HAT-P-9b: A LOW-DENSITY PLANET TRANSITING A MODERATELY FAINT F STAR

We report the discovery of a planet transiting a moderately faint ( V = 12.3 mag) late F star, with an orbital period of 3.92289 ± 0.00004 days. From the transit light curve and radial velocity measurements, we determine that the radius of the planet is R p = 1.40 ± 0.06 R Jup and that the mass is M p = 0.78 ± 0.09 M Jup . The density of the new planet, ρ p = 0.35 ± 0.06 g cm -3 , fits to the low-density tail of the currently known transiting planets. We find that the center of transit is at T c = 2454417.9077 ± 0.0003 (HJD), and the total transit duration is 0.143 ± 0.004 days. The host star has M ⋆ = 1.28 ± 0.13 M ☉ and R ⋆ = 1.32 ± 0.07 R ☉ .