A hypothetical Planet Nine in the distant Kuiper Belt could explain several observed oddities
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The trans-Neptunian objects (TNOs) are icy bodies orbiting in the dark, outer recesses of the Solar System beyond the orbit of Neptune. In recent years, astronomers have noticed a number of oddities about the orbits of some TNOs, which cannot easily be explained by any single current model of Solar System development.
For example, there are a significant number of TNOs with high orbital inclinations, but they lie too far out to have been gravitationally nudged by Neptune. Theories such as the close pass of a rogue planet are also unable to properly explain their presence. Additionally, several ‘extreme’ TNOs have been discovered on exceptionally elliptical orbits at extreme distances from the Sun – such as the dwarf planet Sedna, which has a perihelion distance of 76 AU (where one AU is the distance between the Sun and Earth) and travels a staggering 937 AU out for its aphelion. It’s difficult to explain how these worlds became perturbed into such elongated orbits.
One proposed explanation, although currently still pretty controversial, is that such mysteries with the TNOs could be due to the presence of a large, as-yet undiscovered planet in the Kuiper Belt, referred to by some as Planet Nine. Patryk Sofia Lykawka and Takashi Ito, at Kindai University and the National Astronomical Observatory of Japan, respectively, have been using computer models of the outer Solar System to study how such a hypothetical Kuiper Belt planet might affect the orbital structure of TNOs.
They found that a roughly Earth-sized planet, 1.5–3 times the mass of our planet, on an elliptical orbit with a perihelion of about 200 AU and inclined out of the plane of the Solar System by around 30°, could explain some of these weird properties of the outer Kuiper Belt. What’s more, the presence of such a planet is also compatible w
