HD 38973 b - a cold Saturn orbiting a Sun-like star

Abstract We report the detection of a long-period companion to the nearby solar-type star HD 38973 using precision radial-velocity measurements. The radial-velocity data reveal a coherent Keplerian signal with a period of ∼3000 days and moderate eccentricity, yielding a minimum mass in the sub-Jovian regime. We complement the radial-velocity analysis with astrometric constraints from the Hipparcos–Gaia Catalog of Accelerations (HGCA). Although no significant proper-motion anomaly is detected for HD 38973, the absence of an astrometric signal provides an informative upper limit on the companion mass. By combining the radial-velocity posterior with the HGCA likelihood, we rule out high-mass solutions at low inclinations and derive a robust upper bound on the true companion mass. We find the best-fitting true mass to be $0.240_{-0.040}^{+0.102}\,M_{\rm Jup}$ , on an orbit with period $2733^{+210}_{-190}$ days, making HD 38973b a likely cold Saturn. This study highlights the diagnostic power of astrometric non-detections when combined with precision radial velocities, demonstrating that meaningful constraints on companion masses can be obtained even in the absence of a detected astrometric signal.