Wild Weather: Iron Rain on Failed Stars

Ever since their discovery 11 years ago, brown dwarfs have baffled scientists. First it was the question of how to categorize them. These celestial orbs are too massive to be a planet and not massive enough to be a star. Now scientists are investigating astonishing weather patterns on brown dwarfs that could rival Jupiter's Great Red Spot and even Earth's intense hurricanes.

Sometimes called "failed stars," brown dwarfs are too small to trigger the fusion of hydrogen that keeps stars like our sun shining for billions of years. Instead, over tens of millions of years brown dwarfs slowly cool and fade.

Meanwhile, the weather on these strange objects is some of the wildest in the galaxy.

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Studying brown dwarfs has proven difficult, however. "These objects are very far away and everything is just a point of light," explained Adam Burgasser of the Massachusetts Institute of Technology.

To get a better "look" at brown dwarfs, scientists including Burgasser, and Katharina Lodders, a senior research scientist in the Planetary Chemistry Lab at Washington University in St. Louis have relied on infrared-capable telescopes and mathematical models. Here's what the picture looks like:

The atmospheres contain primarily gases, including gaseous iron and silicate. At hotter temperatures-3,140 degrees Fahrenheit (2,000 degrees Kelvin)-typical of younger brown dwarfs, the iron remains in its gaseous phase. Over time as the glowing body cools, the iron condenses to form iron-rich clouds and droplets of liquid-iron rain.

This by itself was unexpected.

"We think of clouds as being on planets like Earth or Jupiter so it's kind of weird to think of a cloud on a star," Burgasser said recently.

And when it rains it pours: Not only did Burgasser and his team find these metallic clouds, but they also spotted evidence of violent storms. Thermodynamics would predict that as brown dwarfs release heat, they would dim-similar to a cooling ember. But the astronomers found that the older, and cooler, brown dwarfs shined brighter than the warmer, younger ones.

"These brown dwarfs are much brighter than they should be, and the only way for that to happen is for the clouds to disappear very suddenly so you're left with a very hot brown dwarf, which emits a lot of light, but doesn't have the same clouds," Burgasser said.

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The most plausible explanation is an off-the-charts hurricane pummeling through the area and clearing the clouds. If the clouds were to clear naturally as the object cooled, that could take roughly 1 billion years, leaving a chilly brown dwarf, Burgasser told SPACE.com. He expects these clearings must be 100 times as quick, a "lightning-fast" 10 million years.

But what causes the cloudy disturbance? With cooling, a brown dwarf's cloud layer sinks closer to its surface.

"As the clouds form deeper in the atmosphere they are more sensitive to the winds and convective motions in the atmosphere," said Mark Marley, a scientist at NASA's Ames Research Center.  

"This dynamics or 'weather' probably leads to the destruction of the clouds," Marley said by email, adding that he and others are working on promising ideas to explain the cloud-clearing mechanism.

But there are still many unanswered questions, the astronomers said. For instance, what do these storms look like? "We don't have telescopes that are big enough to resolve the surfaces of these objects. So there's really no way of watching the clouds on brown dwarfs," Burgasser said.

Burgasser predicts that in the next 10 to 15 years very large telescopes will exist, making it possible to gather more detailed information about these storms. So we'll have to stay tuned.

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This article is part of SPACE.com's weekly Mystery Monday series.