What the Air Is Like on 33 Objects That Are Neither Star nor Planet

Somewhere between the smallest stars and the largest planets sits a population of objects that never settled the question. They condense out of collapsing gas clouds the way stars do, then stall: too light to sustain hydrogen fusion in their cores, they glow with the heat of their formation, slowly cooling over time. Many drift without a star to orbit. Astronomers discovered the first brown dwarfs in the 1990s, but their atmospheres remain challenging to characterize.

"They're kind of goth," Zafar Rustamkulov said of brown dwarfs when NASA's Jet Propulsion Laboratory announced his team's new survey on Oct. 8. "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."

That survey, published Sept. 28, in The Astrophysical Journal, is a benchmarking exercise. Rustamkulov and J. Davy Kirkpatrick, both at Caltech's IPAC in Pasadena, California, and their colleagues present near-infrared spectra of 33 nearby field brown dwarfs observed with a single instrument. The sample spans spectral classes from L0 to Y4, with temperatures ranging from about 2,500 kelvin to 250 kelvin. At the coldest end, that is below the freezing point of water.

The spectra came from a telescope that was not built to study brown dwarfs. SPHEREx, short for Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer, launched in March 2025 and maps the entire sky in 102 infrared colors. Its main goals include mapping hundreds of millions of galaxies to study the history of the universe and detecting interstellar ices. NASA's Jet Propulsion Laboratory, which manages the mission, described the brown dwarf survey as something of a side project. SPHEREx covers wavelengths from 0.75 to 5 micrometers, observing from above Earth's atmosphere.

"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."

In that band, molecules produce broad absorption features in the light leaving a brown dwarf's atmosphere, and those features change as the object cools. The authors report that across the L, T and Y classes, the spectra show water, carbon dioxide, carbon monoxide and methane. The clouds that dominate warm L dwarfs thin out as the objects cool, and methane becomes more prominent; that changeover is called the L/T transition.

The L/T transition is also where theory is weakest, and testing the models is what the paper set out to do. Atmospheric models are known to disagree on the temperatures and radii they infer for brown dwarfs, so the authors fitted each spectrum against five standard forward-model grids: Sonora Diamondback, Elf Owl, BT-Settl, ATMO2020 and ATMO2020++. The grids "continue to struggle" to fit two parts of the spectrum simultaneously: the J, H and K peaks in the near-infrared and the opacity window near 4 micrometers, where the atmosphere absorbs less light. The discrepancies are greatest in objects crossing the L/T transition, with the largest differences around carbon dioxide and carbon monoxide features, which are sensitive to atmospheric chemistry.

That does not mean the models fail outright. The authors also note that "despite these offsets, the models broadly capture their trends across the L/T transition," and the five grids do not perform equally well. For the field brown dwarfs in this sample, the results strongly favor one version of the Elf Owl grid: the one with weak vertical mixing rather than strong mixing. That ranking is part of the result, not a technicality.

The authors point to shortcomings in the models but do not consider the question settled. Their analysis of carbon chemistry across the sample leaves two possibilities open: the objects may genuinely span a range of compositions and chemical conditions, or the models may fall short because they are missing some relevant physics. SPHEREx's absolute brightness calibration is also still being refined, leaving open the possibility that some of the mismatch comes from the data itself.

The word to notice in the paper is "continue." Other instruments have encountered similar discrepancies across the L/T transition, so this is a known weakness tested against new data rather than a new crack in the theory. What is new is the dataset: one instrument and a consistent calibration across the temperature range. SPHEREx covers wavelengths that capture more than 80% of the light emitted by most brown dwarfs, making these spectra valuable for testing atmospheric models. The authors write that these and future SPHEREx data "will help guide improvements to models."

The survey has already measured spectra for thousands of nearby brown dwarfs, and JPL says the dataset is freely available, so other researchers can begin testing models without waiting for this team. The first 33 objects suggest that fitting their spectra will remain challenging. "No two brown dwarfs are alike," Rustamkulov said. "Even at the same temperature, their spectra look quite distinct." Kirkpatrick put the predicament more plainly: "This journey has turned many of us into accidental meteorologists. 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."