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Until recently, infrared observations of exoplanetary atmospheres havetypically been interpreted using models that assumed solar elementalabundances. With the chemical composition fixed, attempts have been made toclassify hot Jupiter atmospheres on the basis of stellar irradiation. However,recent observations have revealed deviations from predictions based on suchclassification schemes, and chemical compositions retrieved from some datasetshave also indicated non-solar abundances. In hot hydrogen-dominatedatmospheres, the C/O ratio critically influences the relative concentrations ofseveral spectroscopically dominant molecules. Between a C/O of 0.5 (solarvalue) and 2, the H2O and CH4 abundances can vary by several orders ofmagnitude in the observable atmosphere, and new hydrocarbon species such as HCNand C2H2 become prominent for C/O >= 1, while the CO abundance remains almostunchanged. Furthermore, a C/O >= 1 can preclude a strong thermal inversion dueto TiO and VO in a hot Jupiter atmosphere, since TiO and VO are naturallyunder-abundant for C/O >= 1. We, therefore, suggest a new two-dimensionalclassification scheme for hydrogen-dominated exoplanetary atmospheres withirradiation (or temperature) and C/O ratio as the two dimensions. We definefour classes in this 2-D space (O1, O2, C1, and C2) with distinct chemical,thermal and spectral properties. We characterize the thermal structures and C/Oratios of six hot Jupiters (XO-1b, CoRoT-2b, WASP-14b, WASP-19b, WASP-33b, andWASP-12b) in the framework of our proposed 2D classification scheme. While thedata for several systems in our sample are consistent with carbon-richatmospheres (i.e. C/O >= 1), new observations are required to conclusivelyconstrain the C/O ratios in their atmospheres. We discuss how observationsusing existing and forthcoming facilities can constrain C/O ratios inexoplanetary atmospheres.

C/O ratio as a Dimension for Characterizing Exoplanetary Atmospheres