Parameters and Predictions for the Long-Period Transiting Planet HD 17156b

We report high-cadence time series photometry of the recently discovered transiting exoplanet system HD 17156, spanning the time of transit on UT 2007 October 1, from three separate observatories. We present a joint analysis of our photometry, previously published radial velocity measurements, and times of transit center for three additional events. Adopting the spectroscopically determined values and uncertainties for the stellar mass and radius, we estimate a planet radius of R_p = 1.01 ± 0.09 R_Jup and an inclination of i = 86.5^{+ 1.1}_−0.7 deg. We find a time of transit center of T_c = 2,454,374.8338 ± 0.0020 HJD and an orbital period of P = 21.21691 ± 0.00071 days and note that the four transits reported to date show no sign of timing variations that would indicate the presence of a third body in the system. Our results do not preclude the existence of a secondary eclipse, but imply that there is only a 9.2% chance for this to be present and an even lower probability (6.9%) that the secondary eclipse would be a nongrazing event. Due to its eccentric orbit and long period, HD 17156b is a fascinating object for the study of the dynamics of exoplanet atmospheres. To aid such future studies, we present theoretical light curves for the variable infrared emission from the visible hemisphere of the planet throughout its orbit.