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2019-03-05. There’s probably another planet in our solar system.

posted Mar 9, 2019, 12:19 PM by Alan Gould
By MIT Technology Review. [https://www.technologyreview.com/s/613060/theres-probably-another-planet-in-our-solar-system/] For GSS A Changing Cosmos chapter 7. Excerpt: When it comes to exploring the solar system, astronomers have an embarrassing secret. Despite 400 years of stargazing, they have discovered only two large objects that would have been unknown to the ancients: Uranus in 1781 and Neptune in 1846. That’s not for lack of trying. The possibility of an unknown planet just beyond observational reach has attracted astronomers like moths to a flame. A few have been successful. Several astronomers together discovered Neptune after noticing that the other planets were being gravitationally nudged by an unknown mass. Neptune didn’t entirely resolve these discrepancies, and the hunt continued into the 20th century, culminating in Pluto’s discovery in 1930. But Pluto turned out to be so small that it couldn’t account for the nudging. Indeed, it was later humiliatingly demoted to a “dwarf planet.” ...