A PRECISE ESTIMATE OF THE RADIUS OF THE EXOPLANET HD 149026b FROM SPITZER PHOTOMETRY

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 Rp/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 Rp = 0.755 ± 0.040 RJ. These measurements further support models in which the planet is greatly enriched in heavy elements.