Revisiting planetary systems in the Okayama Planet Search Program: A new long-period planet, RV astrometry joint analysis, and a multiplicity-metallicity trend around evolved stars.

This article focuses on the reanalysis of 32 planetary systems around evolved stars observed by the Okayama Planet Search Program (OPSP) and its East Asian Planet Search Network collaboration, using updated radial velocity (RV) data from the Okayama Astrophysical Observatory. The study confirms a new wide-orbit giant planet, 75 Cet c, and identifies five stars exhibiting long-term RV trends indicative of massive outer companions. It also investigates additional RV variations in several stars, some of which may be due to stellar activity or instrumental effects rather than planets. By combining RV and astrometric data, the authors refine orbital parameters for four systems and find that OPSP’s sample is biased toward more metal-poor stars due to a color-selection cutoff. Analysis of a broader sample of evolved stars reveals a positive correlation between host star metallicity and both planetary system multiplicity and total planet mass, particularly for systems with Jupiter-mass planets, supporting the core-accretion model of giant planet formation.