A PRECISE ESTIMATE OF THE RADIUS OF THE EXOPLANET HD 149026b FROMSPITZERPHOTOMETRY

Abstract
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.