Spectral contributions of key molecular species in the 1-5 μm range. The different curves show individual contributions from different molecules to a nominal model transmission spectrum of TOI-270 d shown in black and denoted as Combined. The model assumes a mixing ratio of 10⁻² for CH₄, CO₂, and H₂O, 10⁻³ for C₂H₆ and CS₂, and 10⁻⁵ for all the other species, consistent with our retrieval estimates, and an isothermal temperature profile of 300 K. Each curve corresponds to a transmission spectrum with opacity contributions from a single molecule at a time, in addition to H₂-H₂ and H₂-He collision-induced absorption. The spectral ranges of NIRISS SOSS and NIRSpec G395H are indicated.