Kepler-423b: A half-Jupiter mass planet transiting a very old solar-like star

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 light curve of Kepler-423 exhibits quarter-to-quarter systematic variations of the transit depth, with a peak-to-peak amplitude of ~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 Kepler-423 is a G4 dwarf with M_∗ = 0.85 ± 0.04 M_⊙, R_∗ = 0.95 ± 0.04 R_⊙, T_eff= 5560 ± 80 K, [M/H] = - 0.10 ± 0.05 dex, and with an age of 11 ± 2 Gyr. The planet Kepler-423b has a mass of M_p= 0.595 ± 0.081M_Jup and a radius of R_p= 1.192 ± 0.052R_Jup, yielding a planetary bulk density of ρ_p = 0.459 ± 0.083 g cm^-3. The radius of Kepler-423b 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 2σ confidence level (ΔF_ec = 14.2 ± 6.6 ppm) and found that the orbit might have asmall non-zero eccentricity of 0.019^+0.028_-0.0140.019-0.014+0.028. With a Bond albedo of A_B = 0.037 ± 0.019, Kepler-423b is one of the gas-giant planets with the lowest albedo known so far.