Irregular moons possibly injected from the outer solar system by a stellar flyby

TL;DR

This paper proposes that a stellar flyby could explain the origin of irregular moons and trans-Neptunian objects by injecting a fraction of TNOs into planetary regions, accounting for their orbits and color distributions.

Contribution

It introduces a new hypothesis that a stellar flyby can simultaneously explain irregular moons and TNO dynamics, linking their origins.

Findings

A stellar flyby can inject 7.2% of TNOs into the planetary region.

Most injected TNOs are ejected, but some are captured by planets.

The model explains the orbital and color characteristics of irregular moons.

Abstract

The irregular moons orbit the giant planets on distant, inclined, and eccentric trajectories, in sharp contrast with the coplanar and quasicircular orbits of the regular moons. The origin of these irregular moons is still an open question, but these moons have a lot in common with the objects beyond Neptune (trans-Neptunian objects -- TNOs), suggestive of a common origin. Here, we show that the close flyby of a star may be the connecting element. A stellar flyby can simultaneously reproduce the complex TNO dynamics quantitatively while explaining the origin of the irregular moons and the colour distributions of both populations. This flyby would have catapulted 7.2% of the original TNO population into the region of the planets, many on retrograde orbits. Most injected TNOs would have been subsequently ejected from the solar system (85%). However, a considerable fraction would have had the potential to be captured by the planets. The exclusively distant origin of the injected TNOs may also explain the lack of very red irregular moons.