Disk-Planet Interaction: Triggered Formation and Migration
  We present three-dimensional SPH calculations of giant planets embedded in gaseous disks. Our findings are collected into a map of parameter space, exhibiting four distinct regions: Type I migration, gap formation, triggered formation of more planets, and wholly unstable disks. For Type I migration of the planet due to secular interactions with the disk material, the migration rate depends linearly on the disk mass, and is independent of the initial planet mass. For more massive disks, the planet can disturb the disk strongly enough to trigger the collapse of gas into additional giant planets. When additional planets form, their interaction as point masses dominates the subsequent behavior of the system. This mechanism allows for the rapid formation of Jupiter-mass and higher planets. Migration due to interaction with the disk can significantly change the orbits of giant planets in gas disks. 
Graeme Lufkin, Thomas Quinn and Fabio Governato