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29 PLANETOLOGY MAY-JUNE 2014 its variants; myth astronomy. The Planet X hypothesis did in fact originate as a scientific need following the biggest success of celestial mechanics, namely the discovery of Neptune in 1846, based on the observed gravitational pertur- bations in Uranus' motion. The presence of Neptune did not seem to completely jus- tify the behaviour of Uranus, and the mo- tion of Neptune itself did not appear to