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A 'Planet Nine' far from Earth could explain the odd behaviour of icy bodies beyond Neptune



Astronomers struggle to explain all of the properties of Trans-Neptunian objects (TNOs), which are generally small, icy bodies that orbit the sun beyond the orbit of Neptune, generally with orbits greater than 30 astronomical units (AU). (One AU is the average distance between Earth and the sun: about 93 million miles, or 150 million kilometres.)


TNOs represent the leftover bits from the formation of the solar system that flung to the outer edges of the system during the chaotic early years of planetary creation. Due to the extreme difficulty of discovering and mapping such small, distant and dim objects, the known TNOs (which represent a very small sample of all of the objects orbiting at that distance) have some strange properties that currently defy coherent and consistent explanation.


For one, about 10% of all TNOs are "detached," meaning they have no orbital relationship with Neptune. Even though Neptune is the smallest of the giant planets, it is the dominant gravitational force in the outermost solar system. For so many TNOs to lack movement controlled or influenced by Neptune is strange. Second, there is a large population of TNOs — around a few percent — that have very high orbital inclinations, meaning they orbit the sun at an angle greater than 45 degrees.


Next, there are some TNOs with very large orbits that are simply known as "extreme." The most famous example of these is Sedna, a minor planet whose closest approach to the sun is around 76 AU and over 900 AU away at its farthest point. Lastly, some TNOs are locked into orbital resonances with Neptune, which means that for every whole number of Neptune orbits, they complete a single orbit. Some of these orbits have been stable for billions of years, despite the gravitational nudging that others in the TNO population have received.


The researchers found that a single Earth-scale planet, weighing somewhere between 1.5 and three times Earth's mass, may lurk in the outer solar system, living at an orbit between 250 and 500 AU, within the Kuiper Belt.


This hypothetical large planet, if placed on an orbit with a 45-degree tilt, could explain the known collective behaviour of TNOs: how some of them are detached from Neptune while others are stuck in resonances, the large angles of some of their orbits, and the extreme nature of a small population of TNOs.




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