We report Warm Spitzer full-orbit phase observations of WASP-12b at 3.6 and 4.5 \mu m. This extremely inflated hot Jupiter is thought to be overflowing its Roche lobe , undergoing mass loss , accretion onto its host star , and has been claimed to have a C/O ratio in excess of unity .
We are able to measure the transit depths , eclipse depths , thermal and ellipsoidal phase variations at both wavelengths .
The large amplitude phase variations , combined with the planet ’ s previously-measured day-side spectral energy distribution , is indicative of non-zero Bond albedo and very poor day–night heat redistribution .
The transit depths in the mid-infrared — ( R _ { p } / R _ { * } ) ^ { 2 } = 0.0123 ( 3 ) and 0.0111 ( 3 ) at 3.6 and 4.5 \mu m , respectively— indicate that the atmospheric opacity is greater at 3.6 than at 4.5 \mu m , in disagreement with model predictions , irrespective of C/O ratio .
The secondary eclipse depths are consistent with previous studies : F _ { day } / F _ { * } = 0.0038 ( 4 ) and 0.0039 ( 3 ) at 3.6 and 4.5 \mu m , respectively .
We do not detect ellipsoidal variations at 3.6 \mu m , but our parameter uncertainties —estimated via prayer-bead Monte Carlo— keep this non-detection consistent with model predictions .
At 4.5 \mu m , on the other hand , we detect ellipsoidal variations that are much stronger than predicted .
If interpreted as a geometric effect due to the planet ’ s elongated shape , these variations imply a 3:2 ratio for the planet ’ s longest : shortest axes and a relatively bright day–night terminator .
If we instead presume that the 4.5 \mu m ellipsoidal variations are due to uncorrected systematic noise and we fix the amplitude of the variations to zero , the best fit 4.5 \mu m transit depth becomes commensurate with the 3.6 \mu m depth , within the uncertainties .
The relative transit depths are then consistent with a Solar composition and short scale height at the terminator .
Assuming zero ellipsoidal variations also yields a much deeper 4.5 \mu m eclipse depth , consistent with a Solar composition and modest temperature inversion .
We suggest future observations that could distinguish between these two scenarios .