Extrasolar Binary Planets I: Formation by tidal capture during planet-planet scattering

TL;DR

This study explores how binary planets can form through tidal capture during planet-planet scattering and examines their long-term orbital evolution, suggesting they could be detectable via transit observations beyond 0.3AU.

Contribution

It presents the first detailed analysis of binary planet formation via tidal capture and their subsequent orbital evolution, combining N-body simulations with analytical tidal evolution models.

Findings

Binary planets form in about 10% of systems with orbital crossing.

Typical binary separations are a few times the planetary radii after circularization.

Binary planets can survive for billions of years if beyond 0.3AU from their star.

Abstract

We have investigated i) the formation of gravitationally bounded pairs of gas-giant planets (which we call "binary planets") from capturing each other through planet-planet dynamical tide during their close encounters and ii) the following long-term orbital evolution due to planet-planet and planet-star {\it quasi-static} tides. For the initial evolution in phase i), we carried out N-body simulations of the systems consisting of three jupiter-mass planets taking into account the dynamical tide. The formation rate of the binary planets is as much as 10% of the systems that undergo orbital crossing and this fraction is almost independent of the initial stellarcentric semi-major axes of the planets, while ejection and merging rates sensitively depend on the semi-major axes. As a result of circularization by the planet-planet dynamical tide, typical binary separations are a few times the sum of the physical radii of the planets. After the orbital circularization, the evolution of the binary system is governed by long-term quasi-static tide. We analytically calculated the quasi-static tidal evolution in later phase ii). The binary planets first enter the spin-orbit synchronous state by the planet-planet tide. The planet-star tide removes angular momentum of the binary motion, eventually resulting in a collision between the planets. However, we found that the binary planets survive the tidal decay for main-sequence life time of solar-type stars (~10Gyrs), if the binary planets are beyond ~0.3 AU from the central stars. These results suggest that the binary planets can be detected by transit observations at >0.3AU.