Planetary Migration: What Does It Mean for Planet Formation?

Gravitational interactions between a planet and its protoplanetary disk change the planet's orbit, causing the planet to migrate toward or away from its star. Migration rates are poorly constrained for low-mass bodies but reasonably well understood for giant planets. In both cases, significant migration will affect the details and efficiency of planet formation. If the disk is turbulent, density fluctuations will excite orbital eccentricities and cause orbits to undergo a random walk. Both processes are probably detrimental to planet formation. Planets that form early in the lifetime of a disk are likely to be lost, whereas late-forming planets will survive and may undergo little migration. Migration can explain the observed orbits and masses of extrasolar planets if giants form at different times and over a range of distances. Migration can also explain the existence of planets orbiting close to their star and of resonant pairs of planets.