There's a fairly mad theory that planet 9 (which probably does not exist) is a primordial black hole, in this case meaning an object far too light to have formed from a collapsed star. I suspect primordial black holes may not exist, and we've already done measurements which show they are at least scarce if they do. Well, here's an experiment which will tell us if any exist in the outer Solar system. In the likely case that it finds none we have another data point which puts an upper bound on ...