Hmm. I have seen planets break away too, but I think I have seen it with happy planets. I also have seen empires with massive rioting not have any planets break away. Unless someone has tested this or seen good evidence, I'll remain skeptical that perhaps this is an unimplemented feature.
The rebelled planets use regular AI files. Meaning they can leave their system and expand. They'll even take over other planets with troops if they get teh chance. (I've actually had this happen.)
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I hear voices in my head... and they're not the ones I normally hear.
I think an PBW empire I took over has it, and there is no way to check.
Choose "save empire". Setup a new game and choose "add existing" in the empire screen, and then choose "edit".
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Assume you have a 1kg squirrel
E=mc^2
E=1kg(3x10^8m/s)^2=9x10^16J
which, if I'm not mistaken, is equivilent to roughly a 50 megaton nuclear bomb.
Fear the squirrel.
I once took over an empire and in ~5 turns I had ~50 planets rioting. I took some measures but anyway after about 15-20 turns there still were some of them rioting. And none of them rebelled.