Volatiles would not have condensed in the inner solar system, however. One obvious suggestion is that some kind of glue was involved. Fluffy aggregates of dust have been made in the lab but these are typically less than a centimeter in size ( Blum, 2000). Although we do not know how stage 2 happens, somehow it must be possible. Planetesimals would need to be about a kilometer in size in order for the gravitationally driven stage 3 to start. The second stage is poorly understood but is necessary in order to build objects that are of sufficient mass for gravity to play a major role. It provides a relatively dense plane of material from which the planets grow. Stage 1 takes place over timescales of thousands of years if there is no turbulence but much longer otherwise. Stochastic growth of larger objects through late-stage collisions. Runaway growth of planetary embryos up to ∼10 3 km in size and 4. Growth of planetesimals up to ∼1 km in size 3. Settling of circumstellar dust to the midplane of the disk 2. In the simplest terms accretion of terrestrial planets is envisaged as taking place in four stages: 1. The most widely accepted model of terrestrial planet formation is the planetesimal theory ( Chambers, 2004). This migration could have continued for several hundred million years after the planets acquired most of their mass, and may have triggered the high rate of impacts that occurred during the late heavy bombardment on the Moon and inner planets ( Gomes et al., 2005). The most likely outcome is that Jupiter moved inwards, while Saturn, Uranus, and Neptune moved out, with the ice giants moving at least several astronomical units in each case ( Hahn and Malhotra, 1999). The giant planets are likely to have migrated significantly due to dynamical interactions with the young Kuiper belt, which was more massive than it is today. The embryos would then have undergone rapid runaway growth rather than slower oligarchic growth, reaching the mass of Uranus in only a million years ( Weidenschilling et al., 2004).Īnother possibility is that Uranus and Neptune formed closer to the Sun, where growth is more efficient, and later moved to their current locations ( Thommes et al., 1999). Uranus and Neptune might have formed when two embryos were injected into the outer solar system from orbits closer to the Sun, at a time when no other large bodies existed there. However, these planets contain significant amounts of hydrogen and helium, which suggests they formed while the nebula was still present. A more severe problem is that the Sun’s gravity is weak here, so gravitational interactions between embryos were capable of ejecting a large fraction of the solid material from the region altogether ( Levison and Stewart, 2001).Ĭalculations show that Uranus and Neptune might have formed within a few million years after the gaseous nebula dispersed, provided that planetesimals were less than a meter in size, perhaps due to collisional fragmentation ( Goldreich et al., 2004). Both these factors mean planetary growth was likely to be slow. The surface density of material in this part of the nebula was low compared to regions closer to the Sun, and orbital periods are long. The origin of the “ice giants,” Uranus and Neptune, poses significant problems for theories of planet formation. Chambers, in Treatise on Geochemistry, 2007 1.17.6.5 Formation of Uranus and Neptune
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