Trajectory of the stellar flyby that shaped the outer solar system

TL;DR

This study identifies a specific stellar flyby scenario that explains the observed distribution and characteristics of trans-Neptunian objects in the outer solar system, suggesting such events are relatively common in our galaxy.

Contribution

It provides the first detailed match between observed TNO properties and a specific stellar flyby model, constraining the flyby's parameters and implications for solar system history.

Findings

A star of 0.8 solar masses passing at 110 au reproduces TNO distributions.

The flyby explains the retrograde TNO population.

Such flybys are likely common in the Milky Way.

Abstract

Unlike the Solar System planets, thousands of smaller bodies beyond Neptune orbit the Sun on eccentric ($e >$ 0.1) and ($i>$ 3$^\circ$) orbits. While migration of the giant planets during the early stages of Solar System evolution can induce substantial scattering of trans-Neptunian objects (TNO), this process cannot account for the small number of distant TNOs ($r_p >$ 60 au) outside the planets' reach. The alternative scenario of the close flyby of another star can instead produce all these TNO features simultaneously, but the possible parameter space for such an encounter is vast. Here, we compare observed TNO properties with thousands of flyby simulations to determine the specific properties of a flyby that reproduces all the different dynamical TNO populations, their location and their relative abundance and find that a 0.8$^{+0.1}_{-0.1}$ $M_{\odot}$ star passing at a distance of $ r_p =$ 110 $\mathbf{\pm}$ 10 au, inclined by $i$ = 70$^\circ$ $^{+5}_{-10}$ gives a near-perfect match. This flyby also replicates the retrograde TNO population, which has proved difficult to explain. Such a flyby is reasonably frequent; at least 140 million solar-type stars in the Milky Way are likely to have experienced a similar one. In light of these results, we predict that the upcoming Vera Rubin telescope will reveal that distant and retrograde TNOs are relatively common.