planetary orbits with a similar to 1 AU and the full range of possible eccentricity (0 6 1). Shorter period orbits (smaller a) are difficult to achieve. To account for the observed eccentric giant planets, we also explore a mechanism that combines dynamical scattering and tidal interactions with a circumstellar disk. This combined model naturally produces the observed range of semimajor axis a and eccentricity epsilon. We discuss the relative merits of the different migration mechanisms for producing the observed eccentric giant planets. (C) 2003 Elsevier Science (USA). All rights reserved.