A Search for Wide Companions to the Extrasolar Planetary System HR 8799

TL;DR

This study investigates the HR 8799 planetary system, ruling out additional wide companions and background objects, and provides insights into the system's architecture and potential for scattered planets beyond 600 AU.

Contribution

First direct comparison of archival imaging data to assess candidate companions' proper motion, establishing no additional bound companions within certain limits.

Findings

Two faint objects are background, not bound to HR 8799.

No companions with H<22 within 5-15 arcsec.

Potential scattered planets are beyond 600 AU or less than 3 Jupiter masses.

Abstract

The extrasolar planetary system around HR 8799 is the first multiplanet system ever imaged. It is also, by a wide margin, the highest mass system with >27 Jupiters of planetary mass past 25 AU. This is a remarkable system with no analogue with any other known planetary system. In the first part of this paper we investigate the nature of two faint objects imaged near the system. These objects are considerably fainter (H=20.4, and 21.6 mag) and more distant (projected separations of 612, and 534 AU) than the three known planetary companions b, c, and d (68-24 AU). It is possible that these two objects could be lower mass planets (of mass ~5 and ~3 Jupiters) that have been scattered to wider orbits. We make the first direct comparison of newly reduced archival Gemini adaptive optics images to archival HST/NICMOS images. With nearly a decade between these epochs we can accurately assess the proper motion nature of each candidate companion. We find that both objects are unbound to HR 8799 and are background. We estimate that HR 8799 has no companions of H<22 from ~5-15 arcsec. Any scattered giant planets in the HR 8799 system are >600 AU or less than 3 Jupiters in mass. In the second part of this paper we carry out a search for wider common proper motion objects. While we identify no bound companions to HR 8799, our search yields 16 objects within 1 degree in the NOMAD catalog and POSS DSS images with similar (+/-20 mas/yr) proper motions to HR 8799, three of which warrant follow-up observations.