Accretion of Uranus and Neptune from inward-migrating planetary embryos blocked by Jupiter and Saturn

¹ Université de Bordeaux, Laboratoire d’Astrophysique de Bordeaux, UMR 5804, 33270 Floirac, France
e-mail: izidoro.costa@gmail.com
² Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Blvd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
³ Capes Foundation, Ministry of Education of Brazil, Brasília/DF 70040-020, Brazil
⁴ CNRS and Université de Bordeaux, Laboratoire d’Astrophysique de Bordeaux, UMR 5804, 33270 Floirac, France

Received: 16 December 2014
Accepted: 9 June 2015

Abstract

Reproducing Uranus and Neptune remains a challenge for simulations of solar system formation. The ice giants’ peculiar obliquities suggest that they both suffered giant collisions during their formation. Thus, there must have been an epoch of accretion dominated by collisions among large planetary embryos in the primordial outer solar system. We test this idea using N-body numerical simulations including the effects of a gaseous protoplanetary disk. One strong constraint is that the masses of the ice giants are very similar – the Neptune and Uranus mass ratio is ~1.18. We show that similar-sized ice giants do indeed form by collisions between planetary embryos beyond Saturn. The fraction of successful simulations varies depending on the initial number of planetary embryos in the system, their individual and total masses. Similar-sized ice giants are consistently reproduced in simulations starting with five to ten planetary embryos with initial masses of ~3–6 M_⊕. We conclude that accretion from a population of planetary embryos is a plausible scenario for the origin of Uranus and Neptune.