We present observations of three distinct transits of HD 17156b obtained with the Fine Guidance Sensors ( FGS ) on board the Hubble Space Telescope ( HST ) Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute , which is operated by the Association of Universities for Research in Astronomy , Incorporated , under NASA contract NAS5-26555 . .
We analyzed both the transit photometry and previously published radial velocities to find the planet-star radius ratio R _ { p } / R _ { \star } = 0.07454 \pm 0.00035 , inclination i = 86.49 ^ { +0.24 } _ { -0.20 } deg , and scaled semi-major axis a / R _ { \star } = 23.19 ^ { +0.32 } _ { -0.27 } .
This last value translates directly to a mean stellar density determination \rho _ { \star } = 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 While this work was in press , the companion work revised this measurement to \rho _ { \star } = 0.5301 \pm 0.0044 ~ { } g~ { } cm ^ { -3 } .
We note that the change , which is less than 0.2 of the error , would translate to a stellar radius change of less than 0.001 { R } _ { \odot } and does not materially affect the results presented in this paper . of \rho _ { \star } = 0.5308 \pm 0.0040 ~ { } g~ { } cm ^ { -3 } .
We compare stellar isochrones to this density estimate and find M _ { \star } = 1.275 \pm 0.018 ~ { } M _ { \odot } and a stellar age of 3.37 ^ { +0.20 } _ { -0.47 } Gyr .
Using this estimate of M _ { \star } 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 ~ { } \pm~ { } 0.0066 ~ { } R _ { J } and the stellar radius R _ { \star } = 1.5007 \pm 0.0076 ~ { } R _ { \odot } .
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 .