Comment by shiroiushi

The problem I see with this scenario is: if you're a biological being, planets with a compatible biosphere are probably extremely rare. Just having a biosphere might not be sufficient, if the makeup of the atmosphere is poisonous to you or it's too hot or cold for the way your species evolved. So why would alien civilizations agree to such a thing instead of just adopting a planet and staying there? The whole premise seems to agree that the aliens are all biologically compatible with each other, despite evolving on different worlds. Perhaps they bio-engineer themselves each time they settle on a new world?