The problem Isaac is trying to address is one that is a real one. If you send a space probe filled with self-replicating machines to another star, where do you send it so that it has the best chance of replicating? If you send it to the inner system there is a lot of energy for replication, but very little hydrogen; if you send it to the outer system there is a lot of hydrogen and not much energy.
In the inner system a self-replicator would probably start by growing photovoltaic cells or some other sort of solar panel and would rely on metals and semiconductors. In the outer system the self-reppers would have to try to build a fusion generator from scratch (unless they brought one with them) or try to use tidal or magnetic energy, but they could use plastics and organics (which may perform certain self-replicating tasks well).
Neither prospect is ideal.
A third option - self-replicating on an Earth-like terrestrial, with plenty of C,H,O, and N, is appealing, until you realise that your self-reppers are trapped on that planet until they can build a launcher capable of reaching escape velocity. Earth-like worlds might be rare, and they are always difficult to launch from, but they do offer plenty of exploitable resources.
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