  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 a silicate-dominated
composition as found on Earth, and the atmospheres 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 vapor and enhanced in methane by over two orders of magnitude
each 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 a 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.
