On brown dwarfs, storms of molten iron are always forecast

NASA/JPL-Caltech/University of Western Ontario/Stony Brook University

Brown dwarfs are much smaller and cooler than our sun. They might not be massive enough to have a core that performs nuclear reactions, but they do have extreme conditions. Using the Spitzer Space Telescope astronomers were able to find evidence that most, if not all, brown dwarfs have intense storms of molten iron moving at high speeds. The results were published in The Astrophysical Journal and were presented this week at the 223rd Meeting of the American Astronomical Society in Washington, D.C.

Because brown dwarfs don’t emit a lot of visible light, they are best viewed with infrared.  Astronomers used the Spitzer Space Telescope to observe over 40 brown dwarfs and searched for any variations in the luminosity as evidence of an active weather system. More than 20 of the stars observed indicated that there was indeed fluctuations in the light visible. The brighter light indicates a cloud-free portion, and the light is dimmer when clouds and storms are in the way. The variations occur as the brown dwarf spins on its axis. These storms are very intense, with hurricanes causing lightning along with high winds that whip around molten iron, sand, and even salt. 

The researchers have likened these storms to Jupiter’s Great Red Spot, which is larger than three Earths and has been going on for over 300 years. Even though they only observed these light fluctuations in about half of the stars, the team began the study under the assumption that half of them may not be oriented in the correct way to see it. This means that there is a possibility that all brown dwarf stars are home to some incredibly hellacious storms.

The team chose to observe weather on brown dwarfs instead of exoplanets merely out of simplicity. Studying the atmosphere of exoplanets can be tricky, given that the glare of the parent star. This was done as part of the “Weather on Other Worlds” project for Spitzer. Future studies will seek to better understand the weather on brown dwarfs and also extend to gas giant exoplanets like Jupiter, which are a lot like much smaller cousins. 

Brown dwarfs are essentially failed stars. During formation, they were not able to accumulate enough mass to sustain nuclear hydrogen fusion in their core. Without these reactions, they are technically already "dead" and won't go supernova like regular stars will. They are anywhere from 13-80 times the mass of Jupiter and were confirmed for the first time in 1995. The difference between a small brown dwarf and a large gas planet comes down to the ability to fuse deuterium. While this is an isotope of hydrogen, which brown dwarfs shouldn't be able to fuse, the extra neutron makes deuterium fusion possible at much lower temperatures and densities.


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