>>11930767Mainly because you need a magnetosphere to protect you from solar radiation.
Of course a ringworld would have the same problem.
Consider this: if the Earths' magnetosphere disappeared for some reason and stayed gone, most life, if not all, would perish. Over a long period of time the solar wind also would probably strip away so much of the atmosphere that even if the magnetosphere reappeared at some point, life wouldn't recover.
