  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. 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. However, the complexity of planet formation requires additional
tracers, as the unambiguous interpretation of the measured C/O ratio is fraught
with complexity. Isotope ratios, such as deuterium-to-hydrogen and 14N/15N,
offer a promising avenue to gain further insight into this formation process,
mirroring their utility within the solar system. For exoplanets only a handful
of constraints on 12C/13C exist, pointing to the accretion of 13C-rich ice from
beyond the disks' CO iceline. 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 of the James Webb Space Telescope. 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. Since young stars
and their planets should be more strongly enriched in the 15N isotope, we
expect that 15NH3 will be detectable in a number of cold, wide-separation
exoplanets.
