Science + Technology

Astronomers Find Jupiter-Like Weather on Brown Dwarfs

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For the first time, researchers have observed planet-like weatheracting as a major influence on objects outside our solar system, scientistsfrom UCLA and NASA report May 23.

The UCLA-NASA team has foundcloudy, stormy atmospheres on brown dwarfs, the celestial bodies that are lessmassive than stars but have more mass than giant planets like Jupiter. Thediscovery will give scientists better tools for interpreting atmospheres andweather on brown dwarfs or on planets around other stars.

"The best analogy to what we witness on these objects are thestorm patterns on Jupiter," said Adam Burgasser, astronomer at UCLA and leadauthor of the study. "But I suspect the weather on these more massive browndwarfs makes the Great Red Spot look like a small squall."

The UCLA-NASA findings will be reported in the June 1 issue of theAstrophysical Journal Letters.

Jupiter's Great Red Spot is a massive storm more than 15,000 milesacross and with winds of up to 270 miles per hour. Burgasser teamed withplanetary scientist Mark Marley, meteorologist Andrew Ackerman of NASA AmesResearch Center in California's Silicon Valley, and other collaborators topropose how weather phenomena could account for puzzling observations of browndwarfs.

"We had been thinking about what storms might do to the appearanceof brown dwarfs," Marley said. "And when Adam showed us the new data, werealized there was a pretty good fit." The team calculated that using a modelwith breaks or holes in the cloudy atmosphere solved the mysterious observationsof cooling brown dwarfs.

Brown dwarfs, only recently observed members of the skies,are "failed stars at best," Ackerman said. Not massive enough to sustain theburning of hydrogen, like stars, brown dwarfs go through cooling stages thatscientists observe with infrared energy-detecting telescopes. They appear as afaint glow, like an ember from a fire that gives off both heat and light energyas it dims.

Astronomers expected brown dwarfs, like most objects in theuniverse, to grow steadily fainter as they cool. However, new observationsshowed that during a relatively short phase, brown dwarfs appear to getbrighter as they cool. The explanation lies in the clouds.

At least 25,000 times fainter than the sun, brown dwarfs are stillincredibly hot, with temperatures as high as 3,140 degrees Fahrenheit (2,000degrees Kelvin). At such high temperatures, substances such as iron and sandare in gaseous form. As brown dwarfs cool, these gases condense in theatmosphere into liquid droplets to form clouds, similar to water clouds onEarth. As the brown dwarf cools further, atmospheric weather patterns cause arapid clearing of the clouds; as the clouds are whisked away by the storms,bright infrared light from the hotter atmosphere beneath the clouds escapes,accounting for the unusual brightening of the brown dwarfs.

"The model developed by the group for the first time matches thecharacteristics of a very broad range of brown dwarfs, but only if cloudclearing is considered," Burgasser said. "While many groups have hinted thatcloud structures and weather phenomena should be present, we believe we haveactually shown that weather is present and can be quite dramatic."

By using Earth's weather as a starting point, Ackerman helped theteam work the storms — including wind, downdrafts and iron rain — into theircalculations. "The astrophysicists needed some help understanding rain becauseit's not an important process in most stars," Ackerman said. "We usedobservations and simulations of terrestrial clouds to estimate the effect ofiron rain on the thickness of an iron cloud."

The study will help researchers determine the makeup ofatmospheres outside our solar system.

"Brown dwarfs have traditionally been studied like stars, but it'smore of a continuum," Marley said. "If you line a mug shot of Jupiter up withthese guys, it is just a very low-mass brown dwarf."

Brown dwarfs serve as a training ground for scientists to learnhow to interpret observations of planet-like objects around other stars, Marleysaid. "Everybody wants to find brown dwarfs that are even colder and have waterclouds just like Earth. Once we find those, that will be a good test of ourunderstanding."

NASA, the National Science Foundation and the Hubble PostdoctoralFellowship funded this study, and supplied much of the data. Othercollaborating institutions include Vanderbilt University, Nashville, Tenn.;Washington University, St. Louis; U.S. Naval Observatory, Washington, D.C.; andCalifornia Institute of Technology, Pasadena, Calif.

More information about this study is available at: www.journals.uchicago.edu/ApJ/journal/contents/ApJL/v571n2.html

-UCLA-

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