Astronomers find Jupiter-like weather on brown dwarfs

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

The UCLA-NASA team has found cloudy, stormy atmospheres on brown dwarfs, the celestial bodies that are less massive than stars but have more mass than giant planets like Jupiter. The discovery will give scientists better tools for interpreting atmospheres and weather on brown dwarfs or on planets around other stars.

“The best analogy to what we witness on these objects are the storm patterns on Jupiter,” said Adam Burgasser, astronomer at UCLA and lead author of the study. “But I suspect the weather on these more massive brown dwarfs makes the Great Red Spot look like a small squall.”

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

• 1 June 2002: Evidence of Cloud Disruption in the L/T Dwarf Transition, Astrophysical Journal

Jupiter’s Great Red Spot is a massive storm more than 15,000 miles across and with winds of up to 270 miles per hour. Burgasser teamed with planetary scientist Mark Marley, meteorologist Andrew Ackerman of NASA Ames Research Center in California’s Silicon Valley, and other collaborators to propose how weather phenomena could account for puzzling observations of brown dwarfs.

“We had been thinking about what storms might do to the appearance of brown dwarfs,” Marley said. “And when Adam showed us the new data, we realized there was a pretty good fit.” The team calculated that using a model with breaks or holes in the cloudy atmosphere solved the mysterious observations of cooling brown dwarfs.

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

Astronomers expected brown dwarfs, like most objects in the universe, to grow steadily fainter as they cool. However, new observations showed that during a relatively short phase, brown dwarfs appear to get brighter as they cool. The explanation lies in the clouds.

At least 25,000 times fainter than the sun, brown dwarfs are still incredibly hot, with temperatures as high as 3,140 degrees Fahrenheit (2,000 degrees Kelvin). At such high temperatures, substances such as iron and sand are in gaseous form. As brown dwarfs cool, these gases condense in the atmosphere into liquid droplets to form clouds, similar to water clouds on Earth. As the brown dwarf cools further, atmospheric weather patterns cause a rapid 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 the characteristics of a very broad range of brown dwarfs, but only if cloud clearing is considered,” Burgasser said. “While many groups have hinted that cloud structures and weather phenomena should be present, we believe we have actually shown that weather is present and can be quite dramatic.”

By using Earth’s weather as a starting point, Ackerman helped the team work the storms – including wind, downdrafts and iron rain – into their calculations. “The astrophysicists needed some help understanding rain because it’s not an important process in most stars,” Ackerman said. “We used observations and simulations of terrestrial clouds to estimate the effect of iron rain on the thickness of an iron cloud.”

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

“Brown dwarfs have traditionally been studied like stars, but it’s more of a continuum,” Marley said. “If you line a mug shot of Jupiter up with these guys, it is just a very low-mass brown dwarf.”

Brown dwarfs serve as a training ground for scientists to learn how to interpret observations of planet-like objects around other stars, Marley said. “Everybody wants to find brown dwarfs that are even colder and have water clouds just like Earth. Once we find those, that will be a good test of our understanding.”

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

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