planetary migration

Planetary migration is a change in a planet's orbit, either smooth migration from interaction with a protoplanetary disk (planet-disk interaction, gas disk migration or just disk migration) or planetesimals, or violent migration, due to close encounters or collisions of planets (planet-planet scattering). The former is classified by underlying mechanism as follows:

• Type I migration is due to torque created by interaction with spiral density waves the planet creates in a disk. Planets of terrestrial mass can be moved this way. Timescale is on the order of 100 million years to migrate from 1 AU into the star so it is almost too efficient, e.g., it seems like it could undercut theories of planet formation since it would seem to destroy Earth and analogous planets on formation.
• Type II migration is due to interaction with material filling a disk gap formed by the planet. Planets with more than 10 Earth masses can move this way and it is a way a hot Jupiter can be positioned.
• Type III migration is due to interaction with large-scale vortices within the disk.

Planet-planet scattering (gravitational scattering) seems necessary to explain some observed orbital inclinations, which include misalignment between planetary orbits and stellar rotation, and even retrograde orbits. Smaller inclinations might be ascribed to a tilt of the star or a tilt of the disk by some unknown means. In simulations, planet scattering has produced larger inclinations, and even coplanar flips leading to retrograde orbits.