Exoplanets Bouncing Between Binary Stars (1201.6582v1)

Abstract: Exoplanetary systems are found not only among single stars, but also binaries of widely varying parameters. Binaries with separations of 100--1000 au are prevalent in the Solar neighborhood; at these separations planet formation around a binary member may largely proceed as if around a single star. During the early dynamical evolution of a planetary system, planet--planet scattering can eject planets from a star's grasp. In a binary, the motion of a planet ejected from one star has effectively entered a restricted three-body system consisting of itself and the two stars, and the equations of motion of the three body problem will apply as long as the ejected planet remains far from the remaining planets. Depending on its energy, escape from the binary as a whole may be impossible or delayed until the three-body approximation breaks down, and further close interactions with its planetary siblings boost its energy when it passes close to its parent star. Until then this planet may be able to transition from the space around one star to the other, and chaotically `bounce' back and forth. In this paper we directly simulate scattering planetary systems that are around one member of a circular binary, and quantify the frequency of bouncing in scattered planets. We find that a great majority (70 to 85 per cent) of ejected planets will pass at least once through the space of it's host's binary companion, and depending on the binary parameters about 45 to 75 per cent will begin bouncing. The time spent bouncing is roughly log-normally distributed with a peak at about $10^4$ years, with only a small percentage bouncing for more than a Myr. This process may perturb and possibly incite instability among existing planets around the companion star. In rare cases, the presence of multiple planets orbiting both stars may cause post-bouncing capture or planetary swapping.

• The paper demonstrates that 70–85% of ejected exoplanets traverse the companion star's space, highlighting a common dynamical process.
• It reveals that 45–75% of these planets exhibit a bouncing behavior, with durations peaking at about 10⁴ years before escaping or stabilizing.
• The research enhances our understanding of gravitational interactions in binaries and informs future strategies for observing such complex systems.

Dynamics of Exoplanets in Binary Star Systems

The paper titled "Exoplanets Bouncing Between Binary Stars" by Moeckel and Veras investigates the complex dynamics of exoplanets in binary star systems. Through numerical simulations, this research explores how planets formed around one star of a binary system can be influenced by their gravitational environment, particularly through the mechanism of planet-planet scattering.

Overview of the Research

Binary star systems are not atypical in our galaxy, and a good number of these systems host exoplanets. This paper focuses on binaries with separations of 100–1000 astronomical units (AU), which are commonly found in the Solar neighborhood. At these distances, planet formation around one member of the binary can proceed similarly to planet formation around a single star. However, during the dynamic evolution of these planetary systems, interactions like planet-planet scattering can result in the ejection of planets. The research employs direct simulations of planetary systems around one member of a binary and observes that ejected planets can bounce back and forth between the two stars.

The simulations reveal that a large fraction (70-85%) of ejected planets pass through the companion star's space at least once. Between 45% and 75% of these planets begin a bouncing behavior, transitioning multiple times between the stars. The duration of this bouncing is approximately log-normally distributed, with a peak at about 10^4 years, and only a few cases extending beyond a million years.

Key Numerical Results and Claims

• Occurrence of Bouncing: A significant majority of ejected planets bounce between stars, an insight that elucidates a common dynamical process in such systems.
• Timescale: Most planets spend a relatively brief period, peaking around 10^4 years, in bouncing before either escaping the binary system or stabilizing.
• Binary Parameters Influence: The paper finds that binary separation and stellar mass ratios significantly affect the dynamics, particularly the likelihood and duration of the chaotic bouncing phase.
• Energy and Motion Interpretation: Utilizing the circular restricted three-body problem (CR3BP) dynamics, the paper analogizes the planet's motion to test particles, explaining the analytical structure governing these transitions.

Implications

The implications of this work are manifold, spanning both theoretical and observational aspects. Theoretically, the paper expands our understanding of gravitational interactions in multi-body systems, particularly in explaining the orbital distributions and longevities of planets in binaries. Practically, the insights could inform future observations of exoplanetary systems, aiding in the recognition of planets that have transitioned between binary stars.

Furthermore, these dynamics could elucidate potential pathways for planets to be captured and maintained in orbits around binary companions under certain conditions, especially when considering dissipative forces like disc interactions.

Prospects for Future Research

The findings invite several avenues for future exploration:

1. Impact of Binary Eccentricity: Future studies could extend these findings to include variations in binary eccentricity, which is expected to significantly affect planet stability and dynamics.
2. Role of Discs and Dissipative Forces: Exploring how gaseous or planetesimal disks around stars in binaries could influence the capture and stabilization of bouncing planets.
3. Stellar Evolution Impacts: The long-term secular evolution of binary systems, especially when one or both stars evolve off the main sequence, could also alter the dynamics observed in this work.
4. Observational Validation: Developing observational strategies to identify and confirm planets engaged in this bouncing behavior could be a transformative step in exoplanetary science.

In summation, this paper provides a rigorous methodological approach and offers new perspectives on the dynamical behaviors of exoplanets in binary systems, advancing our broader understanding of the complex gravitational environments in which planets exist.