Theory Bolsters Search For Brown Dwarfs And Giant Planets

June 28, 1996

Scientists today are publishing a first theoretical study of the newly discovered -- and the only confirmed -- "brown dwarf." Results from their models reproduce actual observations of the brown dwarf, Gliese 229 B.

The study shows that planetary scientists and astronomers have begun to understand these objects in enough detail to accelerate new and successful searches for brown dwarfs and giant extrasolar planets.

The research article, "Atmospheric, Evolutionary and Spectral Models of the Brown Dwarf Gliese 229 B," appears today in SCIENCE. The authors are Mark S. Marley of New Mexico State University; Didier Saumon, Tristan Guillot, William B. Hubbard and Jonathan I. Lunine, all of the Lunar and Planetary Laboratory at The University of Arizona in Tucson; Adam S. Burrows of the UA departments of physics and astronomy; and Richard S. Freedman of the NASA Ames Research Center.

Astronomers for more than 20 years have searched for objects they believed had to exist --- "brown dwarfs" that are too massive to be true planets but too small to burn by nuclear fusion, as do stars. A universe that lacked objects more massive than planets but less massive than stars would be hard to explain. Several brown dwarf discoveries were made, but never confirmed -- until eight months ago.

The brown dwarf, Gliese 229 B, was discovered last October by a team of Cal Tech and Johns Hopkins University observers using the 5-meter Mount Palomar telescope. The object revolves around Gliese 229 A, an unremarkable faint red-dwarf star 20 light years away. The observers directly detected the spectra of methane in the brown dwarf's atmosphere, a gas that could never exist in a much hotter stellar atmosphere.

Gliese 229 B since has been directly imaged by the Hubble Space Telescope and by a team of UA Steward Observatory astronomers using adaptive optics on the 4.5-meter Multiple Mirror Observatory on Mount Hopkins, Ariz.

The research published today shows that water, methane and ammonia are present in Gliese 229 B. If scientists next can learn the relative abundances of these gases in Gliese 229 B, they can begin to understand how brown dwarfs form, said Tristan Guillot of the UA Lunar and Planetary Laboratory.

"It is of crucial importance to determine how brown dwarfs and giant planets form, something we know little about, because this will tell us about the formation of our own solar system, and why we are here," he said.

According to the study published today, Gliese 229 B has an atmosphere similar to that of Jupiter's, but is between 30 and 55 times as massive and is much hotter. Gliese 229 B has an atmosphere of hydrogen and helium (elements which are not directly detectable), the methane detected by observers, ammonia that could be observed at far infrared (10 micron) wavelengths, and water vapor.

Water vapor found on Gliese 229 B is not present in the much colder Jupiter. The temperature of the outermost layers of fluid Jupiter is minus 148 degrees Celsius, or minus 234 degrees Fahrenheit. The new models show temperatures for Gliese 229 B at less than 700 degrees Celsius, or 1,300 degrees Fahrenheit.

What will not be detected in the brown dwarf atmosphere are titanium or vanadium oxides, or iron hydride, all heavy metals, the new study says. The brown dwarf is so much cooler than any star that heavy metals condense from the atmosphere, their study shows. Surface temperatures for stars can be no cooler than 1,400 degrees Celsius, or 2,600 degrees Fahrenheit. Temperatures at the cores of stars, which are fueled by the nuclear fusion of hydrogen, reach 1 million degrees Celsius, or 2 million degrees Fahrenheit. Brown dwarfs never reach such high core temperatures because their mass is too low.

The brown dwarf and its star, Gliese 229 A, formed a billion or more years ago. The theorists modeled the age of the brown dwarf and its temperature to calculate its mass at between 30 times and 55 times the mass of Jupiter. A star must be at least 80 times as massive as Jupiter to support nuclear fusion. Also unlike stars, which either explode as novae or evolve successively into red giant then white dwarf stars at death, the brown dwarf will only cool slowly, unspectacularly, as it dies.

"The fact that we are now able to detect brown dwarfs is extremely important," said Guillot. "It shows that we are able to detect fainter and fainter objects. Our goal is to directly detect extrasolar planets. But before we can do that, we have to understand how these objects radiate energy... The fact that we are able to reproduce (in theoretical models) the direct observations of Gliese 229 B shows that soon we will be able to predict how bright brown dwarfs and giant planets are." Astronomers will then know what strategies and instruments they need to detect brown dwarfs and giant planets around other stars.

To date, astronomers have discovered nine extrasolar planets by indirect observations. Some of these may be brown dwarfs, Guillot said. While brown dwarfs are clearly distinct from stars because they are not fueled from within by nuclear fusion, the distinction between brown dwarfs and giant planets is very vague at this time. The brown dwarfs may form as stars form, by the collapse of a molecular cloud. The planets are thought to form in the gaseous disk surrounding an infant star.
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University of Arizona

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