Planet Nine, is it real and why hasn't anyone ever seen it?
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infoterkiniviral.com
The origin of Planet Nine can be traced back to the early 20th century, when Percival Lowell made groundbreaking contributions in this field
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I thought more recent studies have placed the missing mass as the collective mass of the Oort Cloud, especially as the plutinos have been found to have wildly different orbits than the inner rocky planets and the gas giants.