  The recent inference of a carbon-rich atmosphere, with C/O >= 1, in the hot
Jupiter WASP-12b motivates the exotic new class of carbon-rich planets (CRPs).
We report a detailed study of the atmospheric chemistry and spectroscopic
signatures of carbon-rich giant planets (CRGs) and the compositions of icy
planetesimals required for their formation, and the apportionment of ices,
rock, and volatiles in their envelopes. For C/O >= 1, most of the atmospheric
oxygen is occupied by CO for T > 1400 K and pressure (P) < 1 bar, causing a
substantial depletion in H2O, and an overabundance of CH4 compared to those
obtained by assuming solar abundances (C/O = 0.54) in chemical equilibrium.
These differences in chemistry cause distinctly observable signatures in
spectra. We also find that a C/O >= 1 strongly depletes the abundances of TiO
and VO available to form thermal inversions, which is adequate to rule out
thermal inversions due to TiO/VO even in the most highly irradiated hot
Jupiters, such as WASP-12b. Adopting stellar abundances (C/O = 0.44) for the
primordial disk composition and low-temperature formation conditions (T <= 30
K) for WASP-12b leads to a C/O ratio of 0.27 in accreted planetesimals, and,
consequently, in the planet's envelope. In contrast, a C/O ratio of 1 in the
envelope of WASP-12b requires a substantial depletion of oxygen in the disk, by
a factor of ~0.41 for the same formation conditions. This scenario also
satisfies the constraints on the C/H and O/H ratios reported for WASP-12b. If,
alternatively, hotter conditions prevailed in a stellar composition disk such
that only H2O is condensed, the remaining gas can potentially have a C/O ~ 1.
However, a high C/O in WASP-12b caused predominantly by gas accretion would
preclude super-stellar C/H ratios which also fit the data.

Earth and Planetary Astrophysics