This is the plot of the Expanse books. Ancient alien civilization loads a rock with nano-computers that take over self-replicating molecules on planets with lif

This is the plot of the Expanse books. Ancient alien civilization loads a rock with nano-computers that take over self-replicating molecules on planets with life to build stuff, aim it at Earth two billion years ago, but it never makes it because Saturn captures it and it becomes the moon Phoebe instead.