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NASA’s SPHEREx Telescope Sees Menagerie of Brown Dwarfs
NASA’s SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer) space telescope is shedding light on brown dwarfs, celestial objects that blur the line between stars and exoplanets. New findings published in The Astrophysical Journal show that these dark and cloudy worlds have chemically rich atmospheres not unlike the giant planets in our own solar system.
First discovered in the 1990s, brown dwarfs form from collapsing clouds of gas, like stars do, but they aren’t hefty enough to sustain hydrogen fusion in their cores. These dimly glowing balls of warm gas also share characteristics with Jupiter and Saturn. In contrast to most planets, however, brown dwarfs drift in darkness, ungoverned by a host star and heated entirely from within.
“They’re kind of goth,” said Zafar Rustamkulov, lead author of the new study and a scientist at IPAC, Caltech’s science and data center in Pasadena, California. “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.”
Only a few dozen of these “dark wanderers” have been studied in detail with space-based telescopes. Astronomers want to observe more of them in the wild because much of our understanding of their makeup, storminess, and evolution comes from theoretical models.
Enter NASA’s SPHEREx, an infrared space telescope launched in March 2025 and managed by the agency’s Jet Propulsion Laboratory in Southern California.
The study authors analyzed SPHEREx observations of 37 nearby brown dwarfs spanning the full brown dwarf temperature range, from about 4,000 to minus 10 degrees Fahrenheit (2,200 to minus 20 degrees Celsius). The telescope measures their brightness in 102 different colors, from the deepest red our eyes can see, to the invisible heat of infrared light, creating a spectrum. These spectra revealed to the study authors chemically rich atmospheres harboring water, carbon dioxide, carbon monoxide, and methane.
“We’re seeing the signatures of these molecules and how they change from object to object across the entire temperature regime,” said study coauthor J. Davy Kirkpatrick, a scientist at Caltech’s IPAC. “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.”
“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.”
Spotting brown dwarfs is something of a side project for SPHEREx. The space telescope takes about 3,600 unique images per day to stitch into maps of the entire sky. Knowing where hundreds of millions of galaxies are distributed across the cosmos will help scientists reconstruct what happened in the first billionth of a trillionth of a trillionth of a second after the big bang. As it scans the sky, the telescope also is searching for the chemical ingredients of life and finding interstellar ice that could one day seed oceans on distant worlds.
For years, brown dwarfs have been spotted by other observatories, including NASA’s James Webb Space Telescope and retired Spitzer Space Telescope. But thanks to its spectral coverage, SPHEREx is now imaging thousands of them for the first time in a fruitful region of the electromagnetic spectrum that spans deep red and infrared wavelengths of light.