A fifth companion in the HR 8799 system revealed by Gaia

HR 8799 is the benchmark young multi-planet system, hosting four directly imaged giant planets orbiting between a warm inner belt and a cold outer debris ring. The few-au region around the star has remained observationally inaccessible: it lies beyond the reach of conventional high-contrast imaging and is challenging for radial-velocity techniques because of stellar pulsations.

To probe this gap, the authors combined Gaia absolute astrometry and Hipparcos-Gaia proper-motion anomalies with radial velocities and high-contrast imaging. This combination revealed an additional close-in substellar companion in the system.

The allowed solutions span approximately 0.3–6 au, with posteriors peaking at approximately 2–3 au and masses around 10–14 Jupiter masses. Dynamical modelling shows that such a companion with low eccentricity can coexist with the long-lived resonant chain of the four outer planets, and can sculpt the inner edge of the warm debris belt, linking the planet architecture to the debris-disk morphology.

These results close a long-standing detection gap in HR 8799 and demonstrate the power of Gaia astrometry to uncover massive planets at intermediate separations that may remain undetectable with radial velocities or direct imaging alone.