Emission spectra simulated for various existing lava planets, K2-141-b (green), 55-Cnc-e (blue), TOI-431-b (purple) and HD 213885-b (orange), assuming constant composition (TERRA, Table 2) and their corresponding stellar spectra (Zilinskas et al. 2022). Two diagnostic features for fO₂ determination are highlighted: MgO at 6 μm and SiO at 9 μm. The contrast between the fluxes of the respective bins are shown on the right, for each planet, respectively, once as the contrast expected from a perfect observation (solid coloured lines) and the respective standard deviations (1σ) for various number of observed transits with MIRI (black, simulated with pandexo Batalha et al. 2017).