UW Today

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January 9, 1999

Extrasolar planets favor stars with overabundance of heavy elements

News and Information

AUSTIN, Texas – A three-year spectroscopic survey shows a group of stars near our solar system have a much greater allotment of heavy elements than other nearby stars that are like our sun, a University of Washington astronomer reported today at the national meeting of the American Astronomical Society.

Detailed studies of 12 stars orbited by giant, Jupiter-mass planets show concentrations of heavy elements – those heavier than hydrogen and helium – that are typically two to three times greater than in the sun, said Guillermo Gonzalez, a UW postdoctoral researcher. The stars are parents of recently detected planets outside the solar system.

“Two of the stars in our sample have the highest heavy element abundances reliably measured in any star in the universe,” Gonzalez said.

The discovery of a link between heavy elements and the presence of giant planets supports the theory that stars born in heavy-element-enriched interstellar clouds are more likely to harbor giant planets, he said. It also could mean that solar systems like ours are rare occurrences.

He said it also is possible some of those systems have experienced “planet migration,” in which a giant planet moves toward its parent star, dragging any remaining inner disk material or smaller planets that it encounters along the way. This process could naturally lead to the central star adding to its supply of heavy elements.

Gonzalez and his collaborators – Steven Saar of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.; Andrew D. Vanture, a physics and physical science instructor at Everett Community College in Everett, Wash.; and UW astronomy professor George Wallerstein – began studying extrasolar planets in December 1995, in part as an effort to test planet-formation hypotheses. Their research is supported by grants from the National Science Foundation, the Robert A. Welch Foundation of Houston and the Kennilworth Fund of the New York Community Trust.

The findings have important implications for understanding the formation of giant planets, future searches for extrasolar planets and the Search for Extraterrestrial Intelligence (SETI), Gonzalez said.

The data consist of high-resolution spectroscopic information, most gathered with the 82-inch and 107-inch telescopes at the University of Texas McDonald Observatory, near El Paso. Some of the information also came from the Dominion Astrophysical Observatory in Victoria, British Columbia, and the 400-inch Keck I telescope in Hawaii. Since most stars in the sample are bright enough to be seen with the naked eye, the quality of the scientific data is very high, Gonzalez said.

“From these data we have determined the compositions of the planet-bearing stars with a high level of reliability,” he said. He plans to expand the sample beyond the current 12 stars in the near future, but acknowledges it has been difficult keeping up with the recent pace of extrasolar planet discoveries.

Astronomers devising theoretical models of “planet swallowing” and planet migration have worked closely with Gonzalez and other observers. That exchange has aided both sets of scientists, he said.

He also expects the results to be useful to planet hunters, who can use the knowledge of a star’s heavy element abundance as a criterion in preparing a list of targets.

Because they are so faint, it is not yet possible to detect the light from planets orbiting other stars like our own. Astronomers must glean information from the parent stars, including measurements of luminosity, surface temperature, surface gravity, rotational velocity and surface composition. Of these, a star’s composition has potentially the greatest relevance for the formation of gas giant planets, Gonzalez said.

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For more information, contact Gonzalez at (206) 543-8098 or by e-mail at gonzalez@astro.washington.edu. During the AAS conference, messages may be left at the Radisson Hotel in Austin, (512) 478-9611, or at the AAS press room, (512) 404-4650.