The most likely theories about where ‘Planet Nine’ came from are still pretty crazy

Washington Post 2 hours 54 minutes ago 

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At the start of the new year, scientists announced the best-ever evidence of a ninth planet. Not a vindicated Pluto, but a "massive perturber" — a previously unknown world lurking at the outer edge of our solar system. "Planet Nine" has yet to be truly discovered. No one has seen it with a telescope. Instead, its existence is implied by the orbital behavior of a cluster of small, icy dwarf worlds far, far away. But while some scientists are asking where Planet Nine is, others are asking how it got there — and running models to check the probability of their theories. Researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) may have a few clues. The institution has a paper on the ...