TNO colours provide new evidence for a past close flyby of another star to the Solar System

Thousands of small bodies, known as trans-Neptunian objects (TNOs), orbit the Sun beyond Neptune. TNOs are remnants of the planets' formation from a disc of gas and dust, so it is puzzling that they move mostly on eccentric orbits inclined to the planetary plane and show a complex red-to-grey colour distribution. A close stellar flyby can account for the TNOs' dynamics, but it is unclear if this can also explain the correlation between their colours and orbital characteristics. Assuming an initial red-to-grey colour gradient in the disc, our numerical study finds that the spiral arms induced by the stellar flyby simultaneously lead to the observed TNOs' colour patterns and orbital dynamics. The combined explanation of these TNO properties strengthens the evidence for a close flyby of another star to the young Solar System. Our study predicts that (1) small TNOs beyond 60 au will mostly be grey, and (2) retrograde TNOs will lack the colour most common to high-inclination TNOs. The anticipated TNO discoveries by the Vera Rubin telescope will be able to test these predictions. A confirmed flyby would allow us to reveal the chemical composition of the Solar System's primordial disc.