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 scenarios³. However, the complexity of planet formation requires further tracers, as the unambiguous interpretation of the measured C/O ratio is fraught with complexity⁴. Isotope ratios, such as deuterium to hydrogen and ¹⁴N/¹⁵N, 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 ¹²C/¹³C exist, pointing to the accretion of ¹³C-rich ice from beyond the CO iceline of the disks^8,9. Here we report on the mid-infrared detection of the ¹⁴NH₃ and ¹⁵NH₃ 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 ¹⁴N/¹⁵N 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 ¹⁵N isotope¹⁰, we expect that ¹⁵NH₃ will be detectable in several cold, wide-separation exoplanets.