Weather Patterns on a Cosmic Oddball

The almost-stars known as brown dwarfs have a lot more going on than we knew

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NASA / JPL-Caltech

An artist's illustration showing the atmosphere of brown dwarf 2MASS J22282889-431026

Theorists were talking about them as early as the 1960’s, but the first confirmed observation of a brown dwarf, an object that’s more than a planet but not quite a star, didn’t come until 1994. Since then, astronomers have been making up for lost time, trying their best to understand these cosmic oddballs. Now a team based at the University of Arizona has used not one but two space telescopes to reveal something in a brown dwarf lying 30 light years from Earth that we usually think of as very much a terrestrial phenomenon: weather patterns — including clouds most likely made from particles of iron and sand.

(PhotosScenes From the International Space Station)

The dwarf in question is named (believe it or not) 2MASS J22282889-431026, and the observations, reported in Astrophysical Journal Letters, show an atmosphere in no small amount of turmoil. That’s not surprising: the planet Jupiter, the closest thing in our own Solar System to a brown dwarf, has clouds of ammonia, water vapor and various hydrocarbons, driven around the planet by powerful winds and sucked into gigantic storms like the Great Red Spot that can last for centuries.

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But 2MASS J22282889-431026 is more than just a Jupiter — it’s probably 30 times more massive, which is close to half as big as it would have to be to burst into the nuclear reactions that would qualify it as a star. Still, at perhaps 1,300°F (700°C) on its gaseous “surface,” and even toastier deep inside thanks to heat left over from its formation billions of years ago, it’s got enough energy to get its atmosphere roiling nicely — and by using the powerful tag team of the Hubble and Spitzer space telescopes, lead author Esther Buenzli and her colleagues managed to get an unprecedentedly good look at the brown dwarf’s infrared glow. That, in turn, provided them with a glimpse inside. “This is the first time,” she says, “that we’re starting to get a three-dimensional look at its atmosphere.”

What they see there is a world that rotates at high speed (once every 90 minutes, compared with ten hours for Jupiter), with bright spots that represent glowing clouds made of water and methane — and, according to theoretical models, vaporized iron and silicates as well.  “For some other brown dwarfs,” Buenzli says, “we’ve seen the spectral signature of silicates, which vindicates the theory.” The new observations strengthen that thinking.

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What the scientists didn’t expect was to see the brightness and concentration of various clouds differ as they used longer infrared wavelengths to peer deeper into 2MASS J22282889-431026. Cloud formations seem to show up at higher altitudes depending on longitude, says Buenzli. “We aren’t quite sure how to interpret it. It might have to do with circulation. There are definitely huge winds, so maybe the winds smear out the clouds at different levels. That’s one possibility,” she says, “but the theorists are still trying to understand it.” At the very least, said one of those theorists, Arizona’s Adam Showman, in a press release, “The data suggest regions on the brown dwarf where the weather is cloudy and rich in silicate vapor … with drier conditions at higher altitudes — and vice versa.”

Showman and the rest of the team will soon have more data to work with: the “Extrasolar Storms” project, led by University of Arizona astronomer Daniel Apai, is looking at dozens of brown dwarfs to try and understand how their outer atmospheres change as the bodies themselves age. It’s still too early to conclude much, but, says Buenzli, “we have observations from two more brown dwarfs that are earlier in their evolution. They seem to have thicker clouds, and not so much lag at different altitudes. There really seems to be a difference.”

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