2009ApJ...692..229N

We present Spitzer 8 µm 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 R_p/R _{*}= 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 and a planet radius of R_p= 0.755±0.040 R_J. These measurements further support models in which the planet is greatly enriched in heavy elements.