Synthetic transit spectra of GJ 3090 b for different (hypothetical) atmospheric compositions. (a) Transit spectra for cloud-free atmospheres (30× solar metallicity, 1000x solar metallicity, and pure water vapour). (b) Transit spectra for cloudy atmospheres with 1000× solar metallicity with cloud layers at different altitudes (10⁻², 10⁻³, 10⁻⁴ bar, and cloud-free). (c, d) Two levels of zoom into the water-absorption lines (in the 1.4 µm water-absorption band) at high spectral resolution for a 1000× solar metallicity atmosphere with a cloud top at 10⁻³ bar. The spectral ranges of TESS, HST/WFC3, JWST/NIRSPEC, and JWST/MIRI have been added for reference.