Abstract
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 $2.8{-}7.2{{\ \rm per\ cent}}$ 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 ($0.6{{\ \rm per\ cent}}$). 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 $14.5{{\ \rm per\ cent}}$ 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.
Highlights
The G0 star HR 5183 (M★ = 1.07 ± 0.04 M ), has been found to host a super-Jovian planet (M sin I = 3.23+−00..5585 MJ) on a highly-eccentric orbit (Blunt et al 2019)
The high eccentricity of HR 5183b suggests that significant changes to the planet’s orbit have happened in the past, as planets form on nearcircular orbits1
We show in this paper that this significantly increases the probability of observing a planet at a high eccentricity, above either planet–planet scattering alone or Lidov–Kozai cycles alone
Summary
While the orbit of the planet is wide and its semimajor axis poorly constrained (a = 18+−64 au), good coverage of the planet’s periastron passage both permitted its discovery and provided a good determination of the orbital eccentricity (e = 0.84 ± 0.04). The high eccentricity of HR 5183b suggests that significant changes to the planet’s orbit have happened in the past, as planets form on nearcircular orbits. Planet–planet scattering has been shown to be able to produce high eccentricities (Rasio & Ford 1996; Weidenschilling & Marzari 1996; Chatterjee et al 2008; Jurić & Tremaine 2008). The eccentricity distribution of giant planets implies that around 80% of them formed in unstable multi-planet systems (Jurić & Tremaine 2008; Raymond et al 2011).
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