Irregular Moons Possibly Injected from the Outer Solar System by a Stellar Flyby

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 color 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.