2025AJ....169..260D

We present analyses of Spitzer InfraRed Array Camera (IRAC) 3.6 μm and 4.5 μm phase-curve observations of hot Jupiters WASP-77Ab and WASP-121b. For WASP-121b, we find amplitudes of 1771 ± 95 ppm (3.6 μm) and 2048 ± 109 ppm (4.5 μm), and near-zero offsets of -0$\mathop{.}\limits^{\unicode{x000B0}}$78 ± 1$\mathop{.}\limits^{\unicode{x000B0}}$87 (3.6 μm) and 0$\mathop{.}\limits^{\unicode{x000B0}}$42 ± 1$\mathop{.}\limits^{\unicode{x000B0}}$74 (4.5 μm), consistent within 2.2σ and 1.3σ, respectively, with JWST Near Infrared Spectrograph results. For WASP-77Ab, we find amplitudes of 535 ± 52 ppm (3.6 μm) and 919 ± 40 ppm (4.5 μm), and offsets of 33$\mathop{.}\limits^{\unicode{x000B0}}$45 ± 2$\mathop{.}\limits^{\unicode{x000B0}}$79 (3.6 μm) and 16$\mathop{.}\limits^{\unicode{x000B0}}$28 ± 2$\mathop{.}\limits^{\unicode{x000B0}}$52 (4.5 μm). We report day- and nightside brightness temperatures: for WASP-121b, 2779 ± 40 K (3.6 μm) and 2905 ± 51 K (4.5 μm) (day) and 1259 ± 67 K (3.6 μm) and 1349 ± 54 K (4.5 μm) (night); for WASP-77Ab, 1876 ± 23 K (3.6 μm) and 1780 ± 25 K (day) and 1501 ± 22 K (3.6 μm) and 1234 ± 20 K (4.5 μm) (night). Comparing WASP-121b data to general circulation models, we find evidence for drag inhibiting day-to-night heat transfer, which our model reproduces using magnetic circulation. Further, comparing both planets' data to energy balance models, we show suppressed circulation in WASP-121b and potential evidence for an unusually high Bond albedo in WASP-77Ab. We add both planets to the Spitzer population study of previously identified trends in offset versus orbital period, finding that a positive trend is weakened, but not eliminated, by including these planets.