A planet orbiting in a disk of planetesimals can experience an instability in
which it migrates to smaller orbital radii. Resonant interactions between the
planet and planetesimals remove angular momentum from the planetesimals,
increasing their eccentricities. Subsequently, the planetesimals either collide
with or are ejected by the planet, reducing the semimajor axis of the planet.
If the surface density of planetesimals exceeds a critical value, corresponding
to 0.03 solar masses of gas inside the orbit of Jupiter, the planet will
migrate inward a large distance. This instability may explain the presence of
Jupiter-mass objects in small orbits around nearby stars.Comment: Latex, 14 pages, and 2 postscript figures. Appeared in Science, 2
January; Table 1 originally omitte
