Star - Planet - Debris Disk Alignment in the HD 82943 system: Is planetary system coplanarity actually the norm?

TL;DR

This study finds that the HD 82943 system's planets, debris disk, and star are aligned within a few degrees, supporting the idea that planetary systems are generally coplanar, which has implications for understanding their formation.

Contribution

First direct measurement of star, planet, and debris disk alignment in a radial velocity system at ~AU scales, suggesting primordial coplanarity in planetary systems.

Findings

Planet and debris disk inclinations are consistent within uncertainties.

System-wide alignment appears primordial, not due to secular perturbations.

Evidence of similar alignments in other systems supports the coplanarity hypothesis.

Abstract

Recent results suggest that the two planets in the HD 82943 system are inclined to the sky plane by 20 +/- 4deg. Here, we show that the debris disk in this system is inclined by 27 +/- 4deg, thus adding strength to the derived planet inclinations and suggesting that the planets and debris disk are consistent with being aligned at a level similar to the Solar System. Further, the stellar equator is inferred to be inclined by 28 +/- 4deg, suggesting that the entire star - planet - disk system is aligned, the first time such alignment has been tested for radial velocity discovered planets on ~AU wide orbits. We show that the planet-disk alignment is primordial, and not the result of planetary secular perturbations to the disk inclination. In addition, we note three other systems with planets at >10AU discovered by direct imaging that already have good evidence of alignment, and suggest that empirical evidence of system-wide star - planet - disk alignment is therefore emerging, with the exception of systems that host hot Jupiters. While this alignment needs to be tested in a larger number of systems, and is perhaps unsurprising, it is a reminder that the system should be considered as a whole when considering the orientation of planetary orbits.