Searching for Black Holes in the Outer Solar System with LSST

Planet Nine has been proposed to potentially be a black hole (BH) 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 Legacy Survey of Space and Time (LSST) observing program will be able to either rule out or confirm Planet Nine as a BH within a year. We also find that LSST could rule out or confirm the existence of trapped planet-mass BHs out to the edge of the Oort Cloud, indirectly probing the dark matter fraction in subsolar mass BHs and potentially improving upon current limits by orders of magnitude.