Webb Finds Lightest Brown Dwarfs Yet in Perseus
Webb's new panorama of the IC 348 region in the constellation Perseus reveals brown dwarfs weighing just two Jupiter masses, the lightest confirmed, lowering the known mass limit for these objects.
The European Space Agency (ESA) and NASA released one of the largest images ever taken by the James Webb Space Telescope on September 15, a panorama of the IC 348 star forming region in the constellation Perseus, 1,000 light years from Earth. A team of researchers analyzing the image found brown dwarfs there weighing just two Jupiter masses, the lowest confirmed mass for objects of this kind, pushing down the known lower limit for how small a brown dwarf can be.
Brown dwarfs form the same way stars do, through the collapse of a cloud of molecular hydrogen, but their cores never grow hot enough to fuse hydrogen into helium, even though many of them briefly burn deuterium in their first years. The smallest true stars carry about eight percent of the Sun’s mass, and below that threshold lies the realm of brown dwarfs, whose lower boundary researchers have spent years trying to pin down.
The same research team first examined IC 348 with Webb in 2022 and found three brown dwarfs at the cluster’s center, the lightest estimated at three to four Jupiter masses. The new, deeper survey pushed that limit down to just two Jupiter masses, or 0.19 percent of the Sun’s mass, now the lightest confirmed brown dwarfs known. Earlier this year the same telescope was the subject of a Srpske novine story about the fast changing rings of the small celestial body Chariklo; this time it is a completely different part of its work, the search for the smallest objects the star formation process can produce at all.
How Was the Data Gathered?
The data comes from Webb observation program No. 4866, led by principal investigators K. Luhman and C. Alves de Oliveira. The NIRCam instrument imaged the region in 2024 across four infrared bands, shown in blue at 1.62 and 1.82 micrometers, in green at 3.6 micrometers and in red at 4.44 micrometers, producing an image measuring 30,866 by 38,204 pixels, one of the largest Webb has ever released. Candidates for brown dwarfs were first selected by color and brightness, after which the NIRSpec spectrograph carried out spectroscopic follow up in 2025 to estimate their mass. The results were peer reviewed and published in The Astrophysical Journal Letters under the identifier 10.3847/2041-8213/addc55.
What Else Does the Panorama Reveal?
Around one of the lightest brown dwarfs found, researchers spotted signs of a disk, raising the possibility that planets could form around an object that is itself closer to planetary mass. In the spectra of the lightest brown dwarfs, the team also picked up a trace of hydrocarbons, molecules made only of hydrogen and carbon, previously seen only in the atmospheres of the least massive objects, hinting that these bodies could make up a distinct spectral class of their own. In the upper right corner of the image sits a group of young protostars accompanied by Herbig Haro objects, bright clouds of gas produced by jets from newborn stars. A closer look at HH 797 showed it to be two protostellar sources with nearly parallel jets, while the nearby HH 211 has narrower jets and wider gas outflows. The same image also shows traces of gravitational lensing and a spiral galaxy in the background.
The finding covers a single star forming region and a sample of only a handful of confirmed objects, so it does not by itself show that brown dwarfs of two Jupiter masses or less regularly form elsewhere in the galaxy. That is why Webb program 4866, according to the researchers themselves, plans to next compare populations of these objects across several different star forming regions. Neither the disk signature nor the hydrocarbon trace in the spectrum has yet been confirmed by additional, independent measurements, so both remain open questions for future observations rather than a final conclusion about where the lightest brown dwarfs come from.
