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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 amountsof UV radiation. Many M stars emit strong chromospheric emission in the H ILyman $\alpha$ line (Ly$\alpha$) at 1215.67 \AA, the brightest far-UV emissionline. We show that the effect of incoming Ly$\alpha$ flux can significantlychange the photochemistry of mini-Neptunes' atmospheres. We use GJ 436b as anexample, considering different metallicities for its atmospheric composition.For solar composition, H$_2$O-mixing ratios show the largest change because ofLy$\alpha$ radiation. H$_2$O absorbs most of this radiation, thereby shieldingCH$_4$, whose dissociation is driven mainly by radiation at other far-UVwavelengths ($\sim1300$ \AA). H$_2$O photolysis also affects other species inthe atmosphere, including H, H$_2$, CO$_2$, CO, OH and O. For an atmospherewith high metallicity, H$_2$O- and CO$_2$-mixing ratios show the biggestchange, thereby shielding CH$_4$. Direct measurements of the UV flux of thehost 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 observablespectral features.

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