This paper reports the discovery and characterization of the transiting hot giant exoplanet Kepler-17b .
The planet has an orbital period of 1.486 days , and radial velocity measurements from the Hobby-Eberly Telescope show a Doppler signal of 419.5 ^ { +13.3 } _ { -15.6 } m s ^ { -1 } .
From a transit-based estimate of the host star ’ s mean density , combined with an estimate of the stellar effective temperature T _ { eff } = 5630 \pm 100 from high-resolution spectra , we infer a stellar host mass of 1.06 \pm 0.07 M _ { \sun } and a stellar radius of 1.02 \pm 0.03 R _ { \sun } .
We estimate the planet mass and radius to be M _ { P } = 2.45 \pm 0.11 M _ { J } and R _ { P } = 1.31 \pm 0.02 R _ { J } .
The host star is active , with dark spots that are frequently occulted by the planet .
The continuous monitoring of the star reveals a stellar rotation period of 11.89 days , 8 times the the planet ’ s orbital period ; this period ratio produces stroboscopic effects on the occulted starspots .
The temporal pattern of these spot-crossing events shows that the planet ’ s orbit is prograde and the star ’ s obliquity is smaller than 15 ^ { \circ } .
We detected planetary occultations of Kepler-17b with both the Kepler and Spitzer Space Telescopes .
We use these observations to constrain the eccentricity , e , and find that it is consistent with a circular orbit ( e < 0.011 ) .
The brightness temperatures of the planet the infrared bandpasses are T _ { { 3.6 ~ { } \micron } } = 1880 \pm 100 K and T _ { { 4.5 ~ { } \micron } } = 1770 \pm 150 K. We measure the optical geometric albedo A _ { g } in the Kepler bandpass and find A _ { g } = 0.10 \pm 0.02 .
The observations are best described by atmospheric models for which most of the incident energy is re-radiated away from the day side .