Deep orbital search for additional planets in the HR 8799 system

The HR 8799 system hosts four massive planets orbiting 15 and 80 AU. Studies of the system's orbital stability and its outer debris disk open the possibility of additional planets, both interior to and exterior to the known system. Reaching a sufficient sensitivity to search for interior planets is very challenging due to the combination of bright quasi static speckle noise close to the stellar diffraction core and relatively fast orbital motion. In this work, we present a deep L-band imaging campaign using NIRC2 at Keck comprising 14 observing sequences. We further re-reduce archival data for a total of 16.75 hours, one of the largest uniform datasets of a single direct imaging target. Using a Bayesian modeling technique for detecting planets in images while compensating for plausible orbital motion, we then present deep limits on the existence of additional planets in the HR 8799 system. The final combination shows a tentative candidate, consistent with 4-7 $M_{jup}$ at 4-5 AU, detected with an equivalent false alarm probability better than $3σ$. This analysis technique is widely applicable to archival data and to new observations from upcoming missions that revisit targets at multiple epochs.