Planetary and moon mass black holes are ruled out by gravitational microlensing surveys. Microlensing puts an upper bound on the mass of primordial black holes at ~1/5 the mass of Ceres.

These surveys assume that primordial black holes are distributed uniformly across galaxies, but this may not be the case if they form small dark globular clusters in the outskirts of galaxies. By small, it is meant that with a total mass of 100–1000 M, they would produce no significant lensing, and by dark, that such clusters would consist entirely of black holes.