Planet Nine has been proposed to potentially be a black hole in the outer solar system. We
investigate the accretion flares that would result from impacts of small Oort cloud objects,
and find that the upcoming LSST observing program will be able to either rule out or confirm
Planet Nine as a black hole within a year. We also find that LSST could rule out or confirm
the existence of trapped planet-mass black holes out to the edge of the Oort cloud, indirectly
probing the dark matter fraction in primordial black holes and potentially improving upon …