Forming a jovian-sized planet in a protostellar disk is not an easy thing to do. It is difficult to get the gas and dust in a swirling disk around a young star to coalesce into a giant gas planet before the gas and dust are lost to space and the disk dissolves. Core accretion, whereby particle collisions lead to the formation of solid-body cores and core collisions lead to terrestrial-sized planets, assumes that jovian-sized planets form by accumulating a gaseous atmosphere that then contracts around these cores. Gravitational disk instability, in which instabilities in the disk cause fragmentation and the formation of self-gravitating gaseous regions, assumes that jovian-sized planets form from contraction of these gaseous regions.
Rice et al. combine hydrodynamic simulations of disk fragmentation with orbital integrations of objects created by disk instability and the predominance of self-gravity to follow the evolution of a disk to a planetary system. After about 12,000 years, 83 objects have formed. Over the course of another 21 million years, 74 of these objects are ejected (19 of which have masses equal to or greater than Jupiter), 7 are removed by encounters with other stars, 1 collides with the central star, and 1 (the most massive of the 83 objects) remains bound to the central star. These simulations suggest that the most massive planets (5 to 10 times Jupiter's mass) may preferentially form by disk instabilities and that there may be a lot of free-floating planets in interstellar space that are unaccounted for because they are nearly impossible to detect. — LR
Mon. Not. R. Astron. Soc. 346, L36 (2003).