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The general theory was called planet X before pluto was demoted; X just means 10. https://science.nasa.gov/solar-system/planet-x/ https://science.nasa.gov/sola

• lamontcg 3y ago
  Except you and that article are quite wrong and the naming of the "Planet X" theory happened in 1894 long before the discovery of Pluto in 1930, back when astronomers also thought there was only 8 planets: > In 1894, with the help of William Pickering, Percival Lowell (a wealthy Bostonian) founded the Lowell Observatory in Flagstaff, Arizona. In 1906, convinced he could resolve the conundrum of Uranus's orbit, he began an extensive project to search for a trans-Neptunian planet,[18] which he named Planet X, a name previously used by Gabriel Dallet.[11] The X in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for 10 (at the time, Planet X would have been the ninth planet). https://en.wikipedia.org/wiki/Planets_beyond_Neptune#Planet_X https://en.wikipedia.org/wiki/Planets_beyond_Neptune#Planet_...