Instead of it being a 9th Planet (formed along with others as our solar system was created), it's perhaps just an enormous object (black hole?!) whose gravitation causes the sun to tilt ?
Interestingly enough, the new IAU definition of "planet" makes no reference to origin:
A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
Whether a black hole could be considered a planet would hinge, I believe, primarily on b: Arguably, any black hole has sufficient mass to overcome rigid body forces, but does the notion of hydrostatic equilibrium apply?
More broadly, the definition of planet would allow for bodies of extra-solar origin captured by the sun....
I'm going to assume both of these options have been considered and ruled out. Also, the article mentions that this Planet 9 explains the orbits of Kuiper belt objects as well.
A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
Whether a black hole could be considered a planet would hinge, I believe, primarily on b: Arguably, any black hole has sufficient mass to overcome rigid body forces, but does the notion of hydrostatic equilibrium apply?
More broadly, the definition of planet would allow for bodies of extra-solar origin captured by the sun....