"Planetary Formation: Oligarchy or Monarchy?
It is generally accepted that the terrestrial planets formed by "oligarchic" growth. A swarm of small planetesimals quickly produced protoplanetary embryos by runaway accretion; these had similar masses and roughly equal orbital spacings. On a longer timescale, these embyos merged to form a few planets. This scenario is reasonably successful in accounting for the masses and orbital properties of the terrestrial planets, but in the outer solar system fails to produce bodies large enough to become cores of the gas giants during the lifetime of the solar nebula. Results of the PSI multi-zone accretion code suggest that the mode of planetary growth was qualitatively different in the outer solar system, and that a few planetary embryos formed in relative isolation. The lack of competition allowed rapid growth to larger sizes, and the system of outer planets may be the result of this "monarchical" growth.