JWST found brown dwarfs at 2 Jupiter masses and a molecule no model predicted

Brown dwarfs were supposed to have a floor. The objects that fall between planets and stars — too massive to be planets, too small to sustain the hydrogen fusion that makes a star burn — were thought to require at least a certain threshold of mass to coalesce from a collapsing cloud of gas and dust. JWST has found nine that apparently did not get the memo.

The cluster IC 348, a compact nursery of young stars in the Perseus constellation, was already known to harbor unusually small brown dwarfs. An earlier JWST survey pushed the mass record down to 3 to 4 Jupiter masses. The new survey, led by Kevin Luhman of Pennsylvania State University and Catarina Alves de Oliveira of the European Space Agency, went further: using NIRCam imaging followed by NIRSpec spectroscopy, the team identified nine new substellar objects with masses estimated as low as 2 Jupiter masses — approximately 0.19 percent of the Sun’s mass. One carries a protoplanetary disc — the ring of material from which planets form. It is the least massive object ever found harboring the raw material for planetary companions.

That already overturns the previous boundary. The atmospheric molecule may be the stranger result.

An unidentified compound in the coolest atmospheres

Eight of the nine new members — along with one previously known object — show an absorption feature at 3.4 micrometers attributed to an aliphatic hydrocarbon: a class of organic molecule built around chains of carbon and hydrogen atoms. No existing model of brown dwarf atmospheres predicts this compound should be present. The feature has been seen in the atmospheres of Saturn and Titan, which the team used as spectral reference points, but had never been detected in any brown dwarf outside the Solar System before this survey.

The feature is not random. Its strength correlates systematically with apparent faintness — the cooler and lower-mass the brown dwarf, the more pronounced the signature. That gradient points toward a real physical process operating in these extreme, cool atmospheres rather than contamination or instrumental artifact. Luhman and Alves de Oliveira propose a new spectral classification — class H, for hydrocarbon — to accommodate these objects. The existing taxonomy of cool dwarfs runs from spectral types L through T to Y; none of those classes was designed for this mass regime.

What the molecule is precisely, and why it accumulates in the atmospheres of the lightest brown dwarfs, remains open. The paper states the finding "was not predicted by atmospheric models and was not previously detected in atmospheres outside of the solar system."

How they did it

The observations used two of Webb’s instruments in sequence. NIRCam imaged IC 348 across several near-infrared filters, identifying substellar candidates by their unusual colors and faint apparent brightness — a selection that exploits how very low-mass objects appear distinctly redder than background stars at these wavelengths. NIRSpec spectroscopy then confirmed the masses of the candidate objects by measuring how much light each absorbed at each wavelength and comparing those patterns against spectral models of young brown dwarfs.

For objects at 2 Jupiter masses, the spectral features are close to Webb’s detection floor. IC 348 was chosen deliberately: at roughly 3 million years old, the cluster’s objects are still warm from the energy released during formation — brighter and more spectroscopically accessible than older brown dwarfs of the same mass would be. The same search conducted in a 100-million-year-old cluster would yield nothing — the objects would be too cold and faint for current instruments.

Mass estimates at this extreme carry uncertainties of around 30 percent. An object nominally at 2 Jupiter masses could plausibly fall anywhere from 1.4 to 2.6 under different evolutionary model assumptions.

Why this is harder to explain than it looks

The previous JWST mass floor of 3 to 4 Jupiter masses was already below what most theoretical frameworks had predicted as the practical lower boundary for isolated substellar formation. That limit arises from thermodynamics: for a collapsing gas cloud to produce a stable object rather than fragmenting further, the fragment must reach certain conditions of temperature and density. Various models — depending on assumed cloud temperature, turbulence, and magnetic field — place this threshold anywhere from 3 to 10 Jupiter masses.

Two Jupiter masses falls below all of them. The theory is not disproven in the decisive sense — it is incomplete in the sense of not accounting for something that observably happens in at least one young cluster. The mechanism by which IC 348 produces objects this light is not currently understood. The protoplanetary disc on the lightest candidate adds another layer: planet-forming discs were thought to require substantially more massive host objects, and if they can assemble around a 2 Jupiter-mass body, the boundary between planetary and stellar formation narrows further.

What these findings do not settle

The nine objects come from a single cluster. Whether IC 348 is unusual — perhaps its environment is exceptionally efficient at fragmenting gas clouds at low masses — or whether such objects are common across star-forming regions, is not answerable from this dataset alone. A systematic Webb survey of multiple young clusters would be required.

The hydrocarbon feature requires identification. The absorption at 3.4 micrometers is consistent with several different aliphatic compounds; NIRSpec’s spectral resolution at these wavelengths is insufficient to distinguish them. High-resolution follow-up spectroscopy, or laboratory measurements of candidate molecules under brown dwarf atmosphere conditions, would be needed to name the compound. The authors do not speculate on a specific identification.

The broader question of whether mass estimates are reliable at these extremes — whether the models that convert observed brightness into a mass number break down below 3 Jupiter masses — cannot be resolved without independent methods, such as dynamical mass measurements from binary systems, which are not available for these objects.

Common questions about the lightest brown dwarfs

Can planets orbit a 2 Jupiter-mass brown dwarf?

One of the newly found objects at approximately 2 Jupiter masses shows excess infrared emission consistent with a protoplanetary disc — the material reservoir from which planets form. Whether planets actually assemble and survive around something barely heavier than Jupiter is unknown. No planets have been confirmed orbiting any isolated object near this mass.

What is the difference between a brown dwarf and a giant planet?

Brown dwarfs form by the direct gravitational collapse of a gas cloud, the same process that makes stars. Giant planets form inside a disc of material orbiting an already-existing star. The distinction becomes practically uncertain at very low masses: a 2 Jupiter-mass object found in isolation inside a star-forming cluster is classified as a brown dwarf on the basis of how it likely formed, not on mass alone.

What is an aliphatic hydrocarbon and why is it strange in a brown dwarf atmosphere?

Aliphatic hydrocarbons are organic molecules made of carbon and hydrogen atoms arranged in chains or branching structures — they include compounds found in petroleum on Earth and in the atmosphere of Saturn’s moon Titan. In the outer atmosphere of a brown dwarf, existing models do not account for them. Their presence at 3.4 micrometers in the spectra of the coolest IC 348 objects represents a gap in current understanding of substellar atmospheric chemistry.

How far away is IC 348?

IC 348 lies approximately 1,000 light-years from Earth in the Perseus constellation. It is one of the nearest young star-forming regions and has been a target for stellar population studies for several decades. Its age of around 3 million years makes its member objects still bright enough for spectroscopic analysis with Webb.

The team’s observing program focused specifically on IC 348. Comparable deep spectroscopic surveys of other young clusters — including the Orion Nebula Cluster, Chamaeleon I, and Taurus — are planned or underway with Webb. Results from those programs will test whether the 2 Jupiter-mass floor and the hydrocarbon feature appear more broadly, or whether IC 348 is an outlier. The proposed spectral class H awaits independent confirmation from additional datasets before entering standard use.

Reference: Luhman, K. L. & Alves de Oliveira, C., “A New Spectral Class of Brown Dwarfs at the Bottom of the IMF in IC 348,” The Astrophysical Journal Letters, 986 (1), 2025. DOI: 10.3847/2041-8213/addc55

Tags: astronomy, JWST, star formation, aliphatic hydrocarbon, brown dwarfs, IC 348