5
Jan 11

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

Source: Nature


Observations and model spectra for dayside thermal emission of WASP-12b.

Image credits: Nikku Madhusudhan, Joseph Harrington,Kevin B. Stevenson, Sarah Nymeyer, Christopher J. Campo, Peter J. Wheatley, Drake Deming,Jasmina Blecic, Ryan A. Hardy, Nate B. Lust, David R. Anderson, Andrew Collier-Cameron, Christopher B. T. Britt,William C. Bowman,Leslie Hebb, Coel Hellier, Pierre F. L. Maxted, Richard G. West, Don Pollacco

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 System. The solar C/O is 0.54 . Here we report an analysis of dayside multi-wavelength photometry of the transiting hot-Jupiter WASP-12b 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 atmosphere. (read Nature)

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