Astronomers Detect First Brown Dwarf Candidates Outside the Milky Way.

 

An international team of astronomers has made a groundbreaking discovery using the James Webb Space Telescope (JWST), identifying the first rich population of brown dwarf candidates in the star cluster NGC 602, located in the Small Magellanic Cloud, approximately 200,000 light-years from Earth.

NGC 602, a young star cluster, serves as an analogue for conditions in the early Universe, characterized by low metallicity and the presence of dense dust clouds and ionised gas indicative of ongoing star formation. This environment, along with the cluster’s HII region N90, offers a unique opportunity to study star formation in a setting vastly different from our solar neighborhood.

Led by Peter Zeidler from AURA/STScI for the European Space Agency, the team’s observations revealed candidates for young brown dwarfs—objects too massive to be considered planets but not quite stars, with masses ranging from about 13 to 75 times that of Jupiter. “Only with the incredible sensitivity and spatial resolution of JWST can we detect these objects at such distances,” Zeidler noted, highlighting the unprecedented capabilities of the telescope.

Elena Manjavacas, also from AURA/STScI, emphasized that this discovery marks the first identification of brown dwarfs outside our galaxy, expanding the known population of these objects beyond the approximately 3,000 currently identified within the Milky Way.

The study combined data from both Hubble and Webb, showcasing the complementary strengths of these telescopes. Antonella Nota, a team member and former Webb Project Scientist for ESA, explained that while Hubble indicated the presence of young low-mass stars in NGC 602, it was only with Webb that the team could fully appreciate the formation of substellar objects in the cluster.

Zeidler further noted that the findings support the theory that the mass distribution of bodies below the hydrogen burning limit continues the pattern seen in stellar formation, indicating that brown dwarfs form similarly to stars but do not accumulate enough mass to ignite nuclear fusion.

The team’s observations, conducted in April 2023, included a new image from Webb’s Near-InfraRed Camera (NIRCam), illustrating the cluster’s stars, young stellar objects, and the intricate surrounding gas and dust structures, all while accounting for contamination from background galaxies.

“Studying these young, metal-poor brown dwarfs in NGC 602 brings us closer to understanding the formation of stars and planets in the early Universe,” stated Elena Sabbi of NSF’s NOIRLab and the University of Arizona.

As the first substellar objects detected outside the Milky Way, these findings pave the way for future discoveries that could reshape our understanding of star and planet formation. The research has been published in The Astrophysical Journal as part of the JWST GO programme 2662.