MIGRATING PLANETS

A planet orbiting in a disk of planetesimals can experience an instabi lity in which it migrates to smaller orbital radii. Resonant interacti ons between the planet and plane-tesimals remove angular momentum from the planetesimals, increasing their eccentricities. Subsequently, the planetesimals either collide with or are ejected by the planet, reduc ing the semimajor axis of the planet. If the surface density of the pl anetesimals exceeds a critical value, corresponding to similar to 0.03 solar mass of gas inside the orbit of Jupiter, the planet will migrat e inward a large distance. This instability may explain the presence o f Jupiter-mass objects in small orbits around nearby stars.