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Webb watches water clouds thicken and thin on the coldest known brown dwarf

Eleven hours of observations of WISE 0855, 7.5 light-years away, give the first sign of changing water clouds on a world beyond the solar system, astronomers at the University of Arizona say.

From the editorial desk · TucsonPublished 22 October 2026, 11:10 am IST

An illustration of the brown dwarf WISE 0855 (file photo)Photo: NASA/ESA/CSA JWST NIRCam; Alves de Oliveira et al. & Melina Thévenot / Wikimedia Commons, CC BY-SA 4.0

Key points

WISE 0855 is about 7.5 light-years from the Sun and close to minus 8°C.
Webb took 44 spectra over 11 hours, one every 15 minutes.
The team reports the first detection of variable water clouds outside the solar system.

The James Webb Space Telescope has watched the weather change on WISE 0855, the coldest brown dwarf yet found, and the clouds that come and go there appear to be made of water.

A team led by the University of Arizona says it is the first time variable water clouds have been detected outside the solar system. The study has been accepted for publication in The Astrophysical Journal.

“This is the first time we’ve been able to confirm that water clouds are becoming thinner and thicker on a nearby world,” said Miles, a postdoctoral researcher at the university’s Steward Observatory.

Eleven hours of weather

Webb pointed its Near-Infrared Spectrograph at the object for 11 hours on 2 December 2023, recording a spectrum every 15 minutes, 44 in all. The changes are best explained by a layer of patchy, high-altitude water clouds whose thickness shifts across the surface.

WISE 0855 sits about 7.5 light-years from the Sun. At roughly 265 kelvin, about minus 8°C, it is colder than many winter days on Earth. Its mass is put at about twice that of Jupiter or a few times more, which places it near the line between brown dwarfs, sometimes called failed stars, and giant planets.

Carbon monoxide showed the biggest swings, of about 10% at some wavelengths, driven by temperature differences of only about one kelvin. The team also saw signs of convection, with gases such as carbon monoxide and phosphine rising from deeper layers and sinking as they cool. A signal repeating every 9.33 hours turned up, but the researchers are not yet sure it is the object’s rotation.

“Before JWST, we only had photometry, which mixed up the effects of clouds, chemistry, and temperature all together,” Miles said. Mark Marley, director of the university’s Lunar and Planetary Laboratory and a co-author, compared studying such atmospheres to “looking at the world through a screen door.”