NASA’s SPHEREx measured 37 brown dwarfs, from about 4,000°F down to minus 10°F

Thirty-seven brown dwarfs, ranging from roughly 4,000 degrees Fahrenheit (2,200 degrees Celsius) down to minus 10 degrees Fahrenheit (minus 20 degrees Celsius), now have spectra from NASA’s SPHEREx space telescope. The survey, published in The Astrophysical Journal and announced by NASA’s Jet Propulsion Laboratory on Oct. 8, 2026, shows atmospheres loaded with water, carbon dioxide, carbon monoxide and methane.

Zafar Rustamkulov of Caltech’s IPAC, the paper’s lead author, summed up a sample whose coldest members are chillier than a winter night in much of the northern United States: “No two brown dwarfs are alike.”

Thirty-seven spectra across a 4,000-degree span

Brown dwarfs sit between planets and stars. According to NASA’s brown dwarf page, they are more massive than planets, at least 13 times the mass of Jupiter, but lack the mass to begin nuclear fusion in their cores. Without fusion to sustain them, they cool over time, which is why one survey can capture objects glowing at 4,000 degrees and others colder than a household freezer.

The SPHEREx paper, titled “SPHEREx 0.75 to 5 μm Spectra for a Sequence of Nearby Brown Dwarfs,” covers 37 nearby field brown dwarfs. According to its arXiv listing, submitted July 1, 2026, the sample spans spectral types from L0 to Y4, the hottest to the coldest classes of brown dwarf. NASA’s release puts the temperature range at about 4,000 to minus 10 degrees Fahrenheit across the 37.

Rustamkulov and co-author J. Davy Kirkpatrick, also of IPAC, are among more than 20 authors on the preprint, which includes SPHEREx project scientist Olivier Doré of JPL.

The 102-color spectrum and the chemistry it exposes

SPHEREx measures the brightness of each object in 102 colors, from visible deep red through infrared light, and strings those measurements together into a spectrum. The design comes from the instrument itself: JPL explained in December 2025 that six detectors, each carrying a filter with a gradient of 17 colors, produce the 102 wavelength bands, and that the observatory images the whole sky in all of them about every six months.

That breadth is what lets the team read the chemistry. The spectra show absorption from water, carbon dioxide, carbon monoxide and methane, a mix that resembles the atmospheres of the giant planets in the solar system. Rustamkulov described free-floating brown dwarfs as completely independent celestial objects, which makes their atmospheres a way to study giant-planet-like chemistry without a bright host star in the way.

Some of the dwarfs in the sample are in a transitional stage in which clouds thin out and the atmosphere turns methane-rich. Those objects are where the comparison with theory gets difficult, because a cloud deck that is thinning changes which wavelengths escape the atmosphere and how deep into it an observer sees. Current models capture the general chemical trend across the sequence from hot to cold, according to NASA, but they have trouble reproducing the cloudy transitions in detail.

Sonora, Elf Owl and the other model grids that miss

The preprint compares the 37 spectra against five published model grids: Sonora Diamondback, Elf Owl, BT-Settl, ATMO2020 and ATMO2020++. Its authors report that the models still struggle to fit the J, H and K brightness peaks and the 4-micron window at the same time, especially for objects at the transition between L-type and T-type dwarfs, with the largest deviations near carbon dioxide and carbon monoxide features. The observed dwarfs favor the Elf Owl models with weak vertical mixing.

NASA’s release frames the same result more plainly. Rustamkulov said the state-of-the-art models capture the general chemical trend, but the cloudy transition objects are where they struggle.

Kirkpatrick called the findings “a call to action to explore even more of these dark worlds,” and said he wants to see what bounds the universe places on the variety of brown dwarfs. SPHEREx, which launched on March 11, 2025, is collecting the data for it: NASA says the telescope is imaging thousands of brown dwarfs for the first time in this spectral range, and the dataset is freely available to scientists and the public.

The 37 in this paper are therefore a first cut, and the mission’s two-year primary survey leaves room for far more, since SPHEREx is imaging thousands of brown dwarfs for the first time in this spectral range, as NASA puts it, and the same all-sky maps are also being used to track hundreds of millions of galaxies and to search for interstellar ice. IPAC processes and archives the SPHEREx data, and JPL manages the mission for NASA’s Astrophysics Division in Washington, with Caltech managing JPL for NASA. The mission also takes about 3,600 images a day to build its all-sky maps, which is how a single survey can reach brown dwarfs scattered across the whole sky rather than in one targeted field.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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