If we're limiting to work actually done in the 20th century, yes, I agree Planck might not qualify because of the century boundary. And we also get to split hai

If we're limiting to work actually done in the 20th century, yes, I agree Planck might not qualify because of the century boundary. And we also get to split hairs over whether 1900, when Planck published his quantum hypothesis, is in the 20th century or the 19th. :-)