An Edge-On Orbit for the Eccentric Long-Period Planet HR 5183 b

TL;DR

This study measures the orbit of the eccentric long-period exoplanet HR 5183 b, confirming its extreme orbit and binary association, and suggests that stellar companions influence the formation of highly eccentric planets.

Contribution

First astrometric measurement of HR 5183 b's inclination, revealing its edge-on orbit and the mutual inclination with its binary companion, providing new insights into their dynamical relationship.

Findings

HR 5183 b has an orbital period of about 102 years and eccentricity of 0.87.

The planet's orbit is consistent with being edge-on.

A significant fraction of highly eccentric planets are in multiple star systems.

Abstract

The long-period giant planet HR 5183 b has one of the most extreme orbits among exoplanets known to date, and represents a test for models of their dynamical evolution. In this work we use Hipparcos-Gaia astrometry to measure the orbital inclination of this planet for the first time and find $i=89.9^{+13.3\circ}_{-13.5}$, fully consistent with edge-on. The long orbital period and high eccentricity of HR 5183 b are supported by our results, with $P=102^{+84}_{-34}$ years and $e=0.87 \pm 0.04$. We confirm that HR 5183 forms a physically bound binary with HIP 67291 at a projected separation of 15400 AU, and derive new constraints on the orbit of this pair. We combine these results to measure the mutual inclination between the planetary and binary orbits; we observe significant evidence for misalignment, which remains even after accounting for bias of the prior towards high mutual inclinations. However, our results are too imprecise to evaluate a recent prediction that the mutual inclination should reflect the formation history of HR 5183 b. Further observations, especially the release of the full Gaia astrometric data, will allow for improved constraints on the planet-binary mutual inclination. $52 \pm 16\%$ of known planets with eccentricities $e\geq 0.8$ are found in multiple star systems, a rate that we find to be greater than for the overall planet population to moderate significance ($p=0.0075$). This supports the hypothesis that dynamical interactions with wide stellar companions plays an important role in the formation of highly eccentric exoplanets.