“Only a true Earth twin not yet occupied by life would allow this plan to work, and these may exist somewhere, the galaxy after all is big, but they are too far away from us. Viable planets, if they exist, are simply too—far—away.”

Aram pauses for a moment to collect himself. Then he waves a hand and says more calmly,

“That’s why you aren’t hearing from anyone out there. That’s why the great silence persists. There are many other living intelligences out there, no doubt, but they can’t leave their home planets any more than we can, because life is a planetary expression, and can only survive on its home planet.”

— Aurora by Kim Stanley Robinson