Energetic Particle Impact on the Chemistry of Exoplanet Atmospheres

Exoplanet atmospheres are shaped by the high-energy environments of their host stars. Beyond XUV photons, energetic particles (EPs)—Galactic cosmic rays (GCRs) and stellar energetic particles (SEPs) from flares and CMEs—drive ionisation, heating, and secondary radiation that alter chemistry and observables. With JWST now delivering the first hints of photochemical disequilibrium, robust interpretation requires time-resolved, star-specific EP forcing, which is largely absent from current models.EPICEA will deliver the first end-to-end method linking stellar EP generation to exoplanet spectra. WP1 computes time- and energy-resolved SEP+GCR spectra at the planets magnetosphere boundary, using flare frequency distributions, CME statistics, and Parker-transport modelling across stellar mass, age, and activity sample. WP2 couples these fluxes to wind–magnetosphere simulations and field-aligned transport, producing precipitation/ionisation maps and multi-band auroral synthetic spectra (radio, UV, IR). WP3 propagates these inputs through photochemistry (STAND) and radiative transfer (petitRADTRANS) codes, quantifying disequilibrium tracers (e.g. HCN, C_2H_2, HNO_3, SO_2) and generating synthetic spectra.The result will be a public library of datasets: fluxes, ionisation maps, chemistry responses, and spectral templates, tailored for JWST and Ariel. By explicitly including stellar variability and EP forcing, EPICEA aims to advance beyond scaled-solar approximations, opening new pathways to constrain star–planet interactions, the atmospheric evolution of sub-Neptunes, and the robustness of biosignatures of exoplanets around active stars.