[arthens]

Every point in the sky is on a possible orbit (or on infinite orbits?), so you are not really reducing the problem space.

Our planets' orbit do tend to be somehow aligned, so you could make the (possibly wrong) assumption that the 9th planet is also somehow aligned. This might reduce the search space by 50%+.... but even then you'd still be several orders of magnitude off by the precision needed to go looking for it.