15 NH 3 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 in them are nearly identical 1,2 . Understanding the formation of gas giant planets is challenging, often involving the endeav our to link atmospheric abundance ratios, such as the carbon-to-oxygen (C/O) ratio, to for mation scenarios 3 . However, the complexity of planet formation requires additional tracers, as the unambiguous interpretation of the measured C/O ratio is fraught with complexity 4 . Isotope ratios, such as deuterium-to-hydrogen and 14 N/ 15 N, offer a promising avenue to gain further insight into this formation process, mirroring their utility within the solar system 5,6,7 . For exoplanets only a handful of constraints on 12 C/ 13 C exist, pointing to the accretion of 13 C rich ice from beyond the disks' CO iceline 8,9 . Here we report on the mid-infrared detection of the 14 NH 3 and 15 NH 3 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 of the JamesWebb Space Telescope. As expected, our results reveal a 14 N/ 15 N value consistent with star-like formation by gravitational collapse, demonstrating that this ratio can be accurately constrained. Since young stars and their planets should be more strongly enriched in the 15 N isotope 10 , we expect that 15 NH 3 will be detectable in a number of cold, wide-separation exoplanets.