A PHOTOCHEMICAL MODEL FOR THE CARBON-RICH PLANET WASP-12b

The hot-Jupiter WASP-12b is a heavily irradiated exoplanet in a short-period orbit around a G0-star with twice the metallicity of the Sun. A recent thermochemical equilibrium analysis based on Spitzer and ground-based infrared observations suggests that the presence of CH4 in its atmosphere and the lack of H2O features can only be explained if the carbon-to-oxygen ratio in the planet's atmosphere is much greater than the solar ratio ([C]/[O] = 0.54). Here, we use a one-dimensional photochemical model to study the effect of disequilibrium chemistry on the observed abundances of H2O, CO, CO2, and CH4 in the WASP-12b atmosphere. We consider two cases: one with solar [C]/[O] and another with [C]/[O] = 1.08. The solar case predicts that H2O and CO are more abundant than CO2 and CH4, as expected, whereas the high [C]/[O] model shows that CO, C2H2, and HCN are more abundant. This indicates that the extra carbon from the high [C]/[O] model is in hydrocarbon species. H2O photolysis is the dominant disequilibrium mechanism that alters the chemistry at higher altitudes in the solar [C]/[O] case, whereas photodissociation of C2H2 and HCN is significant in the super-solar case. Furthermore, our analysis indicates that C2H2 is the major absorber in the atmosphere ofWASP-12b and the absorption features detected near 1.6 and 8 mu m may be arising from C2H2 rather than CH4. The Hubble Space Telescope's WFC3 can resolve this discrepancy, as C2H2 has absorption between 1.51 and 1.54 mu m, while CH4 does not.