15NH3 in the atmosphere of a cool brown dwarf

Brown dwarfs serve as ideal laboratories for studying the atmospheres of giant exoplanets on wide orbits, as the governing physical and chemical processes within them are nearly identical(1,2). Understanding the formation of gas-giant planets is challenging, often involving the endeavour to link atmospheric abundance ratios, such as the carbon-to-oxygen (C/O) ratio, to formation scenarios(3). However, the complexity of planet formation requires further tracers, as the unambiguous interpretation of the measured C/O ratio is fraught with complexity(4). Isotope ratios, such as deuterium to hydrogen and N-14/N-15, offer a promising avenue to gain further insight into this formation process, mirroring their use within the Solar System(5-7). For exoplanets, only a handful of constraints on C-12/C-13 exist, pointing to the accretion of C-13-rich ice from beyond the CO iceline of the disks(8,9). Here we report on the mid-infrared detection of the (NH3)-N-14 and (NH3)-N-15 isotopologues in the atmosphere of a cool brown dwarf with an effective temperature of 380 K in a spectrum taken with the Mid-Infrared Instrument (MIRI) of JWST. As expected, our results reveal a N-14/N-15 value consistent with star-like formation by gravitational collapse, demonstrating that this ratio can be accurately constrained. Because young stars and their planets should be more strongly enriched in the N-15 isotope(10), we expect that (NH3)-N-15 will be detectable in several cold, wide-separation exoplanets.