Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's 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 insufficient mass at such small radii and the temperature is too high to allow the formation of rocky or icy planetesimals. It has also become clear that terrestrial-mass planets may be subject to rapid inward migration if they form while the gas disk is still present. This may affect the formation of the cores of the giant planets (which have masses of the order of 10 Earth masses), if those planets form via the core accretion mechanism.
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Types of disk
Gas disk
Protoplanetary gas disks around young stars are observed to have lifetimes of a few million years. If planets with masses of around an Earth mass or greater form while the gas is still present, the planets are thought to transfer angular momentum to the surrounding gas in the protoplanetary disk so that their orbits spiral 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 to be thrown into new orbits. This results in angular momentum exchange between the planets and the planetesimals, and leads to migration (either inward or outward). Outward migration of Neptune is believed to be responsible for the resonant capture of Pluto and other Plutinos into the 3:2 resonance with Neptune.
Types of migration
Type I migration
Terrestrial mass planets drive spiral density waves in the surrounding gas or planetesimal disk. An imbalance occurs in the strength of the interaction with the spirals inside and outside the planet's orbit. In most cases, the outer wave exerts a somewhat greater torque on the planet than the interior wave. This causes the planet to lose angular momentum and the planet then migrates inwards on timescales that are 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
- Solar Nebula (and the nebular theory of solar system formation)
References
- Goldreich, P., and Tremaine, S. 1979, Astrophysical Journal, 233, 857
- Lin, D. N. C., and Papaloizou, J. 1979, Monthly Notices of the Royal Astronomical Society, 186, 799
