Dynamical processes of macro-accretion of Uranus and Neptune: A first look

The process of macro-accretion of Uranus and Neptune is studied using a program which simulates the random effect of gravitational scattering and accretionfor a system of Earth-sized objects. While the exclusion of a population of small planetesimals and the suppression of erosion and fragmentation at hypervelocity impact tend to bias the present result toward the formation and buildup of Earth- to Uranus-size objects, the numerical results are instructive in approximating the time scale (a few times 10⁸ years) of stochastic collisions and hence of the large tilting of the protoplanets; we find that (i) it is probable that the insertion of a small number of Earth-sized objects in the trans-Neptunian region acts as the driver of inward orbital diffusion for a cometary belt located outside the orbit of Neptune and (ii) it is possible that a few large planetesimals were scattered into orbits crossing the asteroid belt and hence gravitationally stirred the orbits of the main-belt asteroids. The dynamical process of macro-accretion of the outer planets thus could have a very extensive effect on the general structure and evolution of the Solar System.