A Precise Estimate of the Radius of HD 149026b

Abstract

Abstract. We present Spitzer 8 μm transit observations of the extrasolar planet system HD 149026b. At this wavelength, transit light curves are weakly affected by stellar limbdarkening, 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 i = 85.• 4 + 0.• 9 −0 .• 8 and a planet radius of 0.755 ± 0.040 RJ . These measurements further support models in which the planet is greatly enriched in heavy elements.

Measuring accurate transit parameters

“…A few noteworthy Spitzer results are the transit of GJ 436 (Gillon et al 2007, Deming et al 2007, the Neptune-sized planet about which we hear so much 102 Joshua N. Winn in this volume, and a recent light curve by Nutzman et al (2008) of the very challenging target HD 149026, for which the transit depth is only a quarter of a percent.…”