Evidence of ¹³CO absorption in PSO J318’s atmosphere. Left panel: best-fit spectrum of the winning pRT retrieval model (amorphous spherical SiO particles) post-processed to high spectral resolution (R = 3200, red solid line) and superimposed on the NIRSpec G395H NRS2 data of PSO J318 at full spectral resolution (black solid line). For comparison, pRT’s best-fit model for PSO J318 is also superimposed on a scaled version of VHS 1256b’s G395H spectrum from Miles et al. (2023) (blue solid line). Right panel: zoomed-in version of the left panel showing the data for PSO J318 as well as the pRT best-fit model, where in one case (cyan solid line) the ¹³CO opacity has been turned off in post-processing.