Determine the circularization mechanism of Planet 9’s orbit

Determine the physical mechanism that circularized the hypothesized Planet 9’s initially highly eccentric orbit to an approximately near-circular orbit at 400–800 astronomical units from the Sun, given simulations indicating initial scattering during giant planet migration.

Background

The paper discusses the hypothesized Planet 9 (also called Planet X), a Neptune-sized body with a mass of 5–10 Earth masses thought to have been scattered during the early solar system’s giant planet migration. Simulations suggest Planet 9 may have started on a highly eccentric orbit and later became circularized to a semimajor axis of roughly 400–800 AU.

The authors highlight that the specific process responsible for this orbital circularization is not identified, explicitly noting that it is unknown. They also suggest that over gigayear timescales, the resultant gravitational force of the Milky Way (largely due to dark matter) could significantly alter distant solar system trajectories, motivating reconsideration of Planet 9’s long-term dynamics.