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

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 10.4% for currently observing 𝑒 0.84 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.

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

“…With its orbital inclination now measured as 89.9 +13.3 −13.5 °, we are able to unambiguously confirm that HR 5183 b is a planet with a true mass of 3.31 +0.18 −0.14 𝑀 𝐽 . The near-equivalence of the minimum and true planetary masses is relevant for our understanding of the dynamics of the HR 5183 system, since the dynamical studies of Kane et al (2019) and Mustill et al (2021) have both implicitly assumed that the planetary minimum mass of 3.23 +0.15 −0.14 𝑀 𝐽 measured by Blunt et al (2019) is a good approximation of the true mass; our results demonstrate that this assumption is valid.…”

“…The possibility of physical association between HR 5183 and HIP 67291 has been posited several times in the literature, and this hypothesis was reviewed in detail by Blunt et al (2019). Those authors were ultimately somewhat agnostic on whether or not the pair form a physical binary (although Mustill et al 2021 have more recently reinforced the argument in favour of physical association), so we aim to fully re-evaluate this hypothesis in this study.…”

“…In their study of the dynamical formation history of HR 5183 b, Mustill et al (2021) predicted that the mutual inclination between the planetary orbit and the orbit of the wide stellar companion HIP 67291 could potentially be used to distinguish between formation mechanisms for HR 5183 b, with planet-planet scattering plus dynamical interactions with the stellar companion leading to a broader range of possible mutual inclinations compared to binary interactions alone. In this study we have measured the orbital inclination and longitude of node of HR 5183 b for the first time, and we have combined these with the corresponding parameters for the orbit of HIP 67291 in an attempt to measure the planet-binary mutual inclination.…”

“…The authors find that orbital excitation of an initially circular planetary orbit by HIP 67291 is unlikely to produce the observed eccentricity as compared to planetplanet scattering, but notably the combination of both of these excitation mechanisms results in the highest probability of reproducing the planetary orbit. As a corollary, Mustill et al (2021) interestingly find that the predicted distribution of mutual inclinations between the stellar and planetary orbits differ depending on the formation scenario, with planet-planet scattering inducing a broader distribution of mutual inclinations, potentially allowing for direct constraints on the planet formation history if the planet-binary mutual inclination can be measured precisely.…”

“…a large sky separation of 490" (∼15400 AU projected separation). The presence of this companion may have dynamically influenced the evolution of the planet HR 5183 b, a possibility that has recently been explored by Mustill et al (2021). The authors find that orbital excitation of an initially circular planetary orbit by HIP 67291 is unlikely to produce the observed eccentricity as compared to planetplanet scattering, but notably the combination of both of these excitation mechanisms results in the highest probability of reproducing the planetary orbit.…”