1- There is the historical data, brought to light primarily by Zacharia Sitchin, who using historical texts, artifacts, drawings and cylinder seals, propounded the theory that a planet called Nibiru is the 10th planet of the solar system, with an elliptical orbit that only brings it home every 3600 years.
2 - There are scientific observations of a large Kuiper Belt object that has been perturbing the orbits of Neptune and Uranus. A search ensued from about the 1840's to discover an object that could explain this gravitational phenomenon (Planet X). Pluto was dicovered in 1930, but it was concluded that it's gravitational influence was too weak to explain the anomoly. In the 1980s, this search was lead by NASA and some papers have been published and others have been witheld. There are also some scientific and astronomic suggestions that the Earth has a dark twin (Nemesis), a brown dwarf or failed star, whose movement might account for the long period comets that swirl into Earth from the Oort Cloud. If the dark star exists, as a companion to our Sun, then it could be conceivable that a planet may orbit in a ellipse around both of them.