Remarks on modeling the formation of Uranus and Neptune

We have studied two scenarios for the in situ formation of Uranus and Neptu ne from a hundred or so sub-Earth-sized planetary embryos initially on low- inclination, nearly circular orbits beyond Saturn. We find that giant plane ts do not form during integrations of such systems. Almost no accretion occ urs at all because the embryos are dynamically excited by each other and th e gravitational effects of Jupiter and Saturn on a timescale that is short compared to the collision timescale. This produces large eccentricities and inclinations that significantly decrease the collisional cross section of the embryos because it decreases the effects of gravitational focusing. As a result, giant planets do not grow. These simulations show that the standa rd model for the formation of the Uranus and Neptune is most likely not cor rect. (C) 2001 Academic Press.