CARNEGIE INSTITUTION OF WASHINGTON
For release 20 June 1997
Contact Alan Boss at 202-686-4370 ext. 4402; or
Tina McDowell at Carnegie news office, 202-745-0780; tmcdowell@ciw.edu

NEW BOOST FOR ONE-STEP GIANT PLANET FORMATION THEORY

In new research announced in the 20 June issue of Science magazine, Alan Boss, a scientist with the Carnegie Institution's Department of Terrestrial Magnetism, makes the strongest case yet for resurrecting a theory explaining how massive planets form. Last October, Boss announced that giant planets as massive as Jupiter (or larger) can be made rapidly in a single step through the self-gravity of the gas in the dust-rich disks that form planetary systems. This theory was first proposed in 1951 but was discarded because it was not believed to lead to the formation of the large ice/rock cores thought to characterize the large planets. Instead, another theory, a two-step "core accretion theory," took its place. This theory holds that the ice/rock cores formed first, through a long period of planetesimal collisions, followed by rapid accretion of gas from the rotating disk. One major problem with this theory is that by the time the core is large enough to accrete gas (a process taking a million years or more), there may not be any gas left in the disk. In some cases, young, solar-type stars are believed to dissipate their gas much earlier.
Now, Boss reports that newly refined models of the compositions of Jupiter and Saturn indicate that the rock/ice cores of these giant planets are smaller than previously thought. If this is true, he writes, then "much of the attraction of the core accretion theory would be lost, because lower-mass cores might not be able to trigger gas accretion." In addition, the one-step theory might now be able to account for the formation of the more modest-sized cores newly inferred for Jupiter and Saturn.
The one-step theory that Boss espouses, whereby the solar nebula breaks up through its own self-gravity into clumps of dust and gas (termed giant gaseous protoplanets, or GGPP), which then contract and collapse to form giant planets, was discarded also because of thermodynamical problems. It seemed to require that disk instability proceed at a fixed temperature at a give orbital radius. Now, Boss has performed three-dimensional hydrodynamics computer simulations showing this is not necessarily true. He finds that GGPP formation can occur in outer protoplanetary disks regardless of the exact thermodynamics of the instability, even when the forming GGPP are allowed to heat up without losing any energy by radiation.
Boss finds that the GGPP model is capable of forming a multiple Jupiter-mass sized giant planet with a modest ice/rock core (in the 3-10 Earth-mass range) in some 1000 years, in contrast to the million or more years required by the core accretion mechanism. Boss concludes, "The one-step model seems to be able to make giant planets like those in our solar system, or like those detected recently around nearby stars."
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The Department of Terrestrial Magnetism (DTM), in Washington, D.C., is one of five research departments of the Carnegie Institution of Washington, a nonprofit science research and educational institution founded in 1902 by Andrew Carnegie. The institution's other centers of research are the Geophysical Laboratory, also in Washington; the Department of Plant Biology in Stanford, California; the Department of Embryology in Baltimore, Maryland; and the Carnegie Observatories, based in Pasadena, California. The institution's Las Campanas Observatory is in Chile. DTM is led by its director, Sean C. Solomon. The institution's president is the biologist Maxine F. Singer.