The thermal emission of the exoplanet WASP-3b

We report the detection of thermal emission from the transiting hot Jupiter WASP-3b at 3.6, 4.5 and 8.0 μm using the Spitzer Space Telescope. We obtain planet-to-star flux ratios of 0.209^{+0.040}_{-0.028}, 0.282 ± 0.012 and 0.328^{+0.086}_{-0.055} per cent at these wavelengths, respectively, implying infrared brightness temperatures of T_{3.6 μ m}=2280^{+210}_{-150} K, T_{4.5 μm} = 2400 ± 80 K and T_{8.0 μ m}=2210^{+390}_{-250} K. We find that WASP-3b falls into an emerging class of highly irradiated planets whose measured temperatures suggest that the planets are dark and redistribute heat around the planet inefficiently. The latter is similarly concluded from 1D atmospheric model comparisons, which also favour the presence of an atmospheric temperature inversion. We compare the WASP-3 system to the proposed inversion-activity relation, finding that it hints at a more complex relation than a simple cut-off in activity implied by previous data. Using eclipse timings we also constrain e cos ω to be -0.0006^{+0.0010}_{-0.0006}, suggesting that the eccentricity of WASP-3b can only be large for a narrow range of ω.