Marginally unstable disks

Planet formation unfolds differently beyond the snowline, where water condensation enhances the surface density. Here massive cores (> 5-10 MEarth) may form rapid enough to accrete directly and retain nebular gas. These massive cores, if formed prior to the dispersal of the gas disk, rapidly reach Jupiter masses, forming giant planets. An alternative mechanism that may be responsible for the formation of some giant planets is gravitational instability in a massive, marginally unstable disk (e.g. Boss 2007 Mayer etal. 2007). 

At present, it is unclear whether the protoplanetary nebula could have evolved to the point where it was marginally unstable, or whether disk instabilities would have redistributed mass in the disk prior to the formation of gravitationally bound clumps. If marginally unstable disks do develop, then giant-planet formation by disk instability seems unavoidable. It is worth noting that current simulations of disk instability tend to generate planets with masses greater than... 

Clump Formation in a Marginally Gravitationally Unstable Disk... 

Mixing processes in marginally gravitationally unstable disks... 

Some progress has been made in two-dimensional hydrodynamic models of a thick disk evolving under the action of a globally defined alpha viscosity, representing the effects of torques in a marginally gravitationally unstable disk (Yorke and Bodenheimer, 1999), but in these models the evolution eventually slows down and leaves behind a fairly massive protostellar disk after 10 Myr. 

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