The hot-Jupiter Kepler-17b: discovery, obliquity from stroboscopic starspots, and atmospheric characterization

Abstract

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 ± 100 from high-resolution spectra, we infer a stellar host mass of 1.06 ± 0.07 M☉ and a stellar radius of 1.02 ± 0.03 R☉. We estimate the planet mass and radius to be M_P = 2.45 ± 0.11 M_J and R_P = 1.31 ± 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, eight times 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°. 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's infrared bandpasses areT_(3.6 µm) = 1880 ± 100 K and T_(4.5 µm) = 1770 ± 150 K. We measure the optical geometric albedo A_g in the Kepler bandpass and find A_g = 0.10 ± 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.

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