James Webb found brown dwarfs barely twice Jupiter’s mass hiding a molecule no model predicted

Astronomers using the James Webb Space Telescope have identified brown dwarfs weighing as little as twice the mass of Jupiter inside the star-forming region IC 348, the least massive examples of these star-like failures ever confirmed. Buried in their spectra was something the team was not looking for: an absorption feature from hydrocarbon molecules that atmospheric models of brown dwarfs had never anticipated. The observations, released by the European Space Agency on September 15, come from the same research program that first pushed into this territory four years earlier.

IC 348 sits roughly 1,000 light-years away in the constellation Perseus, a nursery where collapsing clouds of molecular hydrogen are still assembling new stars and the occasional object too small to ever become one. Webb’s new mosaic of the region, one of the largest single images the telescope has produced for public release, captures that entire spectrum of outcomes in a single frame.

A Four-Year Hunt That Pushed Below Three Jupiter Masses

The search is led by Kevin Luhman of Penn State’s Department of Astronomy and Astrophysics and Catarina Alves de Oliveira, head of the Science Operations Development Division at the European Space Agency, who serve as principal investigators on Webb observing program 4866. The pair first used Webb to study IC 348 in 2022 and came away with brown dwarfs weighing three to four times as much as Jupiter, a record at the time.

Returning to the same cluster, the team used Webb’s NIRCam instrument in 2024 to image faint, cool objects by their color and brightness, then followed up with the NIRSpec spectrograph in 2025 to weigh the most promising candidates. That two-step process is what turned up brown dwarfs at roughly twice Jupiter’s mass, equal to about 0.19 percent of the Sun’s mass, cutting the previous record nearly in half.

Webb observing program 4866 was designed with three stated aims: to study the lowest-mass objects the star-formation process can produce, to understand how populations of planetary-mass brown dwarfs vary between different star-forming regions, and to probe the origins of a hydrocarbon feature that had already turned up in a handful of similarly light objects elsewhere. That third aim turned out to be well timed, since IC 348 delivered a fresh example of exactly the feature the program set out to explain.

A Hydrocarbon Signature the Models Never Predicted

While examining the new spectra, the researchers found an absorption feature in two of the three lowest-mass brown dwarfs that they attributed to hydrocarbons, molecules built only from hydrogen and carbon. The paper reporting the result proposes an entirely new spectral class, labeled “H,” defined by the presence of the 3.4 micron fundamental band of the hydrocarbon, Luhman and Alves de Oliveira wrote in the study published in The Astrophysical Journal Letters.

That feature has shown up before only in the atmospheres of the very lowest-mass brown dwarfs known, and no existing atmospheric model predicted it would appear there at all. Proposing an entirely new spectral class is a stronger claim than simply flagging an anomaly, and it suggests the physics governing these objects’ atmospheres changes in ways theorists had not built into their simulations.

Circumstellar Discs Around Objects the Mass of Planets

One of the survey’s lightest finds, at roughly twice Jupiter’s mass, showed excess infrared emission consistent with a surrounding disc of gas and dust, the same kind of structure that builds planets around ordinary stars. A second object weighing about ten Jupiter masses showed similar excess emission.

Neither of these bodies is a planet by any conventional definition, since both formed the way stars do, through the direct collapse of a molecular cloud rather than by accreting inside a disc around a separate star. Finding disc material around something with a planet’s mass raises the odd possibility that free-floating objects this small could still end up hosting planets of their own.

The same wide mosaic that captured these brown dwarfs also caught a cluster of protostars in one corner of the frame, several trailing jets known as Herbig-Haro objects, where material ejected from a forming star slams into surrounding gas and glows. One of those features, cataloged as HH 797, turned out on close inspection to be two protostars with nearly parallel outflows rather than the single source earlier images suggested, while a nearby propeller-shaped source, HH 211, shows both narrow jets and broader surrounding outflows.

Redrawing the Line Between the Smallest Stars and Brown Dwarfs

The smallest true stars weigh in at around eight percent of the Sun’s mass, the rough threshold below which an object’s core never gets hot enough to fuse hydrogen into helium. Brown dwarfs occupy the space beneath that line; many still briefly fuse deuterium, a heavier form of hydrogen, early in their lives before settling into a long, slow cooling phase.

Webb’s IC 348 census now extends that population down to twice Jupiter’s mass, close enough to giant-planet territory that mass alone can no longer reliably separate the two categories. Luhman and Alves de Oliveira’s program was designed specifically to probe how populations of these planetary-mass objects vary between star-forming regions, and the newly proposed “H” spectral class gives them, and other researchers working with Webb data, a concrete feature to search for in fainter, more distant nurseries.

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

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