Leonard Susskind said "It's thought to be the mass of the smallest black hole that can form" (or something like that when referring to the planck mass in his string theory lectures) and I've always been told that the planck mass is the mass of the least massive possible micro-black without ever researching whether or not it was true. I just came across a paper http://cds.cern.ch/record/519241/files/0109057.pdf (Sec. 1, eq. 7) that says, unless I'm drastically misinterpreting it, that the minimum mass of a black hole is $10^{12}$ kg. Is this the lower limit in the Standard Model? Did the lower limit of 1 planck mass come from string theory or loop wquantum gravity? Or does it just not exist?
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$\begingroup$ I can't answer your specific question, but any statement regarding the Planck scale necessarily depends on a theory of quantum gravity, and we haven't decided on one yet. IOW, according to some theories it might be, but we don't know if those theories are correct. $\endgroup$– JavierCommented Nov 26, 2016 at 4:02
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$\begingroup$ This question was attacked by the following paper: arxiv.org/abs/gr-qc/0609055 $\endgroup$– riemanniumCommented Oct 15, 2018 at 14:49
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In our currently accepted theory of black holes, general relativity, the Planck units do not carry any particular significance. General relativity has no problems with theoretically describing a black hole of mass smaller than the Planck mass. See also e.g. this question and this question for refutations of the common misconception that the Planck units represent lower bounds on anything.
Once you introduce quantum field theory into the mix, it is predicted that black holes will radiate Hawking radiation, and that such micro black holes will evaporate almost instantly, making it practially impossible for them to exist. There is no experimental test of Hawking radiation so far (only of certain hydrodynamic analogues). Again, the Planck units do not carry any particular significance here except that they set the scale for being "very small".
answered Nov 26, 2016 at 14:22