Massive trans-Neptunian disc: an alternative to Planet Nine
Observational campaigns, concurrently with the discovery of exoplanets over the last two decades, have revealed a new mystery related to our own Solar System. Namely, observations of remnant icy bodies orbiting the Sun beyond Neptune have unveiled an unexpected orbital structure. As it turns out, these small bodies – known as trans-Neptunian objects (TNOs) – exhibit clustering in the orientation of their orbits. Such spatially aligned states are anomalous as test-particles under the quadrupole action of the giant planets undergo differential precession, and the orientation of their orbits randomizes over relatively short period of time. As such, the problem presented by observations can not be explained by the current eight-planet Solar System architecture. This has led to the so-called "Planet Nine” hypothesis that a yet undiscovered super-Earth resides in the distant Solar System inducing, and maintaining, the observed configurations.
Here, we provide an alternative mechanism. We show that a relatively massive and moderately eccentric disc of trans-Neptunian objects can effectively counteract the quadrupole action of the giant planets. In the process, such a disc shepherds highly eccentric members of its population into nearly-stationary configurations that faithfully reproduce key orbital properties of the observed anomalous TNO population. Our shepherding disc hypothesis could obviate the need for Planet Nine.
Our results are described in: Sefilian, A. A., & Touma, J. R. 2018, AJ (in revision), arXiv:1804.06859