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Brown dwarf cooks
up a storm

Posted: 13 September 2011

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A team led by astronomers from the University of Toronto have observed noticeable changes in brightness of a nearby brown dwarf, roughly 40 light years away, indicating the presence of a gigantic storm. Since brown dwarfs and giant gaseous planets have similar atmospheres, the team believe that this finding could lead us into understanding more about the weather that rages on the surface of distant worlds.

Astronomers have observed extreme brightness changes on a nearby brown dwarf that may indicate a storm grander than any seen yet on a planet. Image: Art by Jon Lomberg.

Using an infrared camera on the 2.5-metre telescope at Las Campanas Observatory in Chile, the scientists conducted a large survey of nearby brown dwarfs – sub-stellar objects that are too low in mass to sustain hydrogen fusion like main sequence stars – capturing repeated images of brown dwarf 2MASS 2139 over several hours. The team recorded the largest variations in brightness that has ever been witnessed on a cool brown dwarf.

“We found that our target’s brightness changed by a whopping 30 percent in just under eight hours,” says Jacqueline Radigan, lead author of the paper that is to be presented this week at the Extreme Solar Systems II conference in Jackson Hole, Wyoming, USA. “The best explanation is that brighter and darker patches of its atmosphere are coming into our view as the brown dwarf spins on its axis.”

But what causes these atmospheric patches? “Brown dwarfs cool down as they age and this one in particular has a spectral type of T1.5, which puts it near the somewhat warmer L-dwarf and the somewhat cooler T-dwarf classes,” explains co-author Professor Ray Jayawardhana, Canada Research Chair in Observational Astrophysics at the University of Toronto and author of Strange New Worlds: The Search for Alien Planets and Life beyond our Solar System. “T-dwarfs are thought to have clear/cloud-free atmospheres compared to cloudy L-dwarfs, because of dust grains sinking down to the bottom of the atmosphere. 2MASS 2139 is probably at an in-between stage, when its atmosphere is patchy, with some clouds remaining but also large holes developing in the cloud deck.”

Theoretical models state that the clouds that exist in the atmospheres of brown dwarfs and giant planets are made from the condensation of silicates and metals. The weekly to monthly variation in brightness of the brown dwarf suggests that the cloud patterns are evolving with time. “If all of the variability were attributed to a single spot it would have to cover around 15 to 35 percent of the visible hemisphere, so possibly quite a bit bigger than Jupiter’s Great Big Spot,” says Radigan. “On the other hand, the shape, or nature, of cloud features responsible is a bit of guesswork – a combination of smaller storms, or other large asymmetries in the distribution of cloud bands and clearings could also contribute.”

While the physical cause of these storms is not fully understood, the team believe that this phenomena still has much to teach us. “Measuring how quickly cloud features change in brown dwarf atmospheres may allow us to eventually infer atmospheric wind speeds and teach us about how winds are generated in brown dwarf and planetary atmospheres,” concludes Radigan.