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Star aspirations. Like young stars, brown dwarfs are surrounded by protoplanetary disks in this artist conception.

Brown Dwarfs Get Hot

PASADENA, CALIFORNIA--The surprising heat from 63 brown dwarfs is helping astronomers make the case that these puzzling objects are failed stars, and not big planets, as some have argued. But even though they fail to shine, the dwarfs may still leave a legacy. The heat suggests that the dwarfs are wrapped in a dusty disk of the kind that typically give birth to planets, a team reported here on 7 June at a meeting of the American Astronomical Society.

In the past 5 years, a misfit class of gaseous balls has been discovered orbiting nearby stars or floating freely through space (ScienceNOW, 5 October 2000 and 26 February 2001). It's hard to know how they formed: The brown dwarfs seem too heavy to have formed from the slow agglomeration of material, like jumbo-sized planets such as Jupiter. Yet they are too light to ignite the nuclear fusion that powers stars. Confused astronomers named the objects failed stars, superplanets, or simply brown dwarfs. But they admit they didn't really know which were stars and which were planets.

At least part of the question has now been answered: The free-floating brown dwarfs are stars. Although brown dwarfs have no nuclear fire in their belly, they are hot enough to emit infrared radiation, just like a human body. And if they formed from contracted clouds like a star, a warm dusty disk should orbit the dwarf and radiate a lot of additional infrared light. It was precisely this extra light that astrophysicist Charles Lada of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, was looking for when his team surveyed 100 free-floating brown dwarfs in the nearby Trapezium star cluster in March 2000, using the 3.5-meter New Technology Telescope in Chile. They found it in 63 dwarfs. An oversized free-floating planet formed by agglomeration would not have a disk, explains Lada, so these dwarfs must have formed like stars. And the disks could spawn small, inhospitable planets, he says.

"This is compelling evidence," says Geoff Marcy, an astronomer at the University of California, Berkeley. And although he is confident that the disks are real, Marcy points out that astronomers' models of brown dwarfs are still in their infancy, and it's hard to predict exactly how much heat they produce. Further refinements in brown dwarf models should soon reduce that uncertainty, he says.

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Research on brown dwarfs by the University of California, Berkeley