2022PASJ...74.1309T

We report the discovery of a triple-giant-planet system around an evolved star HD 184010 (HR 7421, HIP 96016). This discovery is based on observations from Okayama Planet Search Program, a precise radial velocity survey, undertaken at Okayama Astrophysical Observatory between 2004 April and 2021 June. The star is K0 type and located at the beginning of the red giant branch. It has a mass of $1.35_{-0.21}^{+0.19}\, M_{\odot }$, a radius of $4.86_{-0.49}^{+0.55} R_{\odot }$, and a surface gravity log g of $3.18_{-0.07}^{+0.08}$. The planetary system is composed of three giant planets in a compact configuration: the planets have minimum masses of $M_{\rm {b}}\sin i = 0.31_{-0.04}^{+0.03}\, M_{\rm {J}}$, $M_{\rm {c}}\sin i = 0.30_{-0.05}^{+0.04}\, M_{\rm {J}}$, and $M_{\rm {d}}\sin i = 0.45_{-0.06}^{+0.04}\, M_{\rm {J}}$, and orbital periods of $P_{\rm {b}}=286.6_{-0.7}^{+2.4}\:$d, $P_{\rm {c}}=484.3_{-3.5}^{+5.5}\:$d, and $P_{\rm {d}}=836.4_{-8.4}^{+8.4}\:$d, respectively, which are derived from a triple Keplerian orbital fit to three sets of radial velocity data. The ratio of orbital periods are close to P_d : P_c : P_b ∼ 21 : 12 : 7, which means the period ratios between neighboring planets are both lower than 2 : 1. The dynamical stability analysis reveals that the planets should have near-circular orbits. The system could remain stable over 1 Gyr, initialized from co-planar orbits, low eccentricities (e = 0.05), and planet masses equal to the minimum mass derived from the best-fitting circular orbit fitting. Besides, the planets are not likely in mean motion resonance. The HD 184010 system is unique: it is the first system discovered to have a highly evolved star (log g < 3.5 cgs) and more than two giant planets all with intermediate orbital periods (10² < P < 10³ d).