Anomalous clustering of the apsides and orbit poles of distant Kuiper Belt objects have sparked serious suggestions of an unseen massive planet — a super-Earth or sub-Neptune — orbiting at a heliocentric distance of a few hundred astronomical units (Trujillo & Sheppard 2014, Nature, 507:471; Batygin & Brown 2016, AJ 151:22); however, these anomalous clusterings may simply be an artifact of observational biases (Shankman et al. 2017, AJ 154(2), id.50). Independent (though also only tentative) support for the unseen planet hypothesis lies in the near-resonant orbital periods of some of the most distant Kuiper belt objects (Malhotra, Volk & Wang 2017, AJ 824:L22). If such resonances can be more definitively identified, the orbital geometries and dynamics of resonant orbits can help constrain the orbital plane, the orbital eccentricity, and the mass of the unseen planet, and, most importantly, its current location in its orbital path. The sample of distant Kuiper belt objects that could be resonantly interacting with a distant planet has grown from six to twenty-three in the past four years. We will present an updated assessment of the resonance-based constraints on an unseen distant planet.
We gratefully acknowledge research funding from NSF (grant AST-1824869).