Single-celled organisms don’t just store buttloads of energy and fly through space for no reason. There had to be something Astrophage needed from Venus or it would just stay on the sun. And it needed something from the sun, too, or it would stay on Venus. The sun part was pretty easy: It was there for the energy. Same reason plants grew leaves. Got to get that sweet, sweet energy if you’re going to be a life-form. Makes perfect sense. So what about Venus?
