Observations of extrasolar planetary systems suggest that the phenomenon of migration has played a significant role in their evolution. What is involved? The idea is that once a giant planet has formed outside the ice limit, it may change its position within the system, generally moving inwards, through the effects of interactions with the disk of gas, the disk of planetesimals, other planets, or a companion to the central star. This mechanism was known to planetologists who conceived it to explain the existence of resonant satellite systems. It came late to the history of scenarios for the formation of the planets, because the planets in the Solar System have not undergone extensive migration.
The key stage in the formation of a planet is the formation of a solid core. The time formation takes and the mass of this core depend on the density of solid material in the protoplanetary disk. This density is more significant outside the ice line, the minimum distance for the condensation of water ice, rather than inside, where only rocky material and metals condense. This limit lay at about 5 AU under the conditions in the primitive Solar System. Within the ice limit, the protoplanets were not sufficiently massive to accrete a gaseous envelope. Outside, the greater density of solid material favoured the rapid formation of massive cores, which attracted the surrounding gas (see Chap. 4). The natural location for the formation of giant planets is therefore outside the ice line. In the Solar System, however, the terrestrial planets and the giant planets are, in effect, on opposite sides of this boundary. In addition, Jupiter lies at just the distance from the Sun where the density was greatest. In this scheme, migration did not appear to be required.
It was only with the discovery of trans-Neptunian objects in orbits in resonance with Neptune, and time-scales that were rather too long for the formation of Uranus and Neptune, that the migration mechanism found a place in the story of the Solar System's formation.
The mechanism of migration within the Solar System has been the subject of various models. One example is shown in Fig. 6.6. Although all the models agree that Jupiter's migration has been of very little extent, some predict migration outwards, whereas others suggest migration inwards. All the models predict a migration of several astronomical units outwards for Uranus and Neptune, which therefore formed closer to the Sun. Certain models predict that Neptune was initially
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