Thermal emission at 4.5 and 8μm of WASP-17b, an extremely large planet in a slightly eccentric orbit

We report the detection of thermal emission at 4.5 and 8μm from the planet WASP-17b. We used Spitzer to measure the system brightness at each wavelength during two occultations of the planet by its host star. By combining the resulting light curves with existing transit light curves and radial-velocity measurements in a simultaneous analysis, we find the radius of WASP-17b to be 2.0R_Jup, which is 0.2R_Jup larger than any other known planet and 0.7R_Jup larger than predicted by the standard cooling theory of irradiated gas giant planets. We find the retrograde orbit of WASP-17b to be slightly eccentric, with 0.0012 < e < 0.070 (3σ). Such a low eccentricity suggests that, under current models, tidal heating alone could not have bloated the planet to its current size, so the radius of WASP-17b is currently unexplained. From the measured planet-star flux-density ratios we infer 4.5 and 8μm brightness temperatures of 1881 ± 50 and 1580 ± 150K, respectively, consistent with a low-albedo planet that efficiently redistributes heat from its day side to its night side.