Webb Finds Signs of Patchy Water Clouds on Coldest Known Brown Dwarf
Astronomers using the NASA/ESA/CSA James Webb Space Telescope have captured the first time-resolved, medium-resolution spectra of WISE J085510.83-071442.5, an ultracool brown dwarf just 7.4 light-years away, and found that its atmosphere is anything but static. This artist’s conception shows WISE 0855, the coldest know
Astronomers using the NASA/ESA/CSA James Webb Space Telescope have captured the first time-resolved, medium-resolution spectra of WISE J085510.83-071442.5, an ultracool brown dwarf just 7.4 light-years away, and found that its atmosphere is anything but static. This artist’s conception shows WISE 0855, the coldest known brown dwarf.
Image credit: NASA / JPL-Caltech / Penn State University. “This is the first time we’ve been able to confirm that water clouds are becoming thinner and thicker on a nearby world,” said Dr. Brittany Miles, a postdoctoral researcher at Steward Observatory.
“Before Webb, we only had photometry, which mixed up the effects of clouds, chemistry, and temperature all together. Now we can actually distinguish them.” WISE J085510.83-071442.5 (WISE 0855 for short) is about twice Jupiter’s mass and nearly the same size. At about 265 K (minus 8 degrees Celsius or 17 degrees Fahrenheit), the brown dwarf is cool enough to host water clouds high in its atmosphere, yet warm enough to keep ammonia from condensing.
This makes it a rare natural laboratory for studying how clouds and atmospheric chemistry interact beyond our Solar System. Dr. Miles and colleagues used Webb’s NIRSpec instrument to record WISE 0855’s spectrum every 15 minutes for 11 hours.
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Because the data are spectra rather than simple brightness measurements, they could see which molecules were responsible for the changes, rather than just noting that the object got brighter or dimmer. “The photons go through the atmosphere and escape to space,” said Dr. Mark Marley, an astronomer at the University of Arizona.
“It’s like looking at the world through a screen door, where the screen is filtering out some of the light.” “We’re learning about the world on either side of the screen, but we also have to understand the screen itself.” Comparing the data with atmospheric models, the astronomers found that water clouds are needed to match the overall shape of WISE 0855’s spectrum.
Their best-fitting model was a cloudy world that also included chemical mixing. “We’re seeing water clouds getting thicker and thinner, and deep gases rising and falling, and we can actually watch them change in real time,” Dr. Miles said.
“The real value of this discovery isn’t just what it tells us about WISE 0855 specifically, but what it suggests about planetary atmospheres more broadly.” “The basic physics of convection, clouds and chemistry that governs Jupiter also govern this cold, free-floating world more than 7 light-years away.” “If that physics is universal, it applies to the gas giant exoplanets that astronomers are now beginning to study in earnest with Webb.” “Even though brown dwarfs are not true planets, they exhibit planet-like behavior.” “There is a spectrum of behaviors — not a hard line between brown dwarfs and planets.” “Jupiter and this object look distinctly different, but they have similar weather patterns.” “There are basic physics and chemistry that can be applied across all of these worlds.” The team’s paper will be published in the Astronomical Journal . _____ Brittany E.
Miles et al . 2026. Water Cloud and Chemical Modulations in the Coldest Brown Dwarf.
AJ , in press; arXiv: 2609.20664
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