Isolated brown dwarfs are some of the rarest and hardest to study objects in our stellar neighborhood. Recently, astronomers revealed that one of the most famous isolated brown dwarfs, W1935, is not actually one brown dwarf but two.
Challenging but Fascinating
Astronomers know of only 50 free-floating brown dwarfs within the nearest 200 light-years or so, and even the ones we know about are poorly understood. Since brown dwarfs are frigid compared to their stellar cousins, they consequently shine much less brightly and at longer wavelengths of light. Even taking a picture of these ~500K objects requires long stares with specialized telescopes, and these facilities already have their schedules full supporting a wide range of other areas of research from distant galaxies to exploding supernovae.
An image of Jupiter and its prominent aurora taken with the Hubble Space Telescope. [NASA, ESA, and J. Nichols (University of Leicester); Acknowledgment: A. Simon (NASA/GSFC) and the OPAL team]
Recently, JWST continued its pattern of discovery: the telescope took another look at the brown dwarf W1935, where methane emission was detected for the first time, and again found something surprising.
A New Binary
When JWST first took a look at this brown dwarf, it used its Near Infrared Spectrograph (NIRSpec) to record a spectrum of the object. These data revealed a large spike near 3 microns that perfectly lined up with methane emission from an aurora, similar to what we observe on Jupiter in the solar system. This was a thrilling find, since it hinted at an exciting possibility: Jupiter’s aurora is powered by its nearby moon Io, so the researchers cautiously wondered if W1935’s aurora could be evidence of a companion satellite as well.
Images of W1935 taken with MIRI (left column), the author’s model for those images (center column), and the residuals (right column). The model includes two nearby point sources that are so close together they appear as one slightly elongated source. Click to enlarge. [De Furio et al. 2025]
Doubly Interesting
This makes W1935 just the second-ever known Y+Y dwarf binary and cements its status as one of the most interesting brown dwarf systems discovered to date. So far, it’s not clear which object is responsible for the auroral signal since the objects are so close together that their images blur together without careful modeling and image processing.
De Furio’s team pointed out that JWST has yet another instrument that could prove useful here: its Integral Field Unit spectrometer within NIRSpec. Should JWST revisit this fascinating system, it could study the aurora’s variability over time, pin it to one specific object within the binary, and hopefully shed some light on whether any moons play a role in the methane emission. Given all we have learned each time JWST has turned to these strange worlds, we can only hope that it will do so again soon.
Citation
“Discovery of the Second Y+Y Dwarf Binary System: CWISEP J193518.59-154620.3,” Matthew De Furio et al 2025 ApJL 990 L63. doi:10.3847/2041-8213/adfee1