Ideas on gas-giant planet formation take shape (Forwarded)

Carnegie Institution of Washington
Washington, D.C.

Carnegie contacts:

Alan Boss, 202-478-8858
John Debes, 202-478-8862
Catherine Hier-Majumder, 202-478-8846
Hannah Jang-Condell, 202-478-8863
Alycia Weinberger, 202-478-8852

March 22, 2006

Ideas on gas-giant planet formation take shape

Washington, D.C. -- 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-Condell [1] 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 Boss [2] 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 years [3];
methods to detect water ice, methane ice, and silicate dust in the
planet-forming disks of distant stars [4]; and a method to deduce the
composition of far-off planets based on their mass and radius [5].

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

[1] Hannah 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

[2] Alan 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

[3] Alycia 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

[4] John 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

[5] Catherine 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 the Internet, visit:
http://nai.nasa.gov/