Signatures of X-ray induced chemistry in exoplanetary atmospheres

The high-energy radiation from stars deeply impacts planetary atmospheres, affecting their chemistry and producing departures from chemical equilibrium through photochemical processes. While the upper layers are dominated by ionization provided by extreme ultraviolet radiation, the deeper layers are characterized by X-ray ionization and by energetic photoelectrons produced by photoionization events. These ionization processes give rise to a characteristic chemistry, enhancing the mixing ratio of several molecular species, such as hydrocarbons. As a consequence, we found that in the case of high levels of stellar activity, the ionization process produces spectral signatures, mainly due to C2H2 and HCN. These signatures are potentially observable with the aid of current and future instrumentation at high and low resolution.