The effect of Lyman α radiation on mini-Neptune atmospheres around M stars: application to GJ 436b

Mini-Neptunes orbiting M stars are a growing population of known exoplanets. Some of them are located very close to their host star, receiving large amounts of UV radiation. Many M stars emit strong chromospheric emission in the H I Lyman α line (Lyα) at 1215.67 Å, the brightest far-UV emission line. We show that the effect of incoming Lyα flux can significantly change the photochemistry of mini-Neptunes' atmospheres. We use GJ 436b as an example, considering different metallicities for its atmospheric composition. For solar composition, H₂O-mixing ratios show the largest change because of Lyα radiation. H₂O absorbs most of this radiation, thereby shielding CH₄, whose dissociation is driven mainly by radiation at other far-UV wavelengths (∼1300 Å). H₂O photolysis also affects other species in the atmosphere, including H, H₂, CO₂, CO, OH and O. For an atmosphere with high metallicity, H₂O- and CO₂-mixing ratios show the biggest change, thereby shielding CH₄. Direct measurements of the UV flux of the host stars are important for understanding the photochemistry in exoplanets' atmospheres. This is crucial, especially in the region between 1 and 10^-6 bars, which is the part of the atmosphere that generates most of the observable spectral features.