Below a certain M1M2, gravity would get so weak that the masses wouldn't be able to orbit each other with stability, as other factors would overwhelm them, even in the quietest, stillest places in the universe. Passing gravitational waves, imperfections in symmetry, quantum effects, background particles and cosmic radiation come to mind.
So, what is the smallest M1M2 we can reasonably expect to remain in a stable orbit around each other once they get going?
Are there two boulders, or two pebbles, or two grains, or two molecules somewhere in the universe orbiting each-other gravitationally since the early universe?
I understand multiple variables exist in orbital equations and picking them for ideal conditions might be tricky! I just think this is a very interesting question and wonder if it's even possible to answer it.
Thanks for considering it. It might be good to define first how quiet the quietest/stillest and most peaceful place in the universe actually is first and if that should be a separate question, let me know. Not sure I'd know exactly how to formulate it though!