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Modeling the formation of the ice giants Uranus and Neptune is a long-lastingproblem in planetary science. Due to gas-drag, collisional damping, andresonant shepherding, the planetary embryos repel the planetesimals away fromtheir reach and thus they stop growing (Levison et al. 2010). This problempersists independently of whether the accretion took place at the currentlocations of the ice giants or closer to the Sun. Instead of trying to push therunaway/oligarchic growth of planetary embryos up to 10-15 Earth masses, weenvision the possibility that the planetesimal disk could generate a system ofplanetary embryos of only 1-3 Earth masses. Then we investigate whether theseembryos could have collided with each other and grown enough to reach themasses of current Uranus and Neptune. Our results point to two major problems.First, there is typically a large difference in mass between the first and thesecond most massive core formed and retained beyond Saturn. Second, in manysimulations the final planetary system has more than two objects beyond Saturn.The growth of a major planet from a system of embryos requires strong dampingof eccentricities and inclinations from the disk of gas. But strong dampingalso favors embryos and cores to find a stable resonant configuration, so thatsystems with more than two surviving objects are found. In addition to theseproblems, in order to have substantial mutual accretion among embryos, it isnecessary to assume that the surface density of the gas was several timeshigher than that of the minimum-mass solar nebula. However this contrasts withthe common idea that Uranus and Neptune formed in a gas-starving disk, which issuggested by the relatively small amount of hydrogen and helium contained inthe atmospheres of these planets. Only one of our simulations "by chance"successfully reproduced the structure of the outer Solar System.

Considerations on the accretion of Uranus and Neptune by mutual collisions of planetary embryos in the vicinity of Jupiter and Saturn