Simulations of planet migration driven by the scattering of smaller bodies

Abstract

Planet migration is an important part of the formation of planetary systems, both in the Solar system and in extrasolar systems. When a planet scatters nearby comet- and asteroid-size bodies called planetesimals, a significant angular momentum exchange can occur, enough to cause a rapid, self-sustained migration (change of semi-major axis) of the planet. This migration has been studied for the particular case of the four outer planets of the Solar System, but is not well understood in general.

This thesis used the Miranda computer simulation code to perform a broad parameter-space survey of the physical variables that determine the migration of a single planet in a planetesimal disk. A simple model presented within matched well with the dependencies of the migration rate for low-mass planets in relatively high-mass disks. When the planet's mass exceeded that of the planetesimals within a few Hill radii, the migration rate decreased strongly with planet mass. Other trends were identified with the root-mean-squared eccentricity of the planetesimal disk, the mass of the particles dragged by the planet in the corotation region, and the index of the surface density power law. The issue of resolution was also addressed, and it was shown that many previous works in this field may have suffered from being under-resolved.

The trends were discussed in the context of an analysis of the scattering process itself, which was performed using a large simulation of massless planetesimals. In particular, a bias in scattering timescales on either side of the planet's orbit leads to a very strong tendency for the planet to migrate inwards, instead of outwards.

The results of this work show that planet migration driven by planetesimal scattering should be a widespread phenomenon, especially for low-mass planets such as still-forming protoplanets. The simple model provided here, augmented by many more subtle effects, will prove essential to any future work in this underestimated field.