Deep orbital search for additional planets in the HR 8799 system

TL;DR

This study uses deep L-band imaging and Bayesian modeling to search for additional planets in the HR 8799 system, revealing a tentative candidate near 4-5 AU and demonstrating a method applicable to archival and future data.

Contribution

The paper introduces a Bayesian detection technique that accounts for orbital motion, enabling sensitive searches for hidden planets in direct imaging data.

Findings

Tentative detection of a 4-7 Jupiter mass planet at 4-5 AU.

Deep imaging limits constrain the presence of additional planets.

Method applicable to archival and upcoming multi-epoch observations.

Abstract

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 $3\sigma$. This analysis technique is widely applicable to archival data and to new observations from upcoming missions that revisit targets at multiple epochs.