  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 subsolar mass black holes and
potentially improving upon current limits by orders of magnitude.
