Ideas on gas-giant planet formation take shape

Rocky planets such as Earth and Mars are born when small particles smash together to form larger, planet-sized clusters in a planet-forming disk, but researchers are less sure about how gas-giant planets such as Jupiter and Saturn form. Is core accretion--the process that creates their smaller, terrestrial cousins--responsible? Or could an alternate model known as disk instability--in which the planet-forming disk itself actually fragments into a number of planet-sized clumps--be at work? Could both be possible under different circumstances?

Recent work from the Carnegie Institution's Department of Terrestrial Magnetism explores both possibilities. This and other relevant work regarding planet formation is presented at the NASA Astrobiology Science Conference (AbSciCon) 2006 at the Ronald Reagan Building in Washington, D.C. March 26-30. See http://abscicon2006.arc.nasa.gov/ for details.

Carnegie Fellow Hannah Jang-Condell1 has devised a method to catch the early stages of gas-giant core accretion in the act. If actively accreting cores exist, they should leave a gravitational "dimple" in the planet-forming disk--even if the cores are only a fraction the size of Jupiter. Since disk instability would result in planet-sized fragments straight away, the existence of these young, intermediate-sized cores would be a clear indicator of core accretion.

The telltale gravitational dimples resemble craters on the Moon with sunlight shining in from the side: the inside of the edge nearest the star is shadowed, while the star-facing edge is illuminated. The bright side heats up and the shadowed side remains cool, yielding a distinct thermal pattern that an Earth-based observer should be able to see in the infrared spectrum. "If we could detect this signature in a protoplanetary disk, it would indicate the presence of a young planetary body that could go on to form a gas-giant via core accretion," Jang-Condell said.

In some situations, however, core accretion seems an unlikely model for gas-giant planet formation. For example, theoretical computer models by DTM staff member Alan Boss2 suggest that disk instability best explains planet formation around M dwarf stars, which have masses from one tenth to one half that of the Sun. Core accretion would likely take more than 10 million years around these small, gravitationally weak stars, while disk instability happens quickly enough to yield gas-giant planets in as little as 1,000 years.

"M dwarf stars dominate the stellar population in the solar neighborhood, and so are attractive targets for searching for habitable planets," Boss said. "The models show that gas-giant planets are indeed likely to form…at distances sufficiently large enough to permit the later formation of habitable, terrestrial planets."

Other talks and posters on planet formation at the conference include: A study of organic matter in the planet-forming disks of three young stars, ranging in age from less than one million to over 300 million years3; methods to detect water ice, methane ice, and silicate dust in the planet-forming disks of distant stars4; and a method to deduce the composition of far-off planets based on their mass and radius5.

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Talk and poster schedule subject to change. See http://abscicon2006.arc.nasa.gov/agenda.php for the latest information.

1Hannah Jang-Condell, "Planet Shadows in Disks as Signatures of Core Formation"
Tuesday, March 28, 2006, 10:40am
Reagan Center, Horizon B conference room, Session 12: Exploring Planets Around Other Stars II

2Alan Boss, "Planetary System Formation Around M Dwarf Stars"
Wednesday, March 29, 2006, 10:40am
Reagan Center, Polaris B conference room, Session 24: Astronomical Topics General Session

3Alycia Weinberger et al., "Observations of Hydrocarbons in Circumstellar Disks"
Tuesday, March 28, 2006, 5:05pm
Reagan Center, Polaris B conference room, Session 19: Extraterrestrial Prebiotic Chemistry I

4John Debes et al., "Dust and Ices in the Scattered Light of Planet Forming Disks"
Poster displayed throughout the conference. Poster session Monday night, March 27, 2006, 6-8pm
Reagan Center, Atrium Hall

5Catherine Hier-Majumder and Sara Seager, "Mass-Radius Relations for Planets"
Poster displayed throughout the conference. Poster session Monday night, March 27, 2006, 6-8pm
Reagan Center, Atrium Hall

The Carnegie Institution of Washington has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments throughout the U.S. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science. See www.carnegieinstitution.org

This work is supported by the NASA Astrobiology Institute (NAI). The NAI, founded in 1998, is a partnership between NASA, 16 major U.S. teams and six international consortia. NAI's goal is to promote, conduct, and lead integrated multidisciplinary astrobiology research and to train a new generation of astrobiology researchers. For more information about the NAI on on the Internet, visit: http://nai.nasa.gov/


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