Planetary architectures under the influence of a stellar binary

TL;DR

The paper investigates how a distant stellar binary perturbs S-type planetary systems and whether such perturbations can generate highly eccentric planets. It uses N-body simulations with three Jupiter-mass planets around a $1\,M_\odot$ primary and a $0.8\,M_\odot$ companion, exploring a grid of binary semi-major axes $a_{ m B}$, eccentricities $e_{ m B}$, and inclinations $i_{ m B}$ using the REBOUND IAS15 integrator, tracking collisions and ejections over up to $5\times10^8$ years. The results show that planet-planet scattering and secular vZLK interactions induced by the binary cooperate to produce abrupt orbital changes, with the outcome strongly dependent on $a_{ m B}$, $e_{ m B}$, and $i_{ m B}$; the binary eccentricity mainly determines how many planets survive, while the inclination governs the final eccentricities and alignment with the binary plane, enabling highly eccentric single-planet configurations. High eccentricities ($e_p\ge0.8$) predominantly arise in single-planet outcomes for highly inclined and eccentric binaries, whereas multiplanet systems tend to retain low $e_p$ and align with the binary plane; several observed highly eccentric systems are reproduced under plausible parameters, though some observed systems require different masses or very wide companions where additional physics could be important. Overall, the work provides a dynamical pathway by which binaries sculpt planetary architectures and yields predictive constraints for interpreting current and future observations of planets in binary systems.

Abstract

Context. The presence of a stellar companion can strongly influence the architecture and long-term stability of planetary systems. Motivated by the discovery of exoplanets exhibiting extremely high eccentricities (e >= 0.8) in systems with a binary companion, we investigate how planetary orbits around one star (S-type configuration) evolve under the gravitational perturbations of the companion. Aims. We aim to assess the role of a stellar companion in shaping the orbital evolution of S-type planets and to explore whether dynamical interactions in such environments can account for the formation of highly eccentric planets. Methods. We performed a suite of N-body simulations, modeling systems initially composed of three Jupiter-mass planets on nearly circular, coplanar orbits around the primary star. We systematically varied the semi-major axis, eccentricity, and inclination of the stellar companion, to characterize the conditions under which extreme eccentricities can be excited. Results. Our results show that dynamical processes such as planet-planet scattering and secular mechanisms--including the von Zeipel-Kozai-Lidov effect induced by the binary--often act together to produce abrupt and significant changes in planetary orbital evolution, with the outcome strongly dependent on the binary separation. The binary's eccentricity primarily dictates the number of surviving planets, while its inclination not only governs the final eccentricities of those survivors but also drives their orbits to align with the binary plane. Our simulations successfully reproduce the high eccentricities and compact orbits observed in four observed systems, showing close agreement between the modeled configurations and the actual systems.

Figures (9)

• Figure 1: Left: Semi-major axis versus eccentricities of exoplanets in S-type orbits. Circles and stars represent systems with one or multiple planets, respectively. The color bar represents the semi-major axis of the binary star ($a_{\rm B}$) for planets with eccentricities greater than 0.8. Shaded regions highlight the hot, warm and cold Jupiter populations. Data extracted from the Encyclopedia of Exoplanets (https://exoplanet.eu/catalog/). Right: Orbital stability limits for systems with $e \geq 0.8$. The effects of a binary stellar companion with $M_{\rm B} = 0.8 \, M_{\odot}$ are shown for two eccentricities: $e_{\rm B} = 0.05$ (purple) and $e_{\rm B} = 0.9$ (yellow). The stars indicate the observed semi-major axis $a_{\rm B}$ of the binary, while the horizontal lines extend from periapsis $q_{\rm B} =a_{\rm B} (1 - e_{\rm B} )$ to apoapsis $Q_{\rm B} = a_{\rm B} (1 + e_{\rm B} )$. Similarly, the horizontal black circles and lines depict the semi-major axis, peri- and apoapsis of the planet, respectively. The purple and yellow bars extend to the critical semi-major axis $a_{\rm crit}$ for the planetary orbit, above which it would not be dynamically stable following Holman_1999, for each binary eccentricity. Filled bars indicate that the system is dynamically unstable under the corresponding binary eccentricity, while open bars indicate stability.
• Figure 2: Survival probability of a single planet in a binary system that initially hosted three planets, across various orbital configurations. Each panel corresponds to a specific value of the binary's semi-major axis $a_{\rm B}$, with the x- and y-axes showing the binary's inclination $i_{\rm B}$ and eccentricity $e_{\rm B}$, respectively. Shaded regions with crosses denote dynamically unstable configurations where no planets survive, while diagonal hatches indicate stable configurations in which all three initial planets remain bound throughout the simulation.
• Figure 3: Planetary eccentricities as a function of binary inclinations for a fixed separation, $a_{\rm B} = 200$ au. Each panel correspond to a specific value of $e_{\rm B}$. Purple boxes indicate the maximum eccentricities when more than two planets survive, while the yellow boxes show the eccentricities of systems with a single surviving planet.
• Figure 4: Normalized distribution of planetary eccentricities from simulations of systems with varying binary inclinations (left) and binary eccentricities (right). Each row shows results for different survival outcomes: 1, 2, or 3 remaining planets. Colored curves correspond to individual values of $i_{\rm B}$ or $e_{\rm B}$, while the black dash-dotted line indicates the reference case without a binary companion.
• Figure 5: Mean planetary inclinations. For each $a_{\rm B}$, all $e_{\rm B}$ are considered. The control case without a binary (NB) is shown as a cross. Each panel corresponds to a different survival outcome: systems ending with a single planet (top), or with two or three planets remaining (bottom). Symbols denote the initial inclination, while the colored bar indicates the averaged attained planetary eccentricity.