HIP 102152b: A low-mass planet candidate around an old solar twin

We present the discovery of a short-period low-mass exoplanet candidate around the 8-Gyr solar twin HIP 102152 as part of the Solar Twin Planet Search survey. The system was observed over 7 years with the ESO/HARPS spectrograph. This time series was used in the past to determine the star’s rotational period as P_rot = 35.7\(\pm\)1.4 days (Lorenzo-Oliveira et al. 2020). Besides the rotational modulations of stellar activity, we did not find correlations with the proposed orbital period of the candidate exoplanet. To probe the physical and orbital characteristics of HIP 102152b and assess the significance of red noise that account for stellar activity signatures, we employed a joint Keplerian and quasi-periodic Gaussian Process models. The resulting model is consistent with a mini-Neptune (\(m\sin{i} = 8\pm1~M_\oplus\))on a circular orbit with P_b,orb = 23 days, and no radial velocity slope was observed due to acceleration from an outer massive companion in this system. We compared the different four methods using the Bayesian/Akaike Information Criterion (BIC/AIC) indexes, finding out that the resulting model is favoured by a \(\Delta\)BIC/AIC = -13.4/-10.0 in comparison to the second most significant. Even exceptionally similar to the Sun in terms of refractory/volatile species abundances, HIP 102152 is severely lithium-depleted, a possible indication that this heavy material from the protoplanetary disc was used in the formation of planetesimals and other planets including HIP 102152b.