Webb watched water clouds thicken and thin on a world 7.4 light-years away

The coldest brown dwarf ever found has weather. The James Webb Space Telescope stared at WISE 0855, a dim, Jupiter-sized world just over seven light-years from the Sun, for eleven hours and watched its water clouds grow thicker and thinner as it turned. Below those clouds, carbon monoxide and phosphine rose and fell in waves, dredged up from the hot interior.

That makes WISE 0855 the first body outside the solar system where astronomers say they can see water clouds changing, rather than infer that they might exist. “This is the first time we’ve been able to confirm that water clouds are becoming thinner and thicker on a nearby world,” said Brittany Miles of the University of Arizona’s Steward Observatory, who led the study. It also shows that the physics running Jupiter’s sky seems to work the same way on a lonely object with no star to warm it.

Brown dwarfs form like stars but never gather enough mass to ignite hydrogen fusion, so they glow faintly with leftover heat and cool forever. WISE 0855 has cooled further than any other known: the team puts it at about 265 kelvin, roughly minus 8 °C, with 3 to 10 times the mass of Jupiter. Only the Alpha Centauri system, Barnard’s Star and the brown dwarf pair Luhman 16 lie nearer to the Sun.

How Webb read the weather on a single point of light

To Webb, WISE 0855 is a single point of light. But as it spins, warmer and cooler patches and thicker and thinner clouds rotate in and out of view, changing the light that reaches us.

Webb’s NIRSpec spectrograph recorded 44 spectra, one every 15 minutes, across infrared wavelengths from 2.87 to 5.27 microns, where this cold object shines brightest. Earlier studies with NASA’s retired Spitzer Space Telescope had already shown that WISE 0855 flickers, but Spitzer only measured brightness in two broad colour bands. “Before JWST, we only had photometry, which mixed up the effects of clouds, chemistry, and temperature all together. Now we can actually distinguish them,” Miles said.

Splitting the light finely enough to separate individual molecular bands let the team track each gas on its own. The biggest swings came from carbon monoxide, whose absorption changed by up to 10% from peak to trough at some wavelengths. The water vapour bands, by contrast, barely moved.

To sort the signal, the astronomers used principal component analysis, a statistical method that finds the few patterns hiding inside a large set of changes. One pattern accounted for 60.5% of the variability and behaves like a shift in temperature or cloud cover. The next, worth 4.7%, mainly tracks carbon monoxide, and the changes in carbon monoxide and phosphine move together.

“It’s like looking at the world through a screen door, where the screen is filtering out some of the light,” said co-author Mark Marley, who heads the University of Arizona’s Lunar and Planetary Laboratory. Compared against grids of atmosphere models, only those with water clouds matched the average spectrum, and the best fit was a cloudy model at 250 kelvin.

Two layers of weather stacked on top of each other

The two signals come from different heights. The water clouds sit high in the atmosphere, where the pressure is lower, and their thickness changes as the object turns. The carbon monoxide and phosphine swings start much deeper, in the churning convective zone.

There, heat from the interior pushes gas upward faster than chemical reactions can convert it into the forms expected at the cold top of the atmosphere. Chemists call this disequilibrium chemistry, and the deep mix gets frozen in on its way up. Jupiter does the same thing: phosphine, a molecule that should not survive in its cold upper air, is found there because convection keeps hauling it up from below. On WISE 0855 the amount of deep gas reaching the top seems to rise and fall in pulses, like a pot of soup boiling unevenly.

The large carbon monoxide signal also hints at geometry. The team suggests WISE 0855 is probably seen edge-on, with its equator facing Earth, and that most of its latitudes are shaped by long east-west wind bands rather than isolated storms, the arrangement that paints Jupiter in stripes.

Why the coldest brown dwarf is the one that matters

Water clouds are the reason WISE 0855 has drawn so much telescope time. Hotter brown dwarfs carry clouds of mineral dust and molten iron. Models predicted that only the very coldest, below about 375 kelvin, would be cold enough for water to condense near the top of the atmosphere, and WISE 0855 is the only known object cold enough to test that while still being warm enough to keep ammonia from freezing out.

The case for clouds had been building without closing. Its colours gave a first hint, and a spectrum from the Gemini North telescope in Hawaii found water vapour and signs of clouds, yet early Webb spectra could still be fitted by cloudless models with a distorted temperature profile. Watching individual molecules change over hours tipped the balance.

A second Webb study announced days later points the same way from a different object. A Trinity College Dublin team applied the same pattern analysis to SIMP 0136, a warmer brown dwarf about 20 light-years away, and found that temperature and cloud height explain almost all of its changing light, with three recurring weather states over more than a dozen rotations.

“Even though brown dwarfs are not true planets, they exhibit planet-like behavior,” Miles said. That matters for the giant exoplanets Webb now studies around other stars, which are much harder to observe because their host stars drown them out.

What the study does not settle

The rotation period is still unknown. Within Webb’s eleven-hour window the data show significant periods of 5.4 to 5.6 hours, but longer periods also fit the data, and two earlier Spitzer campaigns showed no consistent rhythm. Without a firm period, nobody can map where the clouds sit on the globe.

The models are also strained. No single atmospheric model reproduces the whole infrared spectrum of WISE 0855 from 1 to 22 microns. Fits to Webb spectra gave a radius of 0.79 to 0.89 times Jupiter’s, smaller than the roughly one Jupiter radius that theory expects for an object of this temperature and age. The measured phosphine is lower than chemistry predicts, a deficit seen in other brown dwarfs, and the strength of vertical mixing inferred from different molecules does not agree, which the authors say may require models with a different mix of heavy elements.

The edge-on orientation is an inference rather than a measurement, and all the time-series data come from a single eleven-hour visit. The study says nothing about life, and WISE 0855 offers no surface and no starlight to support any.

Common questions about WISE 0855

What is WISE 0855?

WISE 0855, full name WISE J085510.83−071442.5, is the coldest brown dwarf known and the fourth-closest known system to the Sun, in the constellation Hydra. It drifts alone, with no star, and glows faintly in infrared light.

How cold is WISE 0855?

About 265 kelvin according to this study, roughly minus 8 °C or 17 °F. Other estimates put it near 276 kelvin, just above freezing. No other known brown dwarf is as cold.

Does WISE 0855 have water clouds?

Yes, according to the Webb data. Models without water clouds cannot match its average spectrum, and the way its light changes over hours fits patches of water cloud growing thicker and thinner as the object rotates.

Is a brown dwarf a planet or a star?

Neither, strictly. Brown dwarfs form like stars but are too light to sustain hydrogen fusion. WISE 0855 is estimated at 3 to 10 Jupiter masses, light enough that some astronomers would call it a free-floating planetary-mass object.

How far away is WISE 0855?

About 7.4 light-years, or 2.3 parsecs. Its light takes a little over seven years to reach Earth, and only three known systems are closer to the Sun.

What comes next

Kevin Luhman spotted WISE 0855 in NASA’s WISE survey data in 2013, with the discovery announced in April 2014 and the Gemini North spectrum following in 2016. Webb recorded the eleven-hour series on 2 December 2023. The 27-author study has been accepted for publication and was posted as a preprint in September 2026, and the Trinity team announced its SIMP 0136 results, published in Astronomy & Astrophysics, in October 2026.

Miles hopes to log more hours of Webb observations to pin down the rotation and build a three-dimensional map of the atmosphere. The next goal is to watch several full turns of the nearest world beyond the Sun’s family where the forecast is cloudy with a chance of water.

Reference: Miles et al., “Water Cloud and Chemical Modulations in the Coldest Brown Dwarf,” accepted for publication, 2026. arXiv: 2609.20664

Tags: astronomy, James Webb Space Telescope, JWST, brown dwarfs, University of Arizona, water clouds