NASA's SPHEREx telescope finds water and methane on lonely brown dwarfs that defy the models

Lonely brown dwarfs have water and methane in their skies, but NASA's SPHEREx data show the best models can't yet explain their clouds.

Brown dwarfs blur the line between stars and planets. A new study finds that the best computer models of their atmospheres get the broad chemistry right and miss the details, especially when a brown dwarf's clouds begin to clear. For now, the measurements are sharper than the theories used to interpret them.

Each brown dwarf starts as a collapsing cloud of gas, the way a star does, but never gets massive enough to keep hydrogen fusion going in its core.

In other ways, these objects resemble Jupiter and Saturn. Unlike most planets, though, a brown dwarf drifts through space with no star to warm it, so all of its heat comes from inside.

Zafar Rustamkulov, a scientist at IPAC, Caltech's science and data center in Pasadena, California, led the study.

"They're kind of goth," Rustamkulov said. "Unlike exoplanets, free-floating brown dwarfs are completely independent celestial objects that will fade into eternity alone. We're still learning how complex they are."

SPHEREx telescope scans the skies

Only a few dozen of these objects have been studied in detail with telescopes in space. Much of what astronomers think about their makeup, their storms, and how they develop as they age comes from theory.

NASA's SPHEREx telescope launched in March 2025 and is managed by the agency's Jet Propulsion Laboratory in Southern California.

Its main job is to map the sky with about 3,600 images a day. Those maps will help scientists work out what happened in the first billionth of a trillionth of a second after the big bang.

The telescope is also searching for the ingredients of life, including ice between the stars that could one day end up in oceans on other worlds. Brown dwarfs are a side project.

Brown dwarfs give off colored light

SPHEREx measures how bright an object is in 102 colors, from the deepest red that people can see out into infrared light, which carries heat but is invisible. Astronomers call that set of measurements a spectrum.

With so many colors to work with, SPHEREx catches more than 80 percent of the light that most brown dwarfs give off. Earlier sky surveys measured that light in only a few broad bands.

"From orbit, SPHEREx sees wavelengths of light that are basically impossible to see with telescopes on the ground because water in Earth's atmosphere absorbs them," Rustamkulov said.

"We are picking up light from the deep, red clouds of brown dwarfs all over the sky."

Four molecules appear

The team worked with 37 brown dwarfs that are close to Earth and give off clear signals. Their temperatures range from about 4,000°F (2,200°C) down to about -10°F (-20°C), which is colder than a kitchen freezer.

One of them, roughly 7 light-years away, is the coldest brown dwarf known.

Each molecule absorbs its own set of colors, leaving a telltale pattern of dips in a spectrum. In these spectra, the researchers found the dips left by water, carbon dioxide, carbon monoxide, and methane.

J. Davy Kirkpatrick, also a scientist at IPAC, coauthored the study.

"We're seeing the signatures of these molecules and how they change from object to object across the entire temperature regime," Kirkpatrick said. "Our paper concentrated on just three dozen, but we have thousands more that we are in the process of analyzing."

"I really want to see what bounds the Universe places on the variety of brown dwarfs."

Cooling brings methane

A brown dwarf cools for its whole life, and its atmosphere changes along the way. The hottest ones hold clouds of mineral grains, the stuff of ordinary rock. Below about 2,060°F (1,130°C), those clouds sink too deep to see.

Below about 1,340°F (730°C), the carbon in a brown dwarf's atmosphere tends to end up in methane rather than carbon monoxide.

Some of the brown dwarfs in this study are partway through these changes, with their clouds thinning out and methane becoming more common.

In the SPHEREx spectra, the carbon monoxide signal grew a little stronger from the hottest objects down to about 1,700°F (930°C), and the carbon dioxide signal peaked somewhat cooler.

Below that, both weakened, most steeply between about 1,340°F and 800°F (730°C and 430°C).

In other words, the carbon monoxide and carbon dioxide signals fade fastest over a fairly narrow stretch of a brown dwarf's long cooling.

Models struggle with brown dwarf clouds

The team compared its spectra with five widely used sets of computer models. Each set makes its own assumptions about clouds and chemistry.

All five followed the general shift from carbon monoxide to methane. None matched the spectra across the whole group, though, and they had the most trouble with brown dwarfs in the cloudy stretch between the hot and cold ends.

When the team used the cloudy version of one model family, its match to the carbon monoxide signal got worse.

"The state-of-the-art models are capturing the general chemical trend, but when it comes to these cloudy transitions, the models are struggling to match the data," said Rustamkulov.

"No two brown dwarfs are alike. Even at the same temperature, their spectra look quite distinct."

A lone brown dwarf's size, temperature, and mass can only be estimated by fitting models to its light, which is why the gaps matter. Fit to the coldest object in the set, the models disagreed about its width by a factor of three to four.

More brown dwarf spectra are coming

Picked for their clean signals, the 37 objects showed a fairly smooth shift toward methane. The authors wrote that this could mean the trend really is steady, or that their sample was too small and too carefully picked to show much variety.

The data are also early. At least half of the observations were processed before the mission's software was updated to flag more bad pixels. To compensate, the researchers screened out readings that stood far apart from their neighbors.

"The findings are a call to action to explore even more of these dark worlds. This journey has turned many of us into accidental meteorologists," Kirkpatrick said.

"We know how hard it is to predict weather on our own planet, and we realize it's going to be just as challenging to explain the phenomena we see in these bizarre, cold objects."

What lies ahead

Astronomers don't yet know whether better handling of clouds will close the gap, or whether brown dwarfs vary more than any one set of models can capture.

SPHEREx will scan the whole sky four times during its planned 25-month mission, and each pass will sharpen the spectra.

Its data are free to download from the NASA/IPAC Infrared Science Archive, so other astronomers can study the same thousands of brown dwarfs that the team is still working through.

The full study is published in The Astrophysical Journal.

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