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

N. Madhusudhan, J. Harrington, K.~B. Stevenson, S. Nymeyer, C.~J. Campo, P.~J. Wheatley, D. Deming, J. Blecic, R.~A. Hardy, N.~B. Lust, D.~R. Anderson, A. Collier-Cameron, C.~B.~T. Britt, W.~C. Bowman, L. Hebb, C. Hellier, P.~F.~L. Maxted, D. Pollacco, R.~G. West

Research output: Contribution to journalArticlepeer-review

245 Citations (Scopus)


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 Earth(1); the atmosphere can also differ from those in the Solar System(1,2). The solar C/O is 0.54 (ref. 3). Here we report an analysis of dayside multi-wavelength photometry(4,5) 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 atmospheres(7-9).
Original languageEnglish
Pages (from-to)64-67
Number of pages4
Issue number7328
Publication statusPublished - 6 Jan 2011


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