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 can exchange angular momentum with the surrounding gas in the protoplanetary disk so that their orbits change gradually. Although the sense of migration is typically inwards in locally isothermal disks, outward migration may occur in disks that possess entropy gradients.
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
This type of orbital migration arises from the gravitational force exerted by a sufficiently massive body embedded in a disk on the surrounding disk's gas, which perturbs its density distribution. By the reaction principle of classical mechanics, the gas exerts an equal and opposite gravitational force on the body, which can also be expressed in terms of a torque. This torque alters the angular momentum of the planet's orbit, resulting in a variation of the orbital elements, such as the semi-major axis (but all orbital elements can be affected). An increase over time of the semi-major axis leads to outward migration, i.e., away from the star, whereas the opposite behavior leads to inward migration.
Type I migration
Planets can drive spiral density waves in the surrounding gas or planetesimal disk, via their perturbing gravitational potential. In the Type I regime, the disk response to this perturbation is linear. 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 does the interior wave. This causes the planet to lose orbital angular momentum and the planet then migrates inwards on timescales that are initially short relative to the million-year lifetime of the disk. Torques are also exerted by gas co-rotating with the planet, which shares the same orbit as (or a proximate orbit to) the planet. Co-rotation torques typically raise the angular momentum of the planet, pushing it away from the star. However, in locally isothermal disks and far from steep density gradients, co-rotation torques are generally overpowered by wave, or Lindblad, torques. Outward migration in isothermal disks is also possible, but it requires the presence of steep gradients of surface density and/or temperature. Type I migration was shown to be compatible with the formation of some of the observed Kepler planets.
Type II migration
Planets of more than several tens of Earth's masses start to clear a gap in the disk's density distribution (under typical thermodynamical and viscosity 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 (for sufficiently deep gaps) is of the same order of magnitude as the accretion timescale of the disk. This is one hypothesis for how some or most "hot Jupiters" formed. Unless extreme thermal and viscosity conditions are assumed in a disk, there is an ongoing flux of gas through the gap. As a consequence of this mass flux, torques acting on a planet can be susceptible to local disk properties, akin to torques at work during Type I migration. Therefore, in viscous disks, Type II migration can be typically described as a modified form of Type I migration. The transition between Type I and Type II migration is generally smooth, but deviations from a smooth transition have also been found.
Type III migration
In this regime of migration, planets interact with large-scale vortices within the disc. But other interpretations exist, which are based on the circulation of co-orbital material during the migration of the planet. This regime of migration may only apply to planets that can open partial gaps (via tidal interactions) in the gas surface density of the disk. Originally, it was thought to originate from gas streaming across the orbit of the planet, in the opposite direction as the planet's radial motion. More recently, the torques driving this mode of migration were associated to the gas trapped in the libration regions and radially moving with the planet. In this scenario, the torques arise from a density asymmetry between the gas on the leading and the trailing side of the planet, which develops in response to the planet's radial motion.
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 divergently crossed the 2:1 orbital resonance, in which 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 moving outwards themselves. 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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