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This Q came up while talking about black holes with family: What if an asteroid was heading towards the Earth, but it was so high density that it's a tiny black hole so we wouldn't see it with telescopes? Also what does it look like when a micro black hole asteroid hits a planet?
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The Chandrasekhar limit for white dwarfs is 1.5 M☉, any greater black hole / neutron stars can form. Asteroids vary in size, but plugging it into Schwarzschild radius equation gives us a size of r_s = 3GM☉/c^2 so I guess that could be asteroid sized.
Black holes cannot be detected directly, though we would observe the gravitational effects as the black hole approaches... and then our planet would be destroyed by the immense gravity.