Resonant Trans-Neptunian Objects from Neptune Interactions in the Stellar Flyby Scenario

The resonant populations of Trans-Neptunian Objects (TNOs), particularly those in mean-motion resonances with Neptune, provide crucial insights into the early history of the Solar System, specifically the mechanisms that shaped the outer Solar System. Currently, the prevailing model suggests that Kuiper Belt Objects (KBOs) in resonance with Neptune result from resonant capture during planetary migration (e.g., Malhotra, 2019); however, this model requires an additional external force to account for TNOs with orbits far beyond the gravitational influence of the planets. We investigate an alternative scenario, where long-term interactions with Neptune after a potential stellar flyby produce resonant TNO populations (Pfalzner et al., 2024). Using N-body simulations on a compute cluster, we model 4.5 Gyr of orbital evolution of TNOs and the four giant planets, with and without a small primordial population of classical KBOs, considering two different primordial disk sizes (150 AU and 300 AU). A machine learning classifier, similar to the one proposed by Volk & Malhotra (2025), is employed to identify objects in the simulation data that are in mean-motion resonance (MMR) with Neptune. Our initial results indicate that interactions with Neptune quickly produce resonant orbits, including the 5:2 resonance, which is often underpopulated in other models. Unlike the current incomplete picture from astronomical observations, which suggests the 3:2 MMR as the most dense population due to proximity, simulations favour the more distant 2:1 MMR as densest. Additionally, the simulations reveal a few other significant denser populations for farther-out resonances that are affected by the primordial disk size. The inclusion of a small primordial population of classical TNOs with mutual interactions appears to sustain the resonant population over time, probably resulting from processes described in Punzo et al. (2014). Our findings have implications for understanding the mechanisms that shaped the Solar System and the origins of TNO populations.

References:

Malhotra, R. (2019): Resonant Kuiper belt objects: a review. Geosci. Lett. 6, 12. https://doi.org/10.1186/s40562-019-0142-2

Pfalzner, S., Govind, A. & Portegies Zwart, S. (2024): Trajectory of the stellar flyby that shaped the outer Solar System. Nat. Astron. 8, 1380-1386. https://doi.org/10.1038/s41550-024-02349-x

Volk, K. & Malhotra, R. (2025): Chapter Seven - Machine learning-assisted dynamical classification of trans-Neptunian objects. In: Machine Learning for Small Bodies in the Solar System. Edited by V. Carruba, E. Smirnov & D. Oszkiewicz. ISBN: 978-0-443-24770-5. Elsevier. https://doi.org/10.1016/B978-0-44-324770-5.00012-X

Punzo, D., Capuzzo-Dolcetta, R. & Portegies Zwart, S. (2014): The secular evolution of the Kuiper belt after a close stellar encounter. Mon. Not. R. Astron. Soc. 444, 3, 2808-2819. https://doi.org/10.1093/mnras/stu1650