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G. Bryden, D. N. C. Lin (UCO/Lick Observatory, UCSC, CA, USA)
The solar system's giant planets are all metal-rich relative to solar composition, suggesting that their accretion of gas was somehow inefficient and that the majority of gas available was left unaccreted. Protoplanetary gap formation is effective in determining an upper mass limit for protoplanets undergoing runaway gas accretion. This upper limit is a function of the local disk temperature and viscosity. More massive planets are able to form earlier in the nebula evolution when the disk gas is denser and hotter, while smaller planets such as Uranus and Neptune can only form gaps in the cold outer nebula. The process of gap formation fixes a planet's position relative to the gas, resulting in orbital migration as the gas evolves on a viscous time scale. If the disk were cold enough for proto-Neptune to open a gap, the planet would migrate outward with the gas, capturing planetesimals onto its 3:2 resonance. Under general nebular conditions, gap formation limits planet masses to values comparable to Jupiter's. The predicted distribution of planetary masses is similar to that observed among the extra-solar planets. Gap-forming protoplanets have the freedom to migrate inward and populate the inner regions of their systems, as observed. Overall, by incorporating gap formation and orbital migration into the otherwise standard picture of planetary formation, planetary masses and orbital configurations can be produced which are similar to those observed both within and outside of the solar system.
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