  We present two new in situ core accretion simulations of Saturn with planet
formation timescales of 3.37 Myr (model S0) and 3.48 Myr (model S1), consistent
with observed protostellar disk lifetimes. In model S0, we assume rapid grain
settling reduces opacity due to grains from full interstellar values (Podolak
2003). In model S1, we do not invoke grain settling, instead assigning full
interstellar opacities to grains in the envelope. Surprisingly, the two models
produce nearly identical formation timescales and core/atmosphere mass ratios.
We therefore observe a new manifestation of core accretion theory: at large
heliocentric distances, the solid core growth rate (limited by Keplerian
orbital velocity) controls the planet formation timescale. We argue that this
paradigm should apply to Uranus and Neptune as well.
