Maybe a Sailor Planet X? In the 90's, before the Kuiper Belt was discovered (that wouldn't happen until the early 2000's), there were still astronomers who thought there might be another planet beyond Pluto. This planet was nicknamed "Planet X," a title formerly given to Pluto until Pluto's discovery in the 1920s.
Pluto was thought to be causing irregularities in Neptune's orbit, until it was found to be far too small, leading astronomers to believe that another gas giant planet could be even further out. However, more recently it was calculated that Neptune and Uranus affect each other's orbit, so the need for Planet X was nixed.
Until recently.
The Kuiper Belt was found to end abruptly, leading some to think that there could still be another planet out there clearing the edge of debris. There may also be multiple planets within the Kuiper Belt as large as Mars or Earth that are, for now, too far away to detect. The Oort Cloud, a sphere of comets even more distant than the Kuiper Belt, may also have planets ejected from the Solar System during its early formation.
So taking that into account, there could be dozens of Outer Senshi.
