Picture the outskirts of the Small Magellanic Cloud, a satellite galaxy of our Milky Way, roughly 200 000 light-years from Earth: here lies the young star cluster NGC 602. The local environment of this cluster is similar to the environment of the early Universe, with low abundances of elements heavier than hydrogen and helium. The existence of dark clouds of dense dust and the fact that the cluster is rich in ionised gas also suggest that star formation is taking place. Together with its associated HII [1] region N90, which contains clouds of ionised atomic hydrogen, this cluster provides a valuable opportunity to examine how stars can form under dramatically different conditions from those in the solar neighbourhood.

An international team of astronomers, including Peter Zeidler, Elena Sabbi, Elena Manjavacas and Antonella Nota, used Webb to observe NGC 602 and uncover candidates for the first young brown dwarfs outside our Milky Way.

“Only thanks to the incredible sensitivity and resolution in the right wavelength range we are able to detect these objects at such great distances,” shared lead author Peter Zeidler of AURA/STScI for the European Space Agency. “This has never been possible before and also will remain impossible with telescopes on the ground for the foreseeable future.”

Brown dwarfs are the more massive cousins of giant gas planets (typically ranging from roughly 13 to 75 Jupiter masses, and sometimes lower). They are free-floating, meaning that they are not gravitationally bound to a star as exoplanets are. However, some of them share characteristics with exoplanets, like their atmospheric composition and storm patterns.

“Until now, we’ve known of about 3000 brown dwarfs, but they all live inside our own galaxy,” added team member Elena Manjavacas of AURA/STScI for the European Space Agency.

“This discovery highlights the power of using both Hubble and Webb to study young stellar clusters,” explained team member Antonella Nota, executive director of the International Space Science Institute in Switzerland and the previous Webb Project Scientist for ESA. “Hubble showed that NGC602 harbors very young low mass stars, but only with Webb we can finally see the extent and the significance of the substellar mass formation in this cluster. Hubble and Webb are an amazingly powerful telescope duo!”

“Brown dwarfs seem to form in the same way as stars, they just don’t capture enough mass to become a fully fledged star. Our results fit well with this theory,” remarked Peter.

The team’s data include a new image from Webb’s Near-InfraRed Camera (NIRCam) of NGC 602. In this image, the cluster stars, the young stellar objects, and the surrounding gas and dust ridges are visible. It also shows a large number of background galaxies and other stars in the Small Magellanic Cloud. These observations were made in April 2023.

“These are the first giant exoplanet analogues outside the Milky Way,” added Elena. “We need to be ready for ground-breaking discoveries in these new objects!”

These observations were made as part of the JWST GO programme #2662 (PI: P. Zeidler). The results have been published in The Astrophysical Journal.