Trajectory of the stellar flyby that shaped the outer Solar System

Unlike the Solar System planets, thousands of smaller bodies beyond Neptune orbit the Sun on eccentric (e > 0.1 and i > 3°) orbits. While migration of the giant planets during the early stages of Solar System evolution could have induced substantial scattering of trans-Neptunian objects (TNOs), this process cannot account for the small number of distant TNOs (rp > 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 locations and their relative abundances, and find that a $$0.{8}_{-0.1}^{+0.1}\,{M}_{\odot }$$ star passing at a distance of rp = 110 ± 10 au, inclined by i = 70° $${\,}_{-10}^{+5}$$ , 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. The rocky disk surrounding the young Sun may have experienced a close flyby of another star. Simulations show that a highly inclined flyby of a star slightly smaller than the Sun at 100 au almost perfectly reproduces the orbits of the numerous small objects beyond Neptune.