Dynamical orbital evolution scenarios of the wide-orbit eccentric planet HR 5183b

TL;DR

This paper investigates how the highly eccentric orbit of exoplanet HR 5183b could have formed, analyzing planet-planet scattering and binary-induced Kozai cycles, and finds the combined scattering and Kozai mechanism most likely.

Contribution

It introduces a comprehensive analysis of multiple dynamical scenarios, highlighting the combined scattering and Kozai cycles as the most probable explanation for HR 5183b's orbit.

Findings

Planet-planet scattering alone has a 2.8-7.2% success rate in reproducing the observed eccentricity.

Kozai cycles from the binary alone have a low probability (0.6%) of reaching the observed eccentricity.

Combined scattering and Kozai cycles yield a 14.5% success rate, making it the most plausible scenario.

Abstract

The recently-discovered giant exoplanet HR5183b exists on a wide, highly-eccentric orbit ($a=18$\,au, $e=0.84$). Its host star possesses a common proper-motion companion which is likely on a bound orbit. In this paper, we explore scenarios for the excitation of the eccentricity of the planet in binary systems such as this, considering planet-planet scattering, Lidov-Kozai cycles from the binary acting on a single-planet system, or Lidov-Kozai cycles acting on a two-planet system that also undergoes scattering. Planet-planet scattering, in the absence of a binary companion, has a $2.8-7.2\%$ probability of pumping eccentricities to the observed values in our simulations, depending on the relative masses of the two planets. Lidov-Kozai cycles from the binary acting on an initially circular orbit can excite eccentricities to the observed value, but require very specific orbital configurations for the binary and overall there is a low probability of catching the orbit at the high observed high eccentricity ($0.6\%$). The best case is provided by planet-planet scattering in the presence of a binary companion: here, the scattering provides the surviving planet with an initial eccentricity boost that is subsequently further increased by Kozai cycles from the binary. We find a success rate of $14.5\%$ for currently observing $e\ge0.84$ in this set-up. The single-planet plus binary and two-planet plus binary cases are potentially distinguishable if the mutual inclination of the binary and the planet can be measured, as the latter permits a broader range of mutual inclinations. The combination of scattering and Lidov-Kozai forcing may also be at work in other wide-orbit eccentric giant planets, which have a high rate of stellar binary companions.