Abstract
Gas giant planets are generally believed to form by a two step process. A solid core grows via the accretion of planetesimals and then captures a massive envelope from the solar nebula gas. Simulations based on this model (Pollack et al. 1996) have been successful in explaining many features of giant planets. Recent interior models of Jupiter and Saturn suggest smaller core masses than had been previously predicted. New evolutionary simulations of Jupiter were computed using various values of the grain opacity and the planetesimal surface density (Hubickyj et al. 2004). The implications of halting the accretion solid planetesimals at selected core mass values during the protoplanet's growth, thus simulating the presence of a competing embryo, have been explored.
Results demonstrate that decreasing the grain opacity reduces the evolution time by more than a factor 2. In fact, it is the reduction of the grain opacity in the upper portion of the envelope with temperature T < 500 K that has the largest effect on decreasing the formation time. Decreasing the surface density of the planetesimals lowers the final core mass of the protoplanet, but increases the formation timescale. A core mass cutoff results in the reduction of the time needed for a protoplanet to evolve to the stage of runaway gas accretion, provided the cutoff mass is not too small.
