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Brown Dwarfs May Be Failed Stars, Not Super Planets

Brown dwarfs, first observed about a decade ago, are mysterious gaseous structures that don't shine from sustained nuclear fusion as stars do.

Now an international research team led by University of Florida astronomers has announced (Thursday, June 7) that it has found dusty disks surrounding numerous faint objects believed to be "brown dwarfs" in the Orion Nebula.

Their findings suggest that brown dwarfs may be more similar to "failed stars" than to "super planets" forming from collapsing clouds of interstellar gas.

That means that brown dwarfs, like stars, could have planets rotating around them. However, without a proper sun, the planets probably wouldn't provide an environment conducive to life, said Elizabeth Lada, a UF associate professor of astronomy.

"It is entirely possible that our galaxy contains numerous planetary systems that orbit these cold, dark, failed stars," Lada said. "But even if brown dwarfs do have planetary systems, their planets would not have a stable climate, and thus would be inhospitable to life as we know it."

The team, which announced its findings at the American Astronomical Society meeting in Pasadena, Calif., based its conclusions on observations of likely brown dwarfs in the so-called Trapezium cluster, a group of extremely young stars within the Orion Nebula.

Located about 1,200 light years from Earth, the cluster and nebula appear to the untrained eye as a single central star in the sword of the hunter in the constellation Orion.

The cluster is a kind of stellar nursery, with most of its stars aged less than 1 million years -- in contrast with our middle-aged sun, which is 4 billion years old.

Stars are thought to form when gravity causes a rotating cloud of gas to contract. Before the star is formed, the gas collapses into a rotating disk. Most young stars observed to date have been accompanied by such disks.

Using a state-of-the-art near-infrared camera on a European Southern Observatory (ESO) telescope in the Chilean Andes, the researchers found disks surrounding both young stars and suspected brown dwarfs in the Trapezium cluster. Moreover, the percentage of stars with disks matched the percentage of brown dwarfs with disks.

That suggests that brown dwarfs and stars share a common origin -- one different from planets, which form within disks surrounding stars but don't have disks of their own.

The observation is important because the small size of brown dwarfs -- less than 7 percent the size of our sun -- had led some astronomers to speculate they were related more closely to planets than to stars.

"The high incidence of disks around both young stars and brown dwarfs in this cluster strongly suggests that both stars and brown dwarfs trace their origin to a common physical process and that brown dwarfs are more similar in nature to stars than to planets," said Charles Lada, senior astrophysicist on the staff of the Smithsonian Astrophysical Observatory in Cambridge, Mass. (Charles Lada is Elizabeth Lada's brother.)

The team also achieved another milestone: In analyzing the observations, it identified at least 80 likely brown dwarfs, roughly doubling the number of such objects identified so far.

The brown dwarfs in the Trapezium cluster constitute the largest "population" of brown dwarfs ever observed, the researchers said.

"Even at their brightest, most brown dwarfs are still 100 or more times fainter than our sun, explaining why astronomers find such objects so difficult to detect," said August Muench, a UF doctoral student in astronomy and the project's lead investigator.

The team, which also included Joao Alves of the European Southern Observatory in Garching, Germany, will publish a paper on their results in an upcoming issue of Astrophysical Journal Letters.

The research was funded in part by the National Science Foundation.

(Editor's Note: A fine ESO image of the Orion Nebula and Trapezium Cluster can be viewed at this website.)

[Contact: Elizabeth Lada, Aaron Hoover]






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