# Orbital Evolution of a Circumbinary Planet in a Gaseous Disk

## TL;DR

This paper proposes a new orbital evolution scenario for circumbinary planets, explaining their current positions near instability boundaries through outward migration driven by a balance of gravitational excitation and gas drag, supported by N-body simulations.

## Contribution

It introduces a novel outward transportation mechanism for circumbinary planets migrating from beyond the snow line to their observed close-in orbits.

## Key findings

- Planets more likely to stabilize just beyond unstable regions in less eccentric binaries.
- Orbital stability less sensitive to mass ratio than to eccentricity.
- Simulation results align with observed binary system data.

## Abstract

Sub-Jupiter classed circumbinary planets discovered in close-in binary systems have orbits just beyond the dynamically unstable region, which is determined by the eccentricity and mass ratio of the host binary stars. These planets are assumed to have formed beyond the snow line and migrated to the current orbits rather than forming in situ. We propose a scenario in which a planet formed beyond the snow line and migrated to the inner edge of the circumbinary disk, which was within the unstable area, and then moved to the current orbit through outward transportation. This outward transportation is driven by the balance of orbital excitation of the central stars inside the gravitationally unstable region and damping by the gas-drag force. We carried out N-body simulations with a dissipating circumbinary protoplanetary disk for binary systems with different eccentricities and mass ratios. Planets are more likely to achieve a stable orbit just beyond the unstable region in less eccentric binary systems. This result is not as sensitive to mass ratio as it is to eccentricity. These dependencies are consistent with the data from observed binary systems hosting circumbinary planets. We find CBPs' orbits close to the instability boundaries are explained by our orbital evolution scenario.