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In a paper published in the November 1, 2019 issue of the journal Science, a team of astronomers offers a new way to search for stellar-mass black holes, and shows that it is possible there is a class of black holes smaller than the smallest known black holes in the Universe.

http://www.sci-news.com/astronomy/new-class-low-mass-black-holes-07759.html

Edited November 3, 20196 yr by Strange

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There is no lower limit on the size of Black Holes, just on available mechanisms for their formation;  gravitational collapse of stellar sized objects that lose antagonistic radiation and degenerate pressure, or additional mass acquired by a neutron star from a companion or interstellar gas/dust.
Both methods result in sizeable Black Holes.

However primordial Black Holes, formed in the hot, dense soup shortly after the Big Bang, could form in any size, and could have evolved in two ways.
They could have 'evaporated', due to their high temperature and resulting Hawking Radiation. These small BHs could have or be currently reaching the end of their 'evaporation', where they discard their event horizon and explode in a large gamma ray burst; we don't see any observational evidence of this happening anywhere.
Or they could have become the 'active' seeds of the supermassive BHs ( and their surrounding gas/stars ) we see in most all galaxies. These active BHs are numerous in the distant past ( great distances ) in newly forming galaxies, and are usually termed Quasars.

Interesting research. Hopefully, with more sky survey data from various sources, more possible stellar-mass black hole candidates will be identified.

1 hour ago, MigL said:

There is no lower limit on the size of Black Holes

I think the novelty is that we know have the data to try an identify smaller BHs

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Discussion of electrons as block holes split off to here: https://www.scienceforums.net/topic/120467-electron-as-black-hole-split-from-new-class-of-black-hole/