Model Photochemistry of WASP-12 b


An outline of the paper:  A Photochemical Model for the Carbon-Rich Planet WASP-12 b  (R. Kopparapu et al., 2011)

WASP-12 b is an unusual hot Jupiter in that it has a very short period of about 1 day and it orbits a star with twice solar metallicity. It is also one of the most irradiated known exoplanets. Recent spectral observations with Spitzer have detected the strong presence of methane and a lack of water features (Madhusudhan et al. 2011). Such observations suggest a high carbon to oxygen ratio.

Kopparapu et al. (2011) use a 1D model to study how abundances of molecules containing oxygen and carbon are affected by photochemical disequilibrium chemistry. The authors consider both a solar and a twice solar C/O ratio.  

  • For a solar C/O ratio, most available oxygen is contained in water and carbon monoxide. Both species remain at equilibrium values until below 10^{-5} bar pressure, where water is photolyzed. This process drives the departures from equilibrium chemistry. Water photolysis produces OH molecules, which aid in the production of carbon dioxide, decreasing the water abundance compared to CO and CO_{2} in the upper atmosphere.

    Volume mixing ratios as a function of pressure for a solar C/O ratio.

  • For a C/O ratio twice solar, the whole atmosphere is depleted in water and enhanced in methane and carbon monoxide, confirming the hypotheses drawn from the observations. The switchover between dominance from water and CO to methane and CO happens very abruptly at [C]/[O]=1. Low water abundance means that OH production is negligible and reactions producing C_{2}H_{2} and HCN become more important in both equilibrium and photochemical regions. C_{2}H_{2} is more abundant than either water or methane and so is photolyzed more rapidly than these molecules. It also has a larger optical depth and a feature that overlaps with the methane band at 7-8 microns, and so needs to be taken into account in observations.

    Volume mixing ratios as a function of pressure for a twice solar C/O ratio.

Overall, this work confirms that a high C/O ratio ([C]/[O] \ge 1) is required to explain the observed spectrum of WASP-12 b measured by Madhusudhan et al. (2011). They also find that photochemistry is important to consider below pressures of 10^{-3}-10^{-4} bar, although it could penetrate to higher pressures if the eddy diffusion coefficient is larger.

Despite having high metallicity, the host star has a solar C/O ratio. The authors discuss possible mechanisms for the difference between the planet and the star, including differences due to the position of the ice line relative to where the planet formed (also discussed by Öberg et al. (2011) and summarised here) and  differences in the primordial disc during the planet’s migration.

A final note on this work: it is interesting to model disequilibrium chemistry and non solar metallicity cases. However, Cowan et al. presented results at the Extreme Solar Systems II conference in Wyoming that show no evidence of CO and CO_{2} features in transmission at 3.6 and 4.5 microns in WASP-12 b. This casts doubt on the observed high C/O ratio for this planet. 

Further Reading:

A high C/O ratio and weak thermal inversion in the atmosphere of exoplanet WASP-12 b N. Madhusudhan et al. Observations of a high C/O ratio in WASP-12 b.
Near-infrared Thermal Emission from WASP-12 b: Detections of the Secondary Eclipse in Ks, H, and J  B. Croll et al. Observations of a high C/O ratio in WASP-12 b.

Feature image: Image courtesy of the Chronoscapes by Pery Burge.


About Author

I am undertaking an observational astronomy PhD focusing on the characterisation of planetary atmospheres, working with Dr. David Sing and Prof. Frédéric Pont. I am specialising in using transmission spectroscopy, applied to the hot Jupiter HD 189733b with HST. I am interested in obtaining a more complete picture for the few well-studied exoplanets, comparing observations in different regimes of planetary atmospheres, from secondary eclipse and transmission observations at different wavelengths, both from the ground and from space. (More)