Conclusions Summary of giant planet formation models

The smoothness of the distribution of masses of young M stars, free-floating brown dwarfs, and even free-floating objects somewhat below the deuterium-burning limit, argues strongly that these bodies formed in the same manner, i.e., via collapse, in some cases augmented by fragmentation. In contrast, the mass gap in nearby companions to Sun-like stars (the brown dwarf desert) is convincing evidence that (at least most of) the known giant planets formed in a different manner.

Various models for giant planet formation have been proposed. According to the prevailing core nucleated accretion model, giant planets begin their growth by the accumulation of small solid bodies, as do terrestrial planets. However, unlike terrestrial planets, the growing giant planet cores become massive enough that they are able to accumulate substantial amounts of gas before the protoplanetary disk dissipates. The primary question regarding the core-accretion model is whether planets with small cores can accrete very massive gaseous envelopes within the lifetimes of gaseous protoplanetary disks.

The main alternative giant planet formation scenario is the disk instability model, in which gaseous planets form directly via gravitational instabilities within protoplanetary disks. Formation of giant planets via gas instability has never been demonstrated for realistic disk conditions. Moreover, this model has difficulty explaining the supersolar abundances of heavy elements in Jupiter and Saturn, and it does not explain the origin of planets like Uranus and Neptune. Nonetheless, it is possible that some giant planets form via disk instability.

Most models for extrasolar giant planets suggest that they formed as Jupiter and Saturn are believed to have (in nearly circular orbits, far enough from the star that ice could condense), and subsequently migrated to their current positions, although some models suggest in situ formation. Gas giant planet formation may (or may not) be common, because the gas within most of protoplanetary disks could be depleted before solid planetary cores grow large enough to gravitationally trap substantial quantities of gas. Additionally, an unknown fraction of giant planets migrate into their star and are consumed, or are ejected into interstellar space via perturbations of neighboring giant planets—so even if giant planet formation is common, these planets may be scarce.

This work was supported by NASA's Solar System Origins Program under RTOP 188-07-21-03.

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