Thanks to mathematical modeling and computer simulations by CalTech researchers Konstantin Batygin and Mike Brown, we now have strong evidence that there’s a large planet — about 10x Earth’s size — orbiting in an extremely long orbit in our outer solar system. How long? Batygin and Brown estimate that one orbit takes between 10-20,000 years. They’ve nicknamed it Planet Nine.

Brown notes that the putative ninth planet—at 5,000 times the mass of Pluto—is sufficiently large that there should be no debate about whether it is a true planet. Unlike the class of smaller objects now known as dwarf planets, Planet Nine gravitationally dominates its neighborhood of the solar system. In fact, it dominates a region larger than any of the other known planets—a fact that Brown says makes it “the most planet-y of the planets in the whole solar system.”

…”Still, I was very skeptical,” says Batygin. “I had never seen anything like this in celestial mechanics.”

But little by little, as the researchers investigated additional features and consequences of the model, they became persuaded. “A good theory should not only explain things that you set out to explain. It should hopefully explain things that you didn’t set out to explain and make predictions that are testable,” says Batygin.

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