Disequilibrium Chemistry in Exoplanet Atmospheres observed with the Hubble Space Telescope

Literature on the theory of exoplanet atmospheric disequilibrium chemistry is rich, although its ob- servational counterpart has yet to emerge beyond the hints provided by a few targets in dedicated studies. We report results from an uniform data reduction and analysis for a catalog of 62 Hubble Space Telescope exoplanet transit spectra where we assess the atmospheric model preference for dise- quilibrium chemistry (i.e. water vapor is not the dominant absorption spectral signature) over Thermal Equilibrium Chemistry in a comparative planetology context. Where model preference assessment is possible, we find that disequilibrium occurs in about half of the atmospheres, indicating that disequi- librium processes play an important role in the composition of exoplanet atmospheres. While very hot atmospheres, over 1800K, prefer equilibrium chemistry, we find a clustering of preference for disequi- librium in the 1200K - 1800K temperature range. We suggest that UV-augmented thermochemistry may play a significant role for those atmospheres.