Unveiling the Smallest Stars: Brown Dwarfs and JuMBOs in Focus

How small can a star be? Astronomers using NASA's James Webb Space Telescope (JWST) to study star-forming regions near the Milky Way may have spotted the smallest brown dwarf yet. Brown dwarfs form like stars but don't have enough mass to start hydrogen fusion in their cores.

Previously, three research groups had identified around 12 brown dwarfs with masses between 3 to 8 times that of Jupiter, asserting that smaller brown dwarfs were unlikely. The observed brown dwarfs aligned with the predicted lower mass limit from stellar formation theories, boosting astronomers' confidence that they might have encountered the smallest naturally occurring brown dwarf.

However, a controversial, unpublished study by a team from the European Space Agency claims to have discovered pairs of brown dwarfs nearly the size of Jupiter, challenging established models of star formation. In the Quadrilateral Cluster at the heart of the Orion Nebula, approximately 1,400 light-years away, the team noted 42 brown dwarfs orbiting in pairs, some nearly as light as Jupiter. They dubbed these peculiar celestial bodies "Jupiter Mass Binary Objects (JuMBOs)."

Many astronomers define brown dwarfs based on the material they burn. Celestial bodies with masses greater than 70 times that of Jupiter possess sufficient gravity to trigger hydrogen fusion and are classified as traditional stars. Brown dwarfs with masses between 13 and 70 times that of Jupiter can only fuse a hydrogen isotope called deuterium, sustaining weak nuclear burning for millions of years.

Distinguishing smaller brown dwarfs from planets requires observing how they form. Although their final masses may be as large as 10 Jupiters, planets always arise from gas and dust disks surrounding stars, whereas stars, including brown dwarfs, form independently within vast collapsing gas clouds. Theoretically, the smallest brown dwarfs are believed to have masses approximately 7 times that of Jupiter, or 0.007 times that of the Sun.