Can Neptune's Distant Mean-Motion Resonances Constrain Undiscovered Planets in the Solar System? Lessons from a Case Study of the 9:1

Recent observational surveys of the outer Solar System provide evidence that Neptune's distant n:1 mean-motion resonances may harbor relatively large reservoirs of trans-Neptunian objects (TNOs). In particular, the discovery of two securely classified 9:1 resonators, 2015 KE_172 and 2007 TC_434, by the Outer Solar System Origins Survey is consistent with a population of order 10^4 such objects in the 9:1 resonance with absolute magnitude H_r < 8.66. This work investigates whether the long-term stability of such populations in Neptune's n:1 resonances can be used to constrain the existence of distant 5-10M_⊕ planets orbiting at hundreds of AU. The existence of such a planet has been proposed to explain a reported clustering in the orbits of highly eccentric "extreme" trans-Neptunian objects (eTNOs), although this hypothesis remains controversial. We engage in a focused computational case-study of the 9:1 resonance, generating synthetic populations and integrating them for 1 Gyr in the presence of 81 different test planets with various masses, perihelion distances, eccentricities, and inclinations. While none of the tested planets are incompatible with the existence of 9:1 resonators, our integrations shed light on the character of the interaction between such planets and nearby n:1 resonances, and we use this knowledge to construct a simple, heuristic method for determining whether or not a given planet could destabilize a given resonant population. We apply this method to the currently estimated properties of Planet 9, and find that a large primordial population in the 15:1 resonance (or beyond), if discovered in the future, could potentially constrain the existence of this planet.