We all grew up learning that our sun has eight friends: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Now, researchers who believe that a ninth planet is actually hanging around our galactic neighborhood have a theory as to how it got there. The scientists think that the yet unseen ninth planet may be a “rogue,” which entered out solar system not at its inception, but long after the fact. 

The theory of Planet Nine was born out of the movements of objects in the Kuiper belt, which is a disc-shaped collection of mostly small objects that exists beyond Neptune. In early 2016, Caltech scientists studying the peculiar orbits of objects in the Kuiper belt came to the conclusion that the only reason many of the Kuiper belt objects would behave in such a manner is if a massive planet was actually pulling them with its gravity. The world would have to be on the order of 10 times the size of Earth for the math to work out.

As Space.com reports, a new paper by New Mexico State University undergrad James Vesper, with help from professor Paul Mason, accepts the theory that our solar system has a guest we didn’t know about, and suggests a scenario that explains its existence. After running over 150 computer simulations to test the idea, Vesper claims that the unseen planet may actually be a rogue world that was grabbed by our solar system as it floated aimlessly in space.

There are many, many rogue planets in space, and without a star to orbit they sometimes enter other solar system, either temporarily or for much longer stretches of time. In Vesper’s simulations, a rogue world coming into contact with our solar system ended up sticking around, either by ejecting another planet or simply by sliding in and making a neat little orbit of its own. Further research is ongoing, and astronomers think they’ll be able to either confirm or debunk the Planet Nine theory sometime this year.

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