Migration of Low-Mass Planets

Abstract

Planet migration is the variation over time of a planet’s semimajor axis, leading to either a contraction or an expansion of the orbit. It results from the exchange of energy and angular momentum between the planet and the disk in which it is embedded during its formation and can cause the semimajor axis to change by as much as two orders of magnitude over the disk’s lifetime. The migration of forming protoplanets is an unavoidable process, and it is thought to be a key ingredient for understanding the variety of extrasolar planetary systems. Although migration occurs for protoplanets of all masses, its properties for low-mass planets (those having up to a few Earth masses) differ significantly from those for high-mass planets. The torque that is exerted by the disk on the planet is composed of different contributions. While migration was first thought to be invariably inward, physical processes that are able to halt or even reverse migration were later uncovered, leading to the realization that the migration path of a forming planet has a very sensitive dependence on the underlying disk parameters. There are other processes that go beyond the case of a single planet experiencing smooth migration under the disk’s tide. This is the case of planetary migration in low-viscosity disks, a fashionable research avenue because protoplanetary disks are thought to have very low viscosity, if any, over most of their planet-forming regions. Such a process is generally significantly chaotic and has to be tackled through high-resolution numerical simulations. The migration of several low-mass planets is also is a very fashionable topic, owing to the discovery by the Kepler mission of many multiple extrasolar planetary systems. The orbital properties of these systems suggest that at least some of them have experienced substantial migration. Although there have been many studies to account for the orbital properties of these systems, there is as yet no clear picture of the different processes that shaped them. Finally, some recently unveiled processes could be important for the migration of low-mass planets. One process is aero-resonant migration, in which a swarm of planetesimals subjected to aerodynamic drag push a planet inward when they reach a mean-motion resonance with the planet, while another process is based on so-called thermal torques, which arise when thermal diffusion in the disk is taken into account, or when the planet, heated by accretion, releases heat into the ambient gas.