I have been wondering about rogue planets, size of planet Mars and bigger. I don't expect planets of size Ceres * (1000 km) to much of a problem.
Noting that our solar system has most of the known planets in the 0 degree to 2 degree angle, I don't expect our chances of of detecting rogue planet entering our solar system at 15 digress to 25 digress much. But I am still open for a debate about that issue, as I do not know how surveillance for such object works or is in place to start with.
I fear the risk of rogue planets to fall into the solar system gravity well, if such environment gets created by forces outside our solar system. But as such I don't expect rouge planet to necessary fall into the solar system at 0 degree to 2 degree plane, as our most of the solar system planets are.
* I consider everything a planet if it meets the following. Is a spear and is 1000 km or lager, or has it's own gravitational pull. Some planets might fall outside the 1000 km size limit due to there density.