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The gravitational instability model (disk instability model) is a model of giant planet formation: that the gathering of gas forming a planet begins with instabilities in the gas cloud making up a protoplanetary disk. If the mass becomes sufficiently irregular or patchy, some denser portions may have insufficient pressure to counteract gravity and collapse into planets. A scenario that builds dense regions is a type of disk turbulence (gravitoturbulence) which can produce spiral density waves.
This is the second-most widely accepted general model of gas planet formation, the first being the core accretion model, which presumes a solid core (much like a rocky planet, but possibly ice) starts the process with gas accretion afterward. Both models have problems, both in explaining many extra-solar planet systems and in explaining all the solar system gas giants. These models of planet formation fit into the nebular hypothesis.
The term gravitational instability model likely has also been used for models of other astrophysical phenomena: gravitational instability (GI), the condition of density fluctuations undergoing positive feedback due to gravity, is considered a contributor to the formation of the large scale structures of the universe, such as dark matter halos and galaxy clusters.