Gap opening and planet migration
Phil Armitage
Low mass planets:
Type I migration
Gap formation:
Type II migration
Suppression of accretion
at high planet masses

The frames above are part of a movie (mpeg format, 3.2Mb) that illustrates how planets of different masses interact with the gaseous protoplanetary disk. In the animation, the mass of a planet on a fixed circular orbit grows exponentially in time from an initial mass of 3 Earth masses to a final mass of 10 Jupiter masses. Meanwhile, the response of a viscous protoplanetary disk is calculated using a high resolution 2D hydrodynamic simulation, and displayed in an almost corotating frame. The actual simulation ran for more than 600 orbits (far too many to display in an inertial frame), so the disk response to each mass planet is almost in a steady state. Three distinct stages can be identified:

(1) Low mass planets: For the lowest mass planets, the gravitational interaction between the planet and the gas disk is relatively weak. The planet excites a trailing spiral wave in the disk, and experiences a gravitational torque from the resulting perturbation to the disk surface density. In a real disk, this loss of angular momentum would cause the planet's orbit to decay, a process known as Type I orbital migration. This doesn't happen in the movie because the orbit is artifically kept fixed (and anyway the time scale of the simulation is too short for significant migration). The surface density profile of the disk (shown as function of radius in the inset graph) is not significantly modified by the presence of the planet at this stage.

(2) Gap opening: As the planet mass grows, the strength of the resonant interaction with the gas disk increases. The exchange of angular momentum repels gas away from near the planet's orbit, creating an annular gap where the disk surface density is lower than it would be in the absence of the planet. At first, as shown in the middle frame above, the gap is only partially evacuated of gas. The details of how the remaining gas in the coorbital region interacts with the planet in this regime remain unclear.

(3) Type II migration: Once the planet's mass is high enough (above a Jupiter mass for the particular parameters of this simulation), its gravitational interaction with the disk succeeds in forming a deep, clean gap. Some gas continues to overflow the gap edges and is captured within the Hill sphere of the planet, adding to the planet mass. The rate of mass growth due to accretion across gaps decreases as the planet grows, at least if the orbital eccentricity remains small. The overall exchange of angular momentum between the planet and disk is now governed by the viscous evolution of the disk, and the planet is expected to migrate in the same sense as the disk gas (usually inward at small radii) while maintaining its position within a gap.

For more details see Planetary migration, a review for STScI's 2005 May Symposium "A Decade Of Extrasolar Planets Around Normal Stars".