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. At 4.5 μm, on the other hand, we detect ellipsoidal variations that are much stronger than predicted. We do not detect ellipsoidal variations at 3.6 μm, but our parameter uncertainties-estimated via prayer-bead Monte Carlo-keep this non-detection consistent with model predictions. 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 μm, respectively. The transit depths in the mid-infrared-( R p/ R *) 2 = 0.0123(3) and 0.0111(3) at 3.6 and 4.5 μm, respectively-indicate that the atmospheric opacity is greater at 3.6 than at 4.5 μm, in disagreement with model predictions, irrespective of C/O ratio. The large-amplitude phase variations, combined with the planet's previously measured dayside spectral energy distribution, are indicative of non-zero Bond albedo and very poor day–night heat redistribution. We are able to measure the transit depths, eclipse depths, thermal and ellipsoidal phase variations at both wavelengths. This extremely inflated hot Jupiter is thought to be overflowing its Roche lobe, undergoing mass loss and accretion onto its host star, and has been claimed to have a C/O ratio in excess of unity. We report Warm Spitzer full-orbit phase observations of WASP-12b at 3.6 and 4.5 μm.

Phase 10 variations