Numerical simulations of the accretion of Uranus and Neptune

New numerical simulations of the accretion process of the outer planets are carried out an N-body code. In 21 out of 30 computer runs 2 giant planets from beyond Saturn, whereas 1 forms in 8 cases and 3 in 1 case. The time scale of formation is a few 10(7) years. This rather short time scale seems to be consistent with the content of non-negligible amounts of hydrogen and helium in Uranus and Neptune, which could be accreted before the protoplanetry nebula dispersed. A marked radial shift of the accreting outer planets is found in our simulations in agreement with previous results. The results also seem to confirm that this radial migration is an effective migration mechanism to produce orbital configurations near mean motion resonances. Less than 50% of the solid material originally present in the system as planetesimals contributes to the formed protoplanets, the rest being mostly ejected by Jupiter and Saturn. The phase of strong scattering of bodies from the Uranus-Neptune region to the inner solar system lasts for a period of similar to 4 x 10(7) years. Around 10-20 Earth masses reach the region of the terrestrial planets, although the very short time scale for the massive influx of material(perhaps shorter than the time scale of Earth formation) might pose some difficulties to the theory that comets were major contributors of water to the Earth. (C) Elsevier Science Ltd. All rights reserved.