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Solar System Dynamics and the Kuiper Belt
Solar System Dynamics and the Kuiper Belt
Observational campaigns over the last decades have revealed that a subset of icy bodies orbiting the Sun beyond Neptune exhibit an unexpected orbital architecture. Namely, it has been found that such small bodies – known as detached trans-Neptunian objects (TNOs) – orbit the Sun on highly-elongated paths which appear to be spatially clustered. Such a configuration can not be explained by the current eight-planet solar system architecture, challenging our understanding of the solar system. This has led to the so-called "Planet Nine" hypothesis which suggests the existence of an as-yet undiscovered super-Earth planet in the distant reaches of the solar system as an explanation for the puzzling orbits of TNOs.
In Sefilian & Touma (2019), we presented an alternative to the "Planet Nine" hypothesis. We demonstrated that a (relatively) massive disk of trans-Neptunian objects, lying in the same plane as the giant planets, could be responsible for the observed clustering of trans-Neptunian orbits. In particular, we showed that the gravitational effects of such a disk can effectively counteract the differential precession driven by the giant planets, and in the process shepherd members of its population onto highly elliptical orbits with roughly fixed orbital orientations. In short, this shepherding disk hypothesis faithfully reproduces key orbital properties of the puzzling TNO population, and could obviate the need for Planet Nine altogether.
If interested, you can click here to read more about this in the media, including a featured article by AAS Nova as well as press releases by the University of Cambridge and the American University of Beirut. I have also written a general audience article on this subject, published in The Scholar Magazine (Gates Cambridge Trust, UK).
The stationary orbits, plotted in the orbital semimajor axis-eccentricity space, that are sustained by the combined gravity of an extended massive (10 Earth mass) trans-Neptunian disk and the four giant planets in the Solar System. The stable high-eccentricity family of clustered orbits (blue inverted triangles) follows quite closely the observed trend of the seven clustered TNOs considered in our study in 2019 (yellow diamond symbols). Figure taken from Sefilian & Touma (2019).
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