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 .