[Paper Review] On the Possibility of Tidal Formation of Binary Planets Around Ordinary Stars

Philipp Podsiadlowski, S. Rappaport+2 more|arXiv (Cornell University)|Jul 8, 2010

Astro and Planetary Science10 citations

TL;DR

This paper investigates the tidal capture of two planets during close dynamical encounters as a viable mechanism for forming binary planets or brown dwarfs around ordinary stars. Using N-body simulations with the Fewbody integrator, the authors demonstrate that such systems can form under typical planetary masses and orbital configurations, offering a new pathway for binary planet formation that could be constrained by transit surveys like Kepler.

ABSTRACT

The planet formation process and subsequent planet migration may lead to configurations resulting in strong dynamical interactions among the various planets. Well-studied possible outcomes include collisions between planets, scattering events that eject one or more of the planets, and a collision of one or more of the planets with the parent star. In this work we consider one other possibility that has seemingly been overlooked in the various scattering calculations presented in the literature: the tidal capture of two planets which leads to the formation of a binary planet (or binary brown dwarf) in orbit about the parent star. We carry out extensive numerical simulations of such dynamical and tidal interactions to explore the parameter space for the formation of such binary planets. We show that tidal formation of binary planets is possible for typical planet masses and distances from the host star. The detection (or lack thereof) of planet-planet binaries can thus be used to constrain the properties of planetary systems, including their mutual spacing during formation, and the fraction of close planets in very eccentric orbits which are believed to form by a closely related process.

Motivation & Objective

• To explore whether tidal interactions during close planetary encounters can lead to the formation of stable binary planets or brown dwarfs.
• To assess the viability of tidal capture as an alternative to fission or scattering in forming such systems.
• To constrain planetary system properties—such as mutual spacing and eccentricity distribution—through the detection or non-detection of binary planets.
• To evaluate the dynamical stability and long-term evolution of potential binary planet systems, including mass transfer via Roche lobe overflow.
• To validate the Fewbody integrator against existing scattering simulations, ensuring accuracy in modeling close encounters.

Proposed method

• Employed the Fewbody N-body integrator, optimized for strong small-N gravitational encounters and adaptive, non-symplectic integration during close approaches.
• Conducted extensive numerical simulations of planet-planet scattering events with varying initial masses, eccentricities, inclinations, and semi-major axes.
• Used initial conditions matching those of Ford & Rasio (2008), including planet masses of 10⁻³ M☉ and stellar mass of 1 M☉, to ensure consistency and validation.
• Defined ejection and tidal capture based on orbital energy and binding criteria, distinguishing between unbound ejection and bound binary formation.
• Validated results against published eccentricity distributions from Ford & Rasio (2008), confirming agreement in median eccentricity and overall shape.
• Analyzed tidal energy dissipation and oscillation modes during close encounters to assess energy loss and binding potential.

Experimental results

Research questions

• RQ1 Can tidal capture during close planetary encounters lead to the formation of stable binary planets around ordinary stars?
• RQ2 What are the key orbital and mass parameters that favor tidal capture over ejection or collision?
• RQ3 How does the tidal capture mechanism compare in likelihood to other formation pathways such as fission or disk migration?
• RQ4 What observational signatures might binary planets produce, particularly in transit surveys like Kepler or CoRoT?
• RQ5 To what extent can the absence or presence of binary planets constrain the initial architecture of planetary systems?

Key findings

• Tidal capture of two planets during close encounters can lead to the formation of a stable binary planet system, even for gas giants or brown dwarfs.
• The simulations show that such systems can form under typical planetary masses (e.g., 10⁻³ M☉) and orbital separations, indicating a plausible formation pathway.
• The cumulative eccentricity distribution of surviving planets after ejection matches well with Ford & Rasio (2008), validating the Fewbody integrator’s accuracy in modeling close encounters.
• Tidal energy dissipation during pericenter passages converts orbital energy into oscillation and thermal energy, enabling stable binding post-encounter.
• Binary planet systems could exhibit long-term stability, with potential for mass transfer if Roche lobe contact is achieved, leading to long-lived planetary mass-transfer systems.
• The detection of eclipsing binary planets via transit surveys would provide strong constraints on early planetary system architecture, including initial mutual spacing and eccentricity distribution.