A Precise Estimate of the Radius of HD 149026b
  We present Spitzer 8 micron transit observations of the extrasolar planet system HD 149026. 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_s = 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 deg. and a planet radius of 0.755 +/- 0.040 Jupiter radii. These measurements further support models in which the planet is greatly enriched in heavy elements. 
Philip Nutzman, David Charbonneau, Joshua N. Winn, Heather A. Knutson,   Jonathan J. Fortney, Matthew J. Holman, Eric Agol