Best-fit spectrum with ¹⁵NH₃ included in blue, the best fit removing the ¹⁵NH₃ opacity in red and in black the data with error bars. Panel a: NH₃ feature observed by the MIRI/MRS spectrum of WISE 0855. Panel b: residuals of the fit with and without ¹⁵NH₃ in blue and red respectively. Panel c: ¹⁵NH₃ opacity used for the retrieval at 277 K and for the pressures of the clear retrieval with varying H₂O profile.