Hybrid mechanisms for gas/ice giant planet formation

The effects of gas pressure gradients on the motion of solid grains in the solar nebula substantially enhance the efficiency of forming protoplanetary cores in the standard core accretion model in "hybrid" scenarios for gas/ice giant planet formation. Such a scenario is enhanced core accretion that results from Epstein drag-induced inward radial migration of millimeter-sized grains and subsequent particle subdisk gravitational instability needed to build up a population of 1 km planetesimals. Solid/gas ratios can be enhanced by nearly ~10 times over those in a minimum mass solar nebula (MMSN) distribution in the outer solar nebula (a > 20 AU), increasing the oligarchic core masses and decreasing formation timescales for protoplanetary cores. A 10 M⊕ core can form on ~106-107 yr timescales at 15-25 AU, compared to ~108 yr in the standard model, alleviating the major problem plaguing the core accretion model for gas/ice giant planet formation.