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We describe a detailed study on the properties of alkali atoms in extrasolargiant planets, and specifically focus on their role in generating theatmospheric free electron densities, as well as their impact on the transitdepth observations. We focus our study on the case of HD 209458 b, and we showthat photoionization produces a large electron density in the middle atmospherethat is about two orders of magnitude larger than the density anticipated fromthermal ionization. Our purely photochemical calculations though result in amuch larger transit depth for K than observed for this planet. This result doesnot change even if the roles of molecular chemistry and excited state chemistryare considered for the alkali atoms. In contrast, the model results for thecase of exoplanet XO-2 b are in good agreement with the available observations.Given these results we discuss other possible scenarios, such as changes in theelemental abundances, changes in the temperature profiles, and the possiblepresence of clouds, which could potentially explain the observed HD 209458 balkali properties. We find that most of these scenarios can not explain theobservations, with the exception of a heterogeneous source (i.e. clouds oraerosols) under specific conditions, but we also note the discrepancies amongthe available observations.

Electron densities and alkali atoms in exoplanet atmospheres