AAS 202nd Meeting, May 2003
Session 24 When Do Planets Form?
Topical Oral, Tuesday, May 27, 2003, 8:30-10:00am and 10:45am-12:30pm, 204

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[24.02] Giant Planet Formation by Disk Instability: Observational Consequences

A. P. Boss (DTM, Carnegie Institution)

There are two competing theories for the formation of giant planets, core accretion and disk instability. Core accretion is the generally accepted mechanism, where a multiple-Earth-mass core forms by collisional accumulation of planetesimals and then accretes a massive gaseous envelope. Disk instability envisions the formation of self-gravitating clumps of gas and dust in a marginally gravitationally-unstable disk, followed by coagulation and sedimentation of the dust grains to form a central core. These opposite extremes for giant planet formation have testable predictions: (i) Core accretion seems to require somewhat long-lived disks, implying that gas giants should not be commonplace, while disk instability can occur in even the shortest-lived disk, implying that gas giants should be abundant. Spectroscopic surveys will determine the frequency of gas giants on Jupiter-like orbits within the next decade. (ii) Core accretion takes millions of years to form gas giants, while disk instability forms gaseous protoplanets in thousands of years. Determining the epoch of gas giant planet formation by searching for astrometric wobbles indicative of gas giant companions around young stars with a range of ages (0.1 Myr to 10 Myr) should be possible with the Space Interferometry Mission. Detecting gaps caused by gas giant protoplanets in protoplanetary disks with the Atacama Large Millimeter Array offers another means to age-date the epoch of gas giant planet formation. (iii) Core accretion is hastened by a higher ratio of dust to gas, whereas disk instability occurs equally well for a range of dust opacities. Determining whether a high primordial metallicity is necessary for gas giant planet formation can be accomplished by spectroscopic and astrometric searches for gas giants around metal-poor stars. (iv) Ice giants are formed in the disk instability scenario through photoevaporation of their gaseous envelopes by EUV radiation from nearby OB stars. For a solar-mass star, stripping occurs for protoplanets orbiting outside Saturn's orbit. If ice giants are found to be common inside 10 AU around solar-mass stars by transit photometry (e.g., Kepler Mission), this would suggest their formation by core accretion rather than by disk instability, unless inward orbital migration had occurred.

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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.