Planetary migration
It has also become clear[citation needed] that terrestrial-mass planets may be subject to rapid inward migration if they form while the gas disk is still present.
Observations suggest that gas in protoplanetary disks orbiting young stars have lifetimes of a few to several million years.
By the reaction principle of classical mechanics, the gas exerts an equal and opposite gravitational force on the body, which can also be expressed as a torque.
Small planets undergo Type I disk migration driven by torques arising from Lindblad and co-rotation resonances.
Lindblad resonances excite spiral density waves in the surrounding gas, both interior and exterior of the planet's orbit.
Type I migration in a local isothermal disk was shown to be compatible with the formation and long-term evolution of some of the observed Kepler planets.
In a Type I regime, viscous torques can efficiently counter this effect by resupplying gas and smoothing out sharp density gradients.
[13] In more realistic situations, unless extreme thermal and viscosity conditions occur in a disk, there is an ongoing flux of gas through the gap.
[11][15] In some situations, when planets induce eccentric perturbation in the surrounding disk's gas, Type II migration may slow down, stall, or reverse.
In fact, they can be interpreted and modeled as a single regime of migration, that of Type I appropriately modified by the perturbed gas surface density of the disk.
[17][18] Type III migration is driven by the co-orbital torques from gas trapped in the planet's libration regions and from an initial, relatively fast, planetary radial motion.
[20] In the case of the Solar System, Uranus and Neptune may have been gravitationally scattered onto larger orbits by close encounters with Jupiter and/or Saturn.
[21][22] Systems of exoplanets can undergo similar dynamical instabilities following the dissipation of the gas disk that alter their orbits and in some cases result in planets being ejected or colliding with the star.
[24] As in the Nice model, systems of exoplanets with an outer disk of planetesimals can also undergo dynamical instabilities following resonance crossings during planetesimal-driven migration.
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.
[26] A planetary orbit that is inclined relative to the plane of a binary star can shrink due to a combination of Kozai cycles and tidal friction.
Interactions with the more distant star cause the planet's orbit to undergo an exchange of eccentricity and inclination due to the Kozai mechanism.
The migration of a planet beginning with a similar angular momentum as the disk depends on potential sinks and sources of the planetesimals.
[35] In the grand tack hypothesis the migration of Jupiter is halted and reversed when it captured Saturn in an outer resonance.
[36] The halting of Jupiter's and Saturn's migration and the capture of Uranus and Neptune in further resonances may have prevented the formation of a compact system of super-earths similar to many of those found by Kepler.
[42] The migration of the outer planets is a scenario proposed to explain some of the orbital properties of the bodies in the Solar System's outermost regions.
[43] 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.
[44] According to this scenario the Kuiper belt was originally 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.
Encounters between the planets followed causing Neptune to surge past Uranus and plough into the dense planetesimal belt.
[45] 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.
