Proposed Solutions to the Time-Scale Problem.
- Embryo Atmospheres: (Inaba & Ikoma)
- Embryos can capture extended atmospheres from gas disk (Pollack et al).
- This significantly increases physical cross-section of embryos.
- Particularly if the planetesimals are small.
- Shear-Dominated Accretion:
- Growth can be fast if scale height of particle disk is smaller then Hill sphere (RH).
- Shear-dominated regime.
- Rafikov suggested combining fragmentation with drag to help solve this problem.
- Also see Wetherill & Stewart.
- Collisional Damping: (Goldriech et al.)
- As embryos excite planetesimals collisions will fragmentation.
- planetesimals will adjust size until mean free path is small damping.
- This helps because there is no significant radial migration.
- Evaporation/Condensation Near Snow-Line: (Stevenson & Lunine)
- As planetesimals migrate inward due to drag the evaporate at snow-line.
- Some water diffuses outward and recondenses larger solid surface density.
But none of this has been tested in a full N-body simulation.