The two mechanisms that have been advanced for explaining the formation of giant planets are core accretion (“bottom up”) and disk instability (“top down”). Core accretion, the conventional mechanism, relies on the collisional accumulation of planetesimals to assemble ∼ 10 M⊕ solid cores, which then accrete massive gaseous envelopes from the disk gas (Mizuno, 1980; Lissauer, 1987; Pollack et al., 1996; Kornet et al., 2002; Inaba et al., 2003). In this scenario, the ice-giant planets probably did not form in situ (Levison and Stewart, 2001), but rather formed between Jupiter and Saturn and then were scattered outward to their present orbits (Thommes et al., 1999, 2002).
The alternative to core accretion is disk instability, where gas-giant protoplanets form rapidly through a gravitational instability of the gaseous portion of the disk (Cameron, 1978; Boss, 1997, 1998b, 2000, 2002a,b, 2003, 2004; Gammie, 2001; Mayer et al., 2002, 2004; Nelson, 2000; Nelson et al., 1998, 2000; Pickett et al., 1998, 2000a,b, 2003; Rice and Armitage, 2003, Rice et al., 2003a) and then more slowly contract to planetary densities. Solid cores form simultaneously with protoplanet formation by the coagulation and sedimentation of dust grains in the clumps of disk gas and dust.