Astronomy Tea Talk

  One of the expanding fields of exoplanet study is the detailed characterization of exoplanets, including the properties of their atmospheres. This is currently being done for a growing sample of the so-called hot Jupiters - gas-giant planets orbiting close-in to their host star - a class of planets with no Solar System analog. I will present the results of our atmospheric study of the unique transiting exoplanet Kepler-13Ab. It is one of only two known short-period (1.76 day) planets orbiting a bright (V = 9.95 mag) hot A-type star (Teff = 7,650 K), and it resides in the Kepler field. We have observed the planet’s occultation (secondary eclipse; when the planet moves behind the host star) using data from the optical to the IR, obtained with the Kepler and Spitzer space telescopes, along with a ground-based observation in the near-IR. For the planetary hemisphere facing the star we derive a temperature of 2,750 K, as hot as the smallest main-sequence stars. We find evidence for a high geometric albedo, of about 0.3, which is much higher than for other hot Jupiter exoplanets with a typical albedo of no more than 0.1. We also identify the presence of atmospheric inversion, where the temperature increases with decreasing pressure, consistent with findings for other hot Jupiters orbiting quiet stars. In addition, our revised planetary radius (1.4 Jupiter radius) is significantly smaller than previously thought, and our revised planetary mass, from measuring the beaming effect and ellipsoidal distortion in the Kepler orbital phase curve, is 5 - 8 Jupiter mass. Therefore, this planet is a massive high-density hot Jupiter, with radius similar to those of other hot Jupiters. Finally, we find that the difference between the Kepler occultation time and transit (primary eclipse) time is half a minute shorter than expected from the light travel time delay across the orbit, and discuss possible causes.