Effect of flyby. (a) The preflyby color gradient in the simulated disk is depicted by a false color scheme representing very red to blue–gray TNOs. The same color scheme is used throughout the Letter. (b) Simulation snapshot of the flyby 128 yr after periastron passage. The perturber star approached from the bottom right and already left the shown area. Disk matter is transported inward and outward along the spiral arms, with a fraction of the test particles injected into the planet region. Only the particles remaining bound to the Sun are shown. The movie shows how the flyby induces the formation of spiral arms, eventually leading to observed color mixing within the disk. The disk has initially a radius of 150 au. The movie covers the time up to 12,000 yr after periastron passage.

(An animation of this figure is available in the online article.)

(An animation of this figure is available.)

The video/animation of this figure is available in the online journal.