Planet X is a theoretical trans-Neptunian planet in the Terran Solar System whose existence would explain irregularities in the orbit of Neptune. In Ultima II, the mysterious planet is shown to exist in the twenty-second century of Earth's history as impacted by the enchantress Minax.
Within the history forged by Minax's interventions on Earth, Planet X was a reality. The planet, while still unknown to the majority of terrestrial civilization, had been located by a number of alleged time-travelers dwelling in the prehistorical Pangea. The Stranger, requiring the blessing of the fabled Father Antos, found it necessary to explore this strange new world. It was possible and necessary to visit Planet X (located at coordinates 9, 9, 9) using a rocket ship, which could be acquired at the post-apocalyptic Soviet settlement of Pirates Harbour.
The hero found it to be a planet much like Earth, home to continents and oceans, and inhabited by beings who dwelt in the settlements of Towne Basko and Castle Barataria, under the rule of the enigmatic King Ozymandias.
It is unknown whether a Planet X exists in the universe unaffected by Minax's machinations, but Father Antos later would appear on the world of Britannia, indicating that he, if nothing else, managed to return to Sosaria/Britannia.
Planet X was first named by astronomer Percival Lowell, a historical character whom the Avatar eventually had the privilege of meeting in Martian Dreams. Certain that a ninth planet existed, Lowell built an observatory in Flagstaff (Arizona) and began to look extensively for the theoretical planet. While he failed in this endeavor, Lowell's observations eventually led to the discovery of Pluto, a large dwarf-planet with an orbit beyond Neptune. In contemporary astronomy, the theory of Planet X has been largely discarded, given new observations which would account for the variations in the outer planets' orbits.