  The core accretion theory of planet formation has at least two fundamental
problems explaining the origins of Uranus and Neptune: (1) dynamical times in
the trans-Saturnian solar nebula are so long that core growth can take > 15
Myr, and (2) the onset of runaway gas accretion that begins when cores reach 10
Earth masses necessitates a sudden gas accretion cutoff just as the ice giant
cores reach critical mass. Both problems may be resolved by allowing the ice
giants to migrate outward after their formation in solid-rich feeding zones
with planetesimal surface densities well above the minimum-mass solar nebula.
We present new simulations of the formation of Uranus and Neptune in the
solid-rich disk of Dodson-Robinson et al. (2009) using the initial semimajor
axis distribution of the Nice model (Gomes et al. 2005; Morbidelli et al. 2005;
Tsiganis et al. 2005), with one ice giant forming at 12 AU and the other at 15
AU. The innermost ice giant reaches its present mass after 3.8-4.0 Myr and the
outermost after 5.3-6 Myr, a considerable time decrease from previous
one-dimensional simulations (e.g. Pollack et al. 1996). The core masses stay
subcritical, eliminating the need for a sudden gas accretion cutoff. Our
calculated carbon mass fractions of 22% are in excellent agreement with the ice
giant interior models of Podolak et al. (1995) and Marley et al. (1995). Based
on the requirement that the ice giant-forming planetesimals contain >10% mass
fractions of methane ice, we can reject any solar system formation model that
initially places Uranus and Neptune inside the orbit of Saturn. We also
demonstrate that a large population of planetesimals must be present in both
ice giant feeding zones throughout the lifetime of the gaseous nebula.
