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.


  1. 1

    Bond, J. C., O’Brien, D. P. & Lauretta, D. S. The compositional diversity of extrasolar planets. I. In situ simulations. Astrophys. J. 715, 1050–1070 (2010)

  2. 2

    Kuchner, M. & Seager, S. Extrasolar carbon planets. Preprint at 〈http://arxiv.org/abs/astro-ph/0504214〉 (2005)

  3. 3

    Asplund, M., Grevesse, N. & Sauval, A. in. Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis (eds Barnes, T. G. III & Bash, F. N.) 25–38 (ASP Conf. Ser. 336, 2005)

  4. 4

    Campo, C. et al. On the orbit of exoplanet WASP-12b. Astrophys. J. (in the press). Preprint at 〈http://arxiv.org/abs/1003.2763〉 (2010)

  5. 5

    Croll, B. et al. Near-infrared thermal emission from WASP-12b: detections of the secondary eclipse in Ks, H & J. Astrophys. J. (in the press). Preprint at 〈http://arxiv.org/abs/1009.0071〉 (2010)

  6. 6

    Hebb, L. et al. 2009, WASP-12b: the hottest transiting extrasolar planet yet discovered. Astrophys. J. 693, 1920–1928 (2009)

  7. 7

    Fortney, J. J., Lodders, K., Marley, M. S. & Freedman, R. S. A unified theory for the atmospheres of the hot and very hot Jupiters: two classes of irradiated atmospheres. Astrophys. J. 678, 1419–1435 (2008)

  8. 8

    Knutson, H. A., Howard, A. W. & Isaacson, H. A correlation between stellar activity and hot Jupiter emission spectra. Astrophys. J. 720, 1569–1576 (2010)

  9. 9

    Zahnle, K. et al. Atmospheric sulfur photochemistry on hot Jupiters. Astrophys. J. 701, L20–L24 (2009)

  10. 10

    Werner, M. W. et al. The Spitzer Space Telescope mission. Astrophys. J. Suppl. Ser. 154, 1–9 (2004)

  11. 11

    Madhusudhan, N. & Seager, S. A temperature and abundance retrieval method for exoplanet atmospheres. Astrophys. J. 707, 24–39 (2009)

  12. 12

    Burrows, A., Budaj, J. & Hubeny, I. Theoretical spectra and light curves of close-in extrasolar giant planets and comparison with data. Astrophys. J. 678, 1436–1457 (2008)

  13. 13

    Gilks, W. R., Richardson, S. & Spiegelhalter, D. J. (eds) Markov Chain Monte Carlo in Practice (Chapman & Hall, 1996)

  14. 14

    Swain, M. R. et al. Molecular signatures in the near-infrared dayside spectrum of HD 189733b. Astrophys. J. 690, L114–L117 (2009)

  15. 15

    Lodders, K. & Fegley, B. Atmospheric chemistry in giant planets, brown dwarfs, and low-mass dwarf stars. I. Carbon, nitrogen, and oxygen. Icarus 155, 393–424 (2002)

  16. 16

    Burrows, A. & Sharp, C. M. Chemical equilibrium abundances in brown dwarf and extrasolar giant planet atmospheres. Astrophys. J. 512, 843–863 (1999)

  17. 17

    Spiegel, D. S., Silverio, K. & Burrows, A. Can TiO explain thermal inversions in the upper atmospheres of irradiated giant planets? Astrophys. J. 699, 1487–1500 (2009)

  18. 18

    Atreya, S. K. & Wong, A. S. Coupled clouds and chemistry of the giant planets—a case for multiprobes. Space Sci. Rev. 116, 121–136 (2005)

  19. 19

    Owen, T. et al. A low-temperature origin for the planetesimals that formed Jupiter. Nature 402, 269–270 (1999)

  20. 20

    Seager, S. et al. On the dayside thermal emission of hot Jupiters. Astrophys. J. 632, 1122–1131 (2005)

  21. 21

    Yung, Y. & DeMore, W. B. Photochemistry of Planetary Atmospheres (Oxford University Press, 1999)

  22. 22

    Hubeny, I. & Burrows, A. &. Sudarsky, D. A possible bifurcation in atmospheres of strongly irradiated stars and planets. Astrophys. J. 594, 1011–1018 (2003)

  23. 23

    Fossati, L. et al. A detailed spectropolarimetric analysis of the planet-hosting star WASP-12. Astrophys. J. 720, 872–886 (2010)

  24. 24

    Pollack, J. B. et al. Formation of the giant planets by concurrent accretion of solids and gas. Icarus 124, 62–85 (1996)

  25. 25

    Lodders, K. Jupiter formed with more tar than ice. Astrophys. J. 611, 587–597 (2004)

  26. 26

    Fortney, J. J. & Marley, M. S. &. Barnes, J. W. Planetary radii across five orders of magnitude in mass and stellar insolation: application to transits. Astrophys. J. 659, 1661–1672 (2007)

  27. 27

    Line, M. R., Liang, M. C. & Yung, Y. L. High-temperature photochemistry in the atmosphere of HD 189733b. Astrophys. J. 717, 496–502 (2010)

  28. 28

    Cushing, M. C., Rayner, J. T. & Vacca, W. D. An infrared spectroscopic sequence of M, L, and T dwarfs. Astrophys. J. 623, 1115–1140 (2005)

  29. 29

    Castelli, F. & Kurucz, R. L. New grids of ATLAS9 model atmospheres. Preprint at 〈http://arxiv.org/abs/astro-ph/0405087〉 (2004)

  30. 30

    Lopez-Morales, M. et al. Day-side z′-band emission and eccentricity of WASP-12b. Astrophys. J. 716, L36–L40 (2010)

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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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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.

Correspondence to Nikku Madhusudhan.

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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). https://doi.org/10.1038/nature09602

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