Webb Finds Brown Dwarfs Just Twice Jupiter's Mass, Smaller Than Theory Said Was Possible

Astronomers led by Kevin Luhman used NASA's James Webb Space Telescope to discover brown dwarfs as small as twice Jupiter's mass in IC 348, a star-forming region 1,000 light-years away, NASA announced Tuesday, revealing objects smaller than existing formation models predicted could exist.

What the Record Shows

The discovery comes from Webb General Observer Program 4866, which NASA identifies as the source of the underlying data. The team used Webb's NIRCam (Near-Infrared Camera) in 2024 to capture infrared light from young brown dwarfs and newborn stars in IC 348. They then followed up in 2025 with Webb's NIRSpec (Near-Infrared Spectrograph) to conduct spectroscopic observations, a technique that spreads an object's light into its component wavelengths to reveal physical properties including mass.

Those spectra are where the finding lives. The observations revealed brown dwarfs with masses as low as twice Jupiter's mass, or 0.19 percent of the Sun's mass, according to NASA's account of the research. NASA describes these as the least massive brown dwarfs known.

This is the same research team that used Webb to study IC 348 in 2022 and found brown dwarfs with masses as low as three to four times Jupiter's mass, according to NASA. The new observations pushed that boundary lower still.

Why It Matters

Brown dwarfs occupy an awkward middle ground in the universe. They form the way stars do, through the collapse of cold molecular hydrogen clouds, but their cores never get hot enough to fuse ordinary hydrogen into helium. The smallest stars weigh in at around 8 percent of the Sun's mass, according to NASA. Below that threshold sits this strange class of objects, and the question researchers have been pressing is how far down the mass scale the star-formation process can reach.

The answer, apparently, is further than the models said.

Objects twice Jupiter's mass are, by any reasonable measure, planet-sized. The fact that the star-formation process appears to produce them challenges the theoretical floor that physicists had placed on brown dwarf formation. NASA describes their existence as posing "a challenge to models of how stars form."

One of the lightest newly found brown dwarfs added a second surprise: it showed signs of a disk around it, according to NASA. Disks are where planets form. The implication is that small planets could be assembling around an object that is itself only the mass of a planet. That is a sentence worth reading twice.

An Unidentified Molecule and a Possible New Spectral Class

While reviewing the spectra, the research team found a feature they attributed to an unidentified hydrocarbon, a molecule built only from hydrogen and carbon atoms, according to NASA. The feature has appeared only in the atmospheres of the lowest-mass brown dwarfs observed so far. NASA notes that this pattern suggests these extreme objects might occupy a spectral class of their own, separate from the classification systems already in use for stars and higher-mass brown dwarfs.

The hydrocarbon feature is one of the explicit targets of General Observer Program 4866. The program is also designed to compare populations of planetary-mass objects across different star-forming regions, not just IC 348.

What Else Is in the Image

The panorama of IC 348 that Webb produced is one of the largest images released from the telescope to date, according to NASA. The star-forming region sits in the constellation Perseus.

The upper right corner of the image contains a dense collection of protostars, stars still in the process of forming. Several are accompanied by Herbig-Haro objects, which are glowing regions that appear when jets from newborn stars slam into surrounding gas and dust. Think of them as the visible shock wave from a very slow, very energetic collision that has been running for thousands of years.

Two of those objects are identified in the NASA release. HH 797, a long narrow feature oriented horizontally in the image, turns out on close inspection to be two protostars with nearly parallel outflows. Just to its right is HH 211, shaped like a propeller, featuring both narrow jets and broader outflows.

The image credit, as listed by NASA: NASA, ESA, CSA, Kevin Luhman (PSU), Catarina Alves de Oliveira (ESA), Mahdi Zamani (ESA/Webb).

What Is Next

NASA describes Webb as an international program led by the agency with partners ESA and CSA. General Observer Program 4866 is ongoing, with stated goals that extend beyond the brown dwarf mass question to include the hydrocarbon feature and cross-region comparisons of planetary-mass object populations. NASA has not announced a timeline for the next results from this program in the material released Tuesday.

The lower mass limit of brown dwarf formation is not a settled number. It was three to four Jupiter masses in 2022, according to NASA's account of the earlier work. It is two Jupiter masses today. Whether the next deep look finds something smaller is the question the program was built to answer.

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