The recent inference of a carbon-rich atmosphere , with C/O \geq 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 ) , the possibility of thermal inversions in their atmospheres , the compositions of icy planetesimals required for their formation via core accretion , and the apportionment of ices , rock , and volatiles in their envelopes .
Our results show that CRG atmospheres probe a unique region in composition space , especially at high temperature ( T ) .
For atmospheres with C/O \geq 1 , and T \gtrsim 1400 K in the observable atmosphere , most of the oxygen is bound up in CO , while H _ { 2 } O is depleted and CH _ { 4 } is enhanced by up to two or three orders of magnitude each , compared to equilibrium compositions with solar abundances ( C/O = 0.54 ) .
These differences in the spectroscopically dominant species for the different C/O ratios cause equally distinct observable signatures in the spectra .
As such , highly irradiated transiting giant exoplanets form ideal candidates to estimate atmospheric C/O ratios and to search for CRPs .
We also find that the C/O ratio strongly affects the abundances of TiO and VO , which have been suggested to cause thermal inversions in highly irradiated hot Jupiter atmospheres .
A C/O = 1 yields TiO and VO abundances of \sim 100 times lower than those obtained with equilibrium chemistry assuming solar abundances , at P \sim 1 bar .
Such a depletion is adequate to rule out thermal inversions due to TiO/VO even in the most highly irradiated hot Jupiters , such as WASP-12b .
We estimate the compositions of the protoplanetary disk , the planetesimals , and the envelope of WASP-12b , and the mass of ices dissolved in the envelope , based on the observed atmospheric abundances .
Adopting stellar abundances ( C/O = 0.44 ) for the primordial disk composition and low-temperature formation conditions ( T \lesssim 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 , i.e .
by a factor of \sim 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 H _ { 2 } O is condensed , the remaining gas can potentially have a C/O \sim 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 .