Kepler423b: a half-Jupiter mass planet transiting a very old solar-like star

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We report the spectroscopic confirmation of the Kepler object of interest KOI-183.01 (Kepler-423b), a half-Jupiter mass planet transiting an old solar-like star every 2.7 days. Our analysis is the first to combine the full Kepler photometry (quarters 1-17) with high-precision radial velocity measurements taken with the FIES spectrograph at the Nordic Optical Telescope. We simultaneously modelled the photometric and spectroscopic data-sets using Bayesian approach coupled with Markov chain Monte Carlo sampling. We found that the Kepler pre-search data conditioned (PDC) light curve of KOI-183 exhibits quarter-to-quarter systematic variations of the transit depth, with a peak-to-peak amplitude of about 4.3 % and seasonal trends reoccurring every four quarters. We attributed these systematics to an incorrect assessment of the quarterly variation of the crowding metric. The host star KOI-183 is a G4 dwarf with M⋆=0.85±0.04 MSun, R⋆=0.95±0.04 RSun, Tₑff=5560±80 K, [M/H]=-0.10±0.05 dex, and with an age of 11±2 Gyr. The planet KOI-183b has a mass of Mₚ=0.595±0.081 Mⱼup and a radius of Rₚ=1.192±0.052 Rⱼup, yielding a planetary bulk density of ρₚ=0.459±0.083 g/cm³. The radius of KOI-183b is consistent with both theoretical models for irradiated coreless giant planets and expectations based on empirical laws. The inclination of the stellar spin axis suggests that the system is aligned along the line of sight. We detected a tentative secondary eclipse of the planet at a 2-σ confidence level (ΔFec=14.2±6.6 ppm) and found that the orbit might have a small non-zero eccentricity of e=0.019⁺⁰.⁰²⁸₋₀.₀₁₄. With a Bond albedo of AB=0.037±0.019, KOI-183b is one of the gas-giant planets with the lowest albedo known so far.