Very-wide-orbit planets from dynamical instabilities during the stellar birth cluster phase

TL;DR

This study demonstrates through simulations that very wide-orbit planets can naturally form during the early stellar cluster phase due to dynamical instabilities and stellar flybys, explaining observed distant planets.

Contribution

The paper introduces a new formation mechanism for wide-orbit planets involving early dynamical instabilities and stellar flybys, supported by numerical simulations.

Findings

Wide-orbit planets form via early dynamical instabilities.

Stellar flybys stabilize eccentric wide orbits.

Estimated occurrence rate of such planets is at least 10^-3 per star.

Abstract

Gas giant planets have been detected on eccentric orbits several hundreds of astronomical units in size around other stars. It has been proposed that even the Sun hosts a wide-orbit planet of 5-10 Earth masses, often called Planet Nine, which influences the dynamics of distant Trans-Neptunian objects. However, the formation mechanism of such planets remains uncertain. Here we use numerical simulations to show that very wide-orbit planets are a natural byproduct of dynamical instabilities that occur in planetary systems while their host stars are still embedded in natal stellar clusters. A planet is first brought to an eccentric orbit with an apoastron of several hundred au by repeated gravitational scattering by other planets, then perturbations from nearby stellar flybys stabilise the orbit by decoupling the planet from the interaction with the inner system. In our Solar System, the two main events likely conducive to planetary scattering were the growth of Uranus and Neptune, and the giant planets instability. We estimate a 5-10% likelihood of creating a very wide-orbit planet if either happened while the Sun was still in its birth cluster, rising to 40% if both were. In our simulated exoplanetary systems, the trapping efficiency is 1-5\%. Our results imply that planets on wide, eccentric orbits occur at least $10^{-3}$ per star.