Asteroids within the Sun-Earth Lagrange points should be in stable orbits with reason to believe that they would be doing anything other than leading or following us in orbit.
http://www.december212012.com/articles/PlanetX_Nibiru/NASA_AND_PLANET_X.htm
If it is planet X or the remains of it or a body associated with it, the orbit is elliptical and orbital plane is under our earth, an area of space we have not surveilled until fairly recently