Astronomers see Swirling Weather on the Closest Brown Dwarf

Brown dwarfs are the weird not-planets but not-stars in the universe, and astronomers have wondered for decades if their atmospheres are striped like Jupiter’s, or splotchy like the sun’s. A team of astronomers based at the University of Arizona used NASA’s TESS Observatory to find the answer: if you saw a brown dwarf for yourself, it would look more like a giant planet than a star.

Brown dwarfs are about the size of Jupiter, but much more massive. But they’re not quite massive enough to sustain nuclear fusion of hydrogen in their cores, so they don’t qualify as stars. They float around the galaxy largely undetectable, since they don’t emit copious amounts of radiation.

Still, they’re hot on the inside (generated by leftover heat from their formation), and cold on the outside (because, space), and so heat constantly flows outwards. And when heat flows, weather happens.

But weather can happen in all sorts of ways, like the striped bands of Jupiter, the chaotic swirlings of the Earth, or the random splotches of the sun. What kind do brown dwarfs have?

Astronomers with the University of Arizona turned to NASA’s Transiting Exoplanet Survey Satellite (TESS), which normally hunts for exoplanets, but is exquisitely tuned to also study brown dwarfs. They used TESS to observe the Luhman 16 system, a pair of orbiting brown dwarfs just 6.5 light-years away.

By carefully studying the change in brightness from the Luhman 16 binary as they orbited each other, they could determine the patterns in their atmospheres. The result: striped.

Jupiter gets its stripes from its rapid rotation. Plumes of warm material rise up from the interior, reach the edge of the atmosphere, cool off, and slink back down. The massive planet’s rotation stretches those plumes into rings, giving Jupiter its characteristic alternating stripes of up- and downward moving material.

Presumably something similar is happening in the Luhman 16 system.

“Knowing how the winds blow and redistribute heat in one of the best-studied and closest brown dwarfs helps us to understand the climates, temperature extremes and evolution of brown dwarfs in general,” said lead author Daniel Apai, an associate professor in the Department of Astronomy and Steward Observatory and the Lunar and Planetary Laboratory.

Further research will help astronomers uncover whether this is the status quo for brown dwarfs, or if each one is unique.

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