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E. W. Thommes (UC Berkeley), M. J. Duncan (Queen's University, Ontario), H. F. Levison (SwRI Boulder)
The initial runaway growth in a planetesimal disk transitions to a slower, self-regulating ``oligarchic'' mode when the largest bodies are still much less than an Earth mass (Ida and Makino, Icarus 1993, Kokubo and Ida, Icarus 1998, 2000). We extend the analytic estimates for oligarchic growth rates of Kokubo and Ida (2000) to obtain a simple global model of the planetary embryo formation process throughout the protoplanetary disk. The model predicts a steep front of growth which sweeps outward over time, in a manner which depends sensitively on the disk's surface density. N-body simulation results are consistent with the model, but show that it overestimates the growth rates at later times, when interactions among protoplanets become important. We examine the role that the ``snow line'' (the heliocentric radius at which water ice forms in the disk) may have played in the formation of giant planet cores and the asteroid belt, and discuss implications for extrasolar planetary systems. In agreement with previous work, we find that the in-situ formation of Uranus and Neptune is very difficult to reconcile with oligarchic growth, which suggests that these planets may have originated at heliocentric distances significantly smaller than their present ones (Thommes, Duncan and Levison, Nature 1999).