James Webb has just spotted objects so lightweight that they further blur the boundary between stars and planets.
The discovery is located in IC 348, a region where stars are born about 1,000 light-years from Earth. The space telescope identified brown dwarfs there with masses as low as only twice that of Jupiter. They are the least massive brown dwarfs known to date.
Artist's impression of a brown dwarf, an object formed like a star but too low in mass to sustain hydrogen fusion.
Credit: NASA, ESA, Caltech
A star is born when a cloud of gas collapses under its own weight. Matter gathers, heats up, and eventually allows sustained nuclear reactions in its core. A brown dwarf begins in a similar way, but remains too lightweight to sustain hydrogen fusion like a true star. They are sometimes described as “failed stars.”
How far can this mechanism work with very little matter? That is precisely the question posed by IC 348. In 2022, James Webb had already found brown dwarfs there weighing three to four times Jupiter's mass. New observations have allowed researchers to go even lower.
Astronomers first used the NIRCam infrared camera in 2024. This light makes it possible to detect young objects that are still hot amid clouds of dust. They then analyzed their light with the NIRSpec instrument in 2025. This second step provides information about their nature and makes it possible to estimate their mass.
Mosaic of details from the IC 348 region observed by James Webb, showing embedded stars, jets from young stars, and background galaxies.
Credit: NASA, ESA, CSA, K. Luhman (PSU), C. Alves de Oliveira (ESA), M. Zamani (ESA/Webb)
The lightest objects reach about 0.19% of the Sun's mass. Such a low value poses a challenge to current models of star formation. At this scale, researchers must explain how a cloud of gas can collapse and produce such a small independent object.
Mass alone is not enough to clearly distinguish a planet from a brown dwarf. A planet generally forms in the disk of matter surrounding a star. The brown dwarfs observed here appear instead to have been born directly from the collapse of a cloud, like stars. Their mass can nevertheless be comparable to that of large planets.
One observation makes this boundary even more surprising. One of the brown dwarfs close to twice Jupiter's mass shows signs of a disk around it. This disk contains gas and dust, the materials from which planets can form. However, no planet has been detected there.
Researchers can now search for the minimum mass at which this formation process remains possible. They also want to know whether objects this lightweight can actually form planets of their own. Such a system would then have as its central body an object whose mass was itself comparable to that of a giant planet.