Simulated spectra for the TRAPPIST-1 planet: Emitted (top: MIR, R = 400), reflected (middle: NIR, R = 400), and transmitted light (bottom: NIR-MIR, R = 200). Note. MIR features in emission are not just dependent on the abundance of spectrally active gases, but also the temperature structure of the atmosphere. Left panel: anoxic, abiotic scenario (T_surf = 275 K): CO flux of 10⁹ (black), 10¹⁰ (magenta), and 10¹² molecules cm⁻² s⁻¹ (blue), corresponding to lightning flash rates of ~2, ~35 and ~1500 times modern Earth’s, respectively. Right panel: oxic, biotic scenario (T_surf = 288 K): CO flux of 10⁸ (black) and 10¹¹ molecules cm⁻² s⁻¹ (blue), corresponding to lightning flash rates of ~0.9 and ~680 times modern Earth’s, respectively.