Orbital migration is a major change in a planet's orbit around its host star, either sudden or gradual, caused by interaction with one or more other large bodies (such as neighboring planets) or with the remnants of a protoplanetary nebula, or by some other process.
Outward orbital migration may result in a planet's complete ejection from its orbit to become an unbound rogue planet. Inward orbital migration almost certainly accounts for the unusually small or highly elliptical orbits of many of the first exoplanets to be discovered. Theories of inward migration were hastily put forward following the unexpected discovery of massive planets in extremely small orbits about their central stars. Since it is virtually impossible to understand how such large worlds, which are presumably gas giants like Jupiter, could have formed in their present orbits of radii less than 2 AU, and in some cases less than 0.2 AU, the favored conclusion is that they formed further out and were then caused to move inward. Several processes have been suggested to account for the drastic orbital shift, including near encounters with other massive worlds, resulting in an effect like the scattering of billiard balls, and viscous drag as the new planet plowed its way through the remnants of the dusty nebula from which it was formed, losing energy as it goes. Both theory and observation suggest that the migration time to a small orbit for a giant planet that formed at a radius of 5 AU is less than 1 million yr.