Astronomers discovered brown dwarfs with masses as low as twice the mass of Jupiter using the James Webb Space Telescope, ESA reported. The objects, detected in the star-forming region IC 348, weigh approximately 0.19% of the Sun's mass. That makes them the least massive brown dwarfs known to date, posing a direct challenge to existing models of star formation.
Principal investigators K. Luhman and C. Alves de Oliveira directed the search under Webb observing programme #4866 to test how small star-forming processes can reach. Unlike stars, which require at least 8% of the Sun's mass to fuse hydrogen into helium, brown dwarfs collapse from molecular gas clouds without ever generating enough core heat for sustained fusion. Many burn deuterium early in their lives before cooling.
Researchers initially surveyed IC 348 with Webb in 2022, finding brown dwarfs between three and four times Jupiter's mass. The team returned to the cluster in 2024, using Webb's Near-Infrared Camera to pinpoint candidates by their colour and brightness. Spectroscopic follow-ups conducted with the Near-Infrared Spectrograph in 2025 then verified the masses of the lightweight bodies.
NIRSpec observations revealed further anomalies in the atmospheres and surroundings of the newfound objects. One of the lowest-mass brown dwarfs showed evidence of a circumstellar disc, indicating that planets may form around bodies that themselves possess planetary mass. Spectra from the targets also displayed signatures of hydrocarbons, a feature confined to the lowest-mass brown dwarfs that may place them in their own spectral class.
IC 348 lies 1,000 light-years from Earth in the constellation Perseus, forming part of one of the largest public images Webb has produced. The upper-right corner of the image details active protostars alongside Herbig-Haro objects created by stellar jets hitting nearby gas. These formations include HH 797, which resolves into two protostars with parallel outflows, and the propeller-shaped outflow source HH 211.
ESA provided the Ariane 5 launch service for the Webb mission alongside the NIRSpec instrument and half of the mid-infrared MIRI instrument. The telescope operates as an international partnership among NASA, ESA, and the Canadian Space Agency.
