A Precise Estimate of the Radius of the Exoplanet HD 149026b from Spitzer Photometry

We present Spitzer 8 micron transit observations of the extrasolar planet HD 149026b. At this wavelength, transit light curves are weakly affected by stellar limb-darkening, allowing for a simpler and more accurate determination of planetary parameters. We measure a planet-star radius ratio of Rp/Rs = 0.05158 +/- 0.00077, and in combination with ground-based data and independent constraints on the stellar mass and radius, we derive an orbital inclination of i = 85.4 +0.9/-0.8 degrees and a planet radius of Rp = 0.755 +/- 0.040 R_jup. These measurements further support models in which the planet is greatly enriched in heavy elements.