  Observations conducted with the Fine Guidance Sensor on Hubble Space
Telescope (HST) providing high cadence and precision time-series photometry
were obtained over 10 consecutive days in December 2008 on the host star of the
transiting exoplanet HD 17156b. During this time 10^12 photons (corrected for
detector deadtime) were collected in which a noise level of 163 parts per
million per 30 second sum resulted, thus providing excellent sensitivity to
detection of the analog of the solar 5-minute p-mode oscillations. For HD 17156
robust detection of p-modes supports determination of the stellar mean density
of 0.5301 +/- 0.0044 g/cm^3 from a detailed fit to the observed frequencies of
modes of degree l = 0, 1, and 2. This is the first star for which direct
determination of the mean stellar density has been possible using both
asteroseismology and detailed analysis of a transiting planet light curve.
Using the density constraint from asteroseismology, and stellar evolution
modeling results in M_star = 1.285 +/- 0.026 solar, R_star = 1.507 +/- 0.012
solar, and a stellar age of 3.2 +/- 0.3 Gyr.
