  Until recently, infrared observations of exoplanetary atmospheres have
typically been interpreted using models that assumed solar elemental
abundances. With the chemical composition fixed, attempts have been made to
classify hot Jupiter atmospheres on the basis of stellar irradiation. However,
recent observations have revealed deviations from predictions based on such
classification schemes, and chemical compositions retrieved from some datasets
have also indicated non-solar abundances. In hot hydrogen-dominated
atmospheres, the C/O ratio critically influences the relative concentrations of
several spectroscopically dominant molecules. Between a C/O of 0.5 (solar
value) and 2, the H2O and CH4 abundances can vary by several orders of
magnitude in the observable atmosphere, and new hydrocarbon species such as HCN
and C2H2 become prominent for C/O >= 1, while the CO abundance remains almost
unchanged. Furthermore, a C/O >= 1 can preclude a strong thermal inversion due
to TiO and VO in a hot Jupiter atmosphere, since TiO and VO are naturally
under-abundant for C/O >= 1. We, therefore, suggest a new two-dimensional
classification scheme for hydrogen-dominated exoplanetary atmospheres with
irradiation (or temperature) and C/O ratio as the two dimensions. We define
four 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/O
ratios of six hot Jupiters (XO-1b, CoRoT-2b, WASP-14b, WASP-19b, WASP-33b, and
WASP-12b) in the framework of our proposed 2D classification scheme. While the
data for several systems in our sample are consistent with carbon-rich
atmospheres (i.e. C/O >= 1), new observations are required to conclusively
constrain the C/O ratios in their atmospheres. We discuss how observations
using existing and forthcoming facilities can constrain C/O ratios in
exoplanetary atmospheres.
