Precise Estimates of the Physical Parameters for the Exoplanet System HD-17156 Enabled by HST FGS Transit and Asteroseismic Observations

Abstract: We present observations of three distinct transits of HD 17156b obtained with the Fine Guidance Sensors (FGS) on board the Hubble Space Telescope} (HST). We analyzed both the transit photometry and previously published radial velocities to find the planet-star radius ratio R_p/R_s = 0.07454 +/- 0.00035, inclination i=86.49 +0.24/-0.20 deg, and scaled semi-major axis a/R = 23.19 +0.32/-0.27. This last value translates directly to a mean stellar density determination of 0.522 +0.021/-0.018 g cm^-3. Analysis of asteroseismology observations by the companion paper of Gilliland et al. (2009) provides a consistent but significantly refined measurement of the stellar mean density. We compare stellar isochrones to this density estimate and find M_s = 1.275 +/- 0.018 M_sun and a stellar age of $3.37 +0.20/-0.47 Gyr. Using this estimate of M_s and incorporating the density constraint from asteroseismology, we model both the photometry and published radial velocities to estimate the planet radius R_p= 1.0870 +/- 0.0066 Jupiter radii and the stellar radius R_s = 1.5007 +/- 0.0076 R_sun. The planet radius is larger than that found in previous studies and consistent with theoretical models of a solar-composition gas giant of the same mass and equilibrium temperature. For the three transits, we determine the times of mid-transit to a precision of 6.2 s, 7.6 s, and 6.9 s, and the transit times for HD 17156 do not show any significant departures from a constant period. The joint analysis of transit photometry and asteroseismology presages similar studies that will be enabled by the NASA Kepler Mission.