Spitzer observations of the thermal emission from WASP-43b

WASP-43b is one of the closest-orbiting hot Jupiters, with a semimajor axis of a = 0.01526 ± 0.00018 AU and a period of only 0.81 days.However, it orbits one of the coolest stars with a hot Jupiter (T* = 4520 ± 120 K), giving the planet a modest equilibrium temperature of T_eq = 1440 ± 40 K,assuming zero Bond albedo and uniform planetary energy redistribution.The eclipse depths and brightness temperatures from our jointly fit model are 0.347% ± 0.013% and 1670 ± 23 K at 3.6 μm and0.382% ± 0.015% and 1514 ± 25 K at 4.5 μm. The eclipse timings improved the estimate of the orbital period, P, by a factor of three (P = 0.81347436 ± 1.4 × 10^-7 days) and put an upper limit on the eccentricity (e =0.010^+0.010_-0.007). We use our Spitzer eclipse depths along with four previously reported ground-based photometric observations in the near-infrared to constrain the atmospheric properties of WASP-43b. The data rule out a strong thermal inversion in the dayside atmosphere of WASP-43b. Model atmospheres with no thermal inversions and fiducial oxygen-rich compositions are able to explain all the available data.However, a wide range of metallicities and C/O ratios can explain the data. The data suggest low day-night energy redistribution in the planet, consistent with previous studies, with a nominal upper limit of about 35% for the fraction of energy incident on the dayside that is redistributed to the nightside.