The effect of Lyman alpha 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 alpha line (Ly alpha) at 1215.67 angstrom, the brightest far-UV emission line. We show that the effect of incoming Lya 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 Lya radiation. H2O absorbs most of this radiation, thereby shielding CH4, whose dissociation is driven mainly by radiation at other far-UV wavelengths (similar to 1300 angstrom). H2O photolysis also affects other species in the atmosphere, including H, H-2, 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.