Cold giant discoveries from a joint radial-velocity and astrometry framework

Context. The population of long-period giant planets shapes planetary system architectures and formation pathways, but these cold Jupiters remain relatively unexplored. Radial velocity (RV) surveys lose sensitivity at multi-AU separations and provide only minimum planet masses, while the detection probability of transit surveys at long periods is low. Absolute astrometry from the H IPPARCOS and Gaia missions offers an additional source for stellar motion that can break the orbital inclination degeneracy and strengthen detection confidence. This is especially timely ahead of Gaia DR4 and DR5, which is expected to enable routine astrometric vetting and true-mass measurements for long-period RV planets. Aims. Extending the Chile-Hertfordshire ExoPlanet Survey (CHEPS) by combining RVs spanning up to 16 years with absolute astrometry, we search for and characterise cold giants around metal-rich FGK stars. Methods. We upgraded the EMPEROR framework by incorporating astrometric differencing to jointly fit RVs and astrometry for five CHEPS targets, and we performed a Bayesian model comparison and baseline- and phase-coverage metrics to quantify the astrometric contribution. Results. Our analysis confirms and characterises orbital parameters for two known planets in HIP 21850 and detects five new planets: a warm Jupiter, HIP 10090c, with an orbital period P = 321.8 −0.6 +0.3 d and mass M = 0.85 −0.12 +0.03 M J , and four Jupiter analogues, HIP 8923b with P = 14.1 −0.7 +0.4 yr and M = 9.98 −0.16 +0.78 M J , HIP 10090b with P = 8.1 −0.3 +0.3 yr and M = 3.87 −0.60 +0.65 M J , HIP 39330b with P = 12.7 −0.7 +0.6 yr and M = 1.68 −0.13 +0.16 M J , and HIP 98599b with P = 7.3 −0.1 +0.1 yr and M = 6.85 −0.22 +0.10 M J . The addition of astrometry reduces the period and mass uncertainties by factors between 3 and 10 whilst increasing the Bayes factors by up to ~60. Conclusions. The synergy of long-baseline RVs and absolute astrometry provides a robust pathway for discovering and characterising cold giant planets and metal-rich Solar System analogues. Our results demonstrate that astrometric samples meaningfully improve detection confidence and convert minimum masses into true masses. This approach fills the demographic gap between RV and directimaging sensitivities and prepares the field for forthcoming astrometric missions that will allow us to find stellar systems like our own.