NASA’s SPHEREx reveals chemical diversity across 37 brown dwarfs

NASA reported on October 8 that its SPHEREx space telescope has revealed varied atmospheric chemistry across 37 nearby brown dwarfs, offering researchers new evidence to test models of these objects between stars and planets. Observations made from orbit show molecular signatures across a broad temperature range, while exposing difficulties in explaining how clouds change as brown dwarfs cool.

According to NASA’s announcement, the findings appear in The Astrophysical Journal. The researchers identified signatures of water, carbon dioxide, carbon monoxide and methane. Their analysis covers 37 objects; thousands more brown dwarfs observed by SPHEREx are still being analyzed.

What SPHEREx sees in brown dwarf atmospheres

Brown dwarfs form when clouds of gas collapse, as stars do, but lack enough mass to sustain hydrogen fusion in their cores. Their atmospheres share characteristics with giant planets such as Jupiter and Saturn. The free-floating objects described in NASA’s report have no host star and radiate heat from within, cooling over time.

The sample spans temperatures from approximately 2,200°C to minus 20°C, which NASA describes as the full brown dwarf temperature range. SPHEREx measures brightness in 102 colors, stretching from deep visible red into infrared light. Those measurements produce spectra in which molecules leave distinctive absorption patterns.

The molecular signatures vary from object to object and across temperatures. “We’re seeing the signatures of these molecules and how they change from object to object across the entire temperature regime,” study coauthor J. Davy Kirkpatrick, also at IPAC, said in the announcement.

Cloud transitions challenge atmospheric models

Some of the observed brown dwarfs are passing through a stage in which their clouds thin and methane-rich atmospheres emerge. As the objects age and cool, their molecular absorption patterns change, giving researchers evidence against which to compare theoretical descriptions of their atmospheres.

Rustamkulov said current models capture the general chemical trend but struggle with these cloudy transitions. “Even at the same temperature, their spectra look quite distinct,” he said. The observations therefore present a challenge for models seeking to explain atmospheric differences among objects with similar temperatures.

NASA says only a few dozen brown dwarfs had previously been studied in detail with space-based telescopes. Much of scientists’ understanding of their composition, atmospheric behavior and evolution consequently rests on theoretical models. The new spectra expand the observational evidence available to researchers testing those explanations.

Earlier Webb research tracked changing brown dwarf weather

Separate research reported by the University of Edinburgh on July 19, 2024 illustrates why atmospheric variability matters. An Edinburgh-led team used the James Webb Space Telescope to study WISE 1049AB, a pair of brown dwarfs about six light-years away. This earlier work provides context rather than an independent assessment of the new SPHEREx results.

The team tracked changes in brightness as cloudier and clearer regions rotated into and out of view. Those measurements allowed researchers to reconstruct atmospheric changes over full rotations lasting roughly five to seven hours. Edinburgh described swirling clouds of hot sand and atmospheric temperatures around 950°C.

The Webb researchers also examined wavelength-dependent signals involving water, methane and carbon monoxide. Edinburgh noted that static views of just one side of a brown dwarf have limitations because these objects rotate quickly and their weather can change considerably over time.

That study appeared in Monthly Notices of the Royal Astronomical Society and involved researchers from Trinity College Dublin and the University of Virginia. Edinburgh’s Beth Biller linked its importance to understanding giant exoplanets as well as brown dwarfs.

Thousands more SPHEREx observations await analysis

Launched in March 2025, SPHEREx takes about 3,600 unique images each day as it maps the sky. Brown dwarf research is a secondary use of that survey, whose broader goals include mapping the distribution of galaxies and studying interstellar ice.

Kirkpatrick said the team is analyzing thousands more brown dwarfs to investigate the extent of their diversity. NASA’s account gives no completion timetable. Its description of observations of thousands of objects does not establish that all are newly discovered brown dwarfs.

NASA says the SPHEREx dataset is freely available to scientists and the public, with processing and archiving at IPAC. Scientific analysis involves 13 institutions. Researchers studying substellar atmospheres can use those observations to test models, although the announcement does not quantify how much their accuracy might improve.