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 of WASP-12b and the absorption features detected near 1.6 and 8 μm 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 μm, while CH₄ does not.