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 protoplanetary 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.
Types of 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.
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 orbital 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 (under typical thermodynamical conditions), 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. The timescale of this process corresponds approximately to the accretion timescale of the disk. This is one hypothesis for how some or most "hot Jupiters" form.
Type III migration
Planets interact with large-scale vortices within the disc.
Another possible mechanism that may move planets over large orbital radii is gravitational scattering by larger planets or, in a protoplantetary disk, gravitational scattering by over-densities in the fluid of the disk. In the case of the Solar System, Uranus and Neptune may have been gravitationally scattered in close encounters with Jupiter and/or Saturn. Planetesimals that were present in the early formation of the Solar System called oligarchs are much smaller than Uranus and Neptune and so are likely to have been scattered much further out and be roaming the space between the Kuiper belt and the Oort cloud. Sedna may be the first known example of such oligarch planets. Even smaller objects would have been scattered even further out to become the Oort cloud.
Tides between the star and planet modify the semi-major axis and orbital eccentricity. Disk migration lasts around a million years until the gas dissipates, but tidal migration continues for billions of years. Tidal evolution of close-in planets produces semi-major axes typically half as large as they were at the time that the gas nebula cleared. More massive planets probably undergo much more tidal migration than less massive ones.
In the Solar System
The migration of the outer planets is necessary to account for the existence and properties of the Solar System's outermost regions. Beyond Neptune, the Solar System continues into the Kuiper belt, the scattered disc, and the Oort cloud, three sparse populations of small icy bodies thought to be the points of origin for most observed comets. At their distance from the Sun, accretion was too slow to allow planets to form before the solar nebula dispersed, and thus the initial disc lacked enough mass density to consolidate into a planet. The Kuiper belt lies between 30 and 55 AU from the Sun, while the farther scattered disc extends to over 100 AU, and the distant Oort cloud begins at about 50,000 AU.
Originally, however, the Kuiper belt was much denser and closer to the Sun: it contained millions of planetesimals, and had an outer edge at approximately 30 AU, the present distance of Neptune.
After the formation of the Solar System, the orbits of all the giant planets continued to change slowly, influenced by their interaction with the large number of remaining planetesimals. After 500–600 million years (about 4 billion years ago) Jupiter and Saturn fell into a 2:1 orbital resonance; Saturn orbited the Sun once for every two Jupiter orbits. This resonance created a gravitational push against the outer planets, causing Neptune to surge past Uranus and plough into the dense planetesimal belt. The planets scattered the majority of the small icy bodies inwards, while themselves moving outwards. These planetesimals then scattered off the next planet they encountered in a similar manner, moving the planets' orbits outwards while they moved inwards. This process continued until the planetesimals interacted with Jupiter, whose immense gravity sent them into highly elliptical orbits or even ejected them outright from the Solar System. This caused Jupiter to move slightly inward. This scattering scenario explains the trans-Neptunian populations' present low mass.
The outer two planets of the Solar System, Uranus and Neptune, are believed to have migrated outward in this way from their formation in orbits near Jupiter and Saturn to their current positions, over hundreds of millions of years. Eventually, friction within the planetesimal disc made the orbits of Uranus and Neptune circular again.
In contrast to the outer planets, the inner planets are not believed to have migrated significantly over the age of the Solar System, because their orbits have remained stable following the period of giant impacts.
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