KELT-8b: A HIGHLY INFLATED TRANSITING HOT JUPITER AND A NEW TECHNIQUE FOR EXTRACTING HIGH-PRECISION RADIAL VELOCITIES FROM NOISY SPECTRA

We announce the discovery of a highly inflated transiting hot Jupiter by the KELT-North survey. A global analysis including constraints from isochrones indicates that the V = 10.8 host star (HD 343246) is a mildly evolved, G dwarf with T_eff = 5754_-55⁺⁵⁴ K, log g = 4.078_-0.054^+0.049, [Fe/H] = 0.272 ± 0.038, an inferred mass M_∗ = 1.211 _-0.066^+0.078 M_⊙, and radius R_∗ = 1.67_-0.12^+0.14 R_⊙. The planetary companion has a mass M_P = 0.867_-0.061^+0.065 M_J, radius R_P = 1.86_-0.16^+0.18 R_J, surface gravity log g_P = 2.793_-0.075^+0.072, and density ρ_P = 0.167_-0.038^+0.047 g cm^-3. The planet is on a roughly circular orbit with semimajor axis a = 0.04571_-0.00084^+0.00096 AU and eccentricity e = 0.035_-0.025^+0.050. The best-fit linear ephemeris is T₀ = 2456883.4803 ± 0.0007 BJD_TDB and P = 3.24406 ± 0.00016 days. This planet is one of the most inflated of all known transiting exoplanets, making it one of the few members of a class of extremely low density, highly irradiated gas giants. The low stellar log g and large implied radius are supported by stellar density constraints from follow-up light curves, as well as an evolutionary and space motion analysis. We also develop a new technique to extract high-precision radial velocities from noisy spectra that reduces the observing time needed to confirm transiting planet candidates. This planet boasts deep transits of a bright star, a large inferred atmospheric scale height, and a high equilibrium temperature of T_eq = 1675_-55⁺⁶¹ K, assuming zero albedo and perfect heat redistribution, making it one of the best targets for future atmospheric characterization studies.