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We present observations of three distinct transits of HD 17156b obtained withthe Fine Guidance Sensors (FGS) on board the Hubble Space Telescope} (HST). Weanalyzed both the transit photometry and previously published radial velocitiesto find the planet-star radius ratio R_p/R_s = 0.07454 +/- 0.00035, inclinationi=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 of0.522 +0.021/-0.018 g cm^-3. Analysis of asteroseismology observations by thecompanion paper of Gilliland et al. (2009) provides a consistent butsignificantly refined measurement of the stellar mean density. We comparestellar isochrones to this density estimate and find M_s = 1.275 +/- 0.018M_sun and a stellar age of $3.37 +0.20/-0.47 Gyr. Using this estimate of M_sand incorporating the density constraint from asteroseismology, we model boththe photometry and published radial velocities to estimate the planet radiusR_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 studiesand consistent with theoretical models of a solar-composition gas giant of thesame mass and equilibrium temperature. For the three transits, we determine thetimes of mid-transit to a precision of 6.2 s, 7.6 s, and 6.9 s, and the transittimes for HD 17156 do not show any significant departures from a constantperiod. The joint analysis of transit photometry and asteroseismology presagessimilar studies that will be enabled by the NASA Kepler Mission.

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