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A high C/O ratio and weak thermal inversion in the atmosphere of exoplanet WASP-12b


The carbon-to-oxygen ratio (C/O) in a planet provides critical information about its primordial origins and subsequent evolution. A primordial C/O greater than 0.8 causes a carbide-dominated interior, as opposed to the silicate-dominated composition found on Earth1; the atmosphere can also differ from those in the Solar System1,2. The solar C/O is 0.54 (ref. 3). Here we report an analysis of dayside multi-wavelength photometry4,5 of the transiting hot-Jupiter WASP-12b (ref. 6) that reveals C/O ≥ 1 in its atmosphere. The atmosphere is abundant in CO. It is depleted in water vapour and enhanced in methane, each by more than two orders of magnitude compared to a solar-abundance chemical-equilibrium model at the expected temperatures. We also find that the extremely irradiated atmosphere (T > 2,500 K) of WASP-12b lacks a prominent thermal inversion (or stratosphere) and has very efficient day–night energy circulation. The absence of a strong thermal inversion is in stark contrast to theoretical predictions for the most highly irradiated hot-Jupiter atmospheres7,8,9.

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Figure 1: Observations and model spectra for dayside thermal emission of WASP-12b.
Figure 2: Constraints on the atmospheric composition of WASP-12b.
Figure 3: Thermal profiles of WASP-12b.


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We thank the authors of ref. 5 for sharing their ground-based observations before publication, and Thomas J. Loredo for discussions. N.M. thanks S. Seager for financial support during his stay at MIT, where most of the modelling work was carried out. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech.

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Authors and Affiliations



N.M. conducted the atmospheric modelling and wrote the paper with input on both from J.H.; J.H. and P.J.W. led the observing proposals, data from which have been interpreted in this work; J.H., J.B. and C.J.C. designed the observations with input from P.J.W., D.R.A., A.C.-C., L.H., C.H., P.F.L.M., D.P. and R.G.W.; J.H., K.B.S., S.N., C.J.C., D.D., J.B., R.A.H., N.B.L., D.R.A., A.C.-C., C.B.T.B. and W.C.B. analysed the Spitzer data.

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Correspondence to Nikku Madhusudhan.

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The authors declare no competing financial interests.

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Madhusudhan, N., Harrington, J., Stevenson, K. et al. A high C/O ratio and weak thermal inversion in the atmosphere of exoplanet WASP-12b. Nature 469, 64–67 (2011).

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