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Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellites orbital parameters, especially its semi-major axis. Planetary migration is the most likely explanation for 'hot Jupiters': extrasolar planets with jovian masses, but orbits of only a few days. The generally accepted theory of planet formation from a protostellar accretion disk predicts such planets cannot form so close to their stars, as there is simply insufficient mass in such small orbits. It has also become clear that terrestrial planets may be subject to rapid migration inwards, raising the question of how to explain the existence of terrestrial planets.
Gas Disk During the core accretion phase of solar system formation, a gas giant is thought to transfer angular momentum to surrounding gas in the protoplanetary disk so that its orbit spirals gradually inwards towards the primary. Planetesimal Disk During the late phase of planetary system formation, massive protoplanets and planetesimals gravitationally interact in a chaotic manner causing many planetesimals (particularly lower mass ones) to be thrown into new orbits. Type I Migration Terrestrial mass planets drive spiral density waves in the surrounding gas or planetesimal disk. An imbalance occurs in the shape between the spirals inside and outside the planet's orbit. The inner wave exerts a somewhat greater counterforce on the planet, causing it to migrate inwards on timescales short relative to the million-year lifetime of the disk. Type II Migration Planets of more than about 10 Earth masses clear a gap in the disk, ending Type I migration. However, material continues to enter the gap on the timescale of the larger accretion disk, moving the planet and gap inward on the accretion timescale of the disk. This is presumably how 'hot Jupiters' form. See also | ||||||||
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