Star-planet-debris disc alignment in the HD 82943 system: is planetary system coplanarity actually the norm?

Abstract

Recent results suggest that the two planets in the HD 82943 system are inclined to the sky plane by 20 ± 4°. Here, we show that the debris disc in this system is inclined by 27 ± 4°, thus adding strength to the derived planet inclinations and suggesting that the planets and debris disc are consistent with being aligned at a level similar to the Solar system. Further, the stellar equator is inferred to be inclined by 28 ± 4°, suggesting that the entire star-planet-disc 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-disc alignment is primordial, and not the result of planetary secular perturbations to the disc inclination. In addition, we note three other systems with planets at ≳10 au discovered by direct imaging that already have good evidence of alignment, and suggest that empirical evidence of system-wide star-planet-disc 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.