HD 28185 revisited: an outer planet, instead of a brown dwarf, on a Saturn-like orbit

Abstract

ABSTRACT As exoplanet surveys reach ever-higher sensitivities and durations, planets analogous to the Solar system giant planets are increasingly within reach. HD 28185 is a Sun-like star known to host a $m\sin i=6~M_\mathrm{ J}$ planet on an Earth-like orbit; more recently, a brown dwarf with a more distant orbit has been claimed. In this work, we present a comprehensive re-analysis of the HD 28185 system, based on 22 yr of radial velocity (RV) observations and precision Hipparcos–Gaia astrometry. We confirm the previous characterization of HD 28185 b as a temperate giant planet, with its $385.92^{+0.06}_{-0.07}$ d orbital period giving it an Earth-like incident flux. In contrast, we substantially revise the parameters of HD 28185 c; with a new mass of $m=6.0\pm 0.6~M_\mathrm{ J}$, we reclassify this companion as a super-Jovian planet. HD 28185 c has an orbital period of $24.9^{+1.3}_{-1.1}$ yr, a semimajor axis of $8.50^{+0.29}_{-0.26}$ au, and a modest eccentricity of $0.15\pm 0.04$, resulting in one of the most Saturn-like orbits of any known exoplanet. HD 28185 c lies at the current intersection of detection limits for RVs and direct imaging, and highlights how the discovery of giant planets at $\approx$10 au separations is becoming increasingly routine.