Last year, scientists published a paper with an iconic diagram: a black circle representing, to scale, a 5-solar-mass black hole—and the size of the hypothetical Planet Nine if it were a black hole instead of a planet. So, if Planet Nine really is a black hole, how would we find it?
https://gizmodo.com/say-there-were-a-black-hole-in-our-solar-system-how-wo-1843204014