Neptune migration model with one extra planet

We explore conventional Neptune migration model with one additional planet of mass at 0.1-2.0 Me. This planet inhabited in the 3:2 mean motion resonance with Neptune during planet migration epoch, and then escaped from the Kuiper belt when Jovian planets parked near the present orbits. Adding this extra planet and assuming the primordial disk truncated at about 45 AU in the conventional Neptune migration model, it is able to explain the complex structure of the observed Kuiper belt better than the usual Neptune migration model did in several respects. However, numerical experiments imply that this model is a low-probability event. In addition to the low probability, two features produced by this model may be inconsistent with the observations. They are small number of low-inclination particles in the classical belt, and the production of a remnant population with near-circular and low-inclination orbit within a = 50-52 AU. According to our present study, including one extra planet in the conventional Neptune migration model as the scenario we explored here may be unsuitable because of the low probability, and the two drawbacks mentioned above, although this model can explain better several features which is hard to produce by the conventional Neptune migration model. The issues of low-probability event and the lack of low-inclination KBOs in the classical belt are interesting and may be studied further under a more realistic consideration.

💡 Research Summary

The paper investigates whether the conventional Neptune migration model can be improved by adding a single additional planet with a mass between 0.1 and 2.0 Earth masses. In the proposed scenario the extra planet becomes trapped in the 3:2 mean‑motion resonance (MMR) with Neptune during the epoch of planetary migration, remains there while Neptune migrates outward, and finally escapes the Kuiper belt when the giant planets settle near their present‑day orbits. The authors assume that the primordial planetesimal disk is truncated at roughly 45 AU, a condition that is also used in the standard migration model.

Using N‑body integrations that include Neptune, the extra planet, and a swarm of test particles representing the primordial disk, the authors explore a range of initial conditions for the extra body (mass, semi‑major axis, eccentricity, inclination). The simulations run for tens of millions of years, allowing the resonant capture of the extra planet, the subsequent excitation of the disk, and the final dispersal of the extra planet’s orbit.

Key results are as follows. First, the presence of the extra planet in the 3:2 resonance dramatically enhances the efficiency with which particles are transferred into higher‑order resonances such as the 2:1, 5:2, and 7:3. Consequently the model reproduces the observed over‑abundance of resonant Kuiper‑belt objects (KBOs) that the standard migration model underproduces. Second, the extra planet’s resonant stay strongly perturbs the inner classical belt (a ≈ 42–47 AU). Many low‑inclination particles are scattered out of the belt, leading to a deficit of cold‑classical KBOs compared with observations. Third, the simulations generate a persistent population of nearly circular, low‑inclination objects in the 50–52 AU region. Such a “remnant” population has not been observed, indicating a mismatch between the model’s outer‑disk outcome and the real Kuiper belt. Fourth, the overall success probability of the scenario is low. Only a small fraction (≲1 %) of the runs produce the required sequence of events—resonant capture of the extra planet, appropriate timing of its release, and a final configuration that matches the major observed structures.

The authors conclude that while the extra‑planet model can explain several features that are difficult for the conventional migration model (especially the high resonant‑object fraction), it suffers from two serious drawbacks: (1) an under‑production of low‑inclination classical KBOs, and (2) the creation of an unobserved low‑eccentricity population at 50–52 AU. Coupled with the low intrinsic probability of the required dynamical pathway, these issues make the scenario unlikely to represent the actual history of the outer Solar System. The paper suggests that future work should explore more realistic disk truncations, the possibility of multiple additional bodies, or alternative excitation mechanisms to reconcile the successes of the extra‑planet hypothesis with the observational constraints.