Spitzer Phase Curve Constraints for WASP-43b at 3.6 and 4.5 μm

Previous measurements of heat redistribution efficiency (the ability to transport energy from a planet's highly irradiated dayside to its eternally dark nightside) show considerable variation between exoplanets. Theoretical models predict a positive correlation between heat redistribution efficiency and temperature for tidally locked planets; however, recent Hubble Space Telescope (HST) WASP-43b spectroscopic phase curve results are inconsistent with current predictions. Using the Spitzer Space Telescope, we obtained a total of three phase curve observations of WASP-43b (P = 0.813 days) at 3.6 and 4.5 μm. The first 3.6 μm visit exhibits spurious nightside emission that requires invoking unphysical conditions in our cloud-free atmospheric retrievals. The two other visits exhibit strong day–night contrasts that are consistent with the HST data. To reconcile the departure from theoretical predictions, WASP-43b would need to have a high-altitude, nightside cloud/haze layer blocking its thermal emission. Clouds/hazes could be produced within the planet's cool, nearly retrograde mid-latitude flows before dispersing across its nightside at high altitudes. Since mid-latitude flows only materialize in fast-rotating ($\lesssim 1$ day) planets, this may explain an observed trend connecting measured day–night contrast with planet rotation rate that matches all current Spitzer phase curve results. Combining independent planetary emission measurements from multiple phases, we obtain a precise dayside hemisphere H2O abundance ($2.5\times {10}^{-5}\mbox{--}1.1\times {10}^{-4}$ at 1σ confidence) and, assuming chemical equilibrium and a scaled solar abundance pattern, we derive a corresponding metallicity estimate that is consistent with being solar (0.4–1.7). Using the retrieved global CO+CO2 abundance under the same assumptions, we estimate a comparable metallicity of 0.3–1.7× solar. This is the first time that precise abundance and metallicity constraints have been determined from multiple molecular tracers for a transiting exoplanet.