Misalignment of terrestrial circumbinary planets as an indicator of their formation mechanism

TL;DR

This study uses N-body simulations to explore how the initial misalignment of circumbinary disks influences the formation and final orientation of terrestrial planets around binary stars, revealing conditions that favor coplanar or polar configurations.

Contribution

It demonstrates that terrestrial planets form close to coplanar or polar orientations depending on binary eccentricity and initial disk misalignment, highlighting the role of mergers and gas disks in planet alignment.

Findings

Planets around circular binaries tend to be coplanar.

Planets around eccentric binaries can be coplanar or polar.

Larger planets are more aligned due to mergers.

Abstract

Circumbinary gas disks are often observed to be misaligned to the binary orbit suggesting that planet formation may proceed in a misaligned disk. With N-body simulations we consider the formation of circumbinary terrestrial planets from a particle disk that is initially misaligned. We find that if terrestrial planets form in this way, in the absence of gas, they can only form close to coplanar or close to polar to the binary orbit. Planets around a circular binary form coplanar while planets around an eccentric binary can form coplanar or polar depending on the initial disk misalignment and the binary eccentricity. The more massive a terrestrial planet is, the more aligned it is (to coplanar or polar) since it has undergone more mergers that lead on average to smaller misalignment angles. Nodal precession of particle disks with very large initial inclinations lead to high mutual inclinations between the particles. This produces high relative velocities between particles that leads to mass ejections that can completely inhibit planet formation. Misaligned terrestrial circumbinary planets may be able to form in the presence of a misaligned circumbinary gas disk that may help to nodally align the particle orbits and maintain the inclination of the planets during their formation.