We report the detection of thermal emission at 4.5 and 8 µm 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 2.0 R _ { Jup } , which is 0.2 R _ { Jup } larger than any other known planet and 0.7 R _ { 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 ( 3 \sigma ) .
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 8 µm brightness temperatures of 1881 \pm 50 K and 1580 \pm 150 K , respectively , consistent with a low-albedo planet that efficiently redistributes heat from its day side to its night side .