New evidence points toward Ninth Planet in the solar system

[Gawdzilla]

It could have formed by collection of material.

I think the current view on solar system formation is the original debris disk around the sun is too sparse beyond the orbit of Neptune to support the accretion in situ of a planet sized body, which is why only Pluto sized objects or smaller are expected to form there.

Any larger planet sized object would have had to form closer in the solar system.  If any are in the Keiper belt now, it would have to have been moved out there through gravitational interaction.   Unless the body had multiple chance encounters, any single major gravitation interaction should leave the perihelion close to where it had been.  So this would mean the perihelion of a planet thrown out into the Keiper belt should still be reasonably close to may be the orbit of Pluto.

We've been wrong before.