I well-written and nicely balanced article. Our problem is we have precisely one data-point regarding life: our own planet. Back before we could detect exoplanets we imagined most solar systems would be like our own: small rocky planets nearest to the star, with gas giants further out. After all, there were plausible reasons why this would be so. As it turned out, our solar system appears to be quite anomalous compared to what we now know about exoplanetary systems and their astonishing (and entirely unexpected) variety.

While it’s possible to theorize about spontaneous self-organization ultimately resulting in single-celled organisms adapted to their local environment, the fact is we have only speculation. Our various theories and speculations are all plausible, just as were our speculations about the “usual” type of solar system.

If life does exist elsewhere, I suspect you’re correct in arguing that it is most likely to be prokaryotic. We have no idea at all how eukaryotic life got started on Earth, aside from our present understanding that it was a highly unlikely occurrence. Nick Lane’s book The Vital Question is a great primer on why the development of eukaryotic life is such a puzzle.

As for one day discovering more complex organisms, distance alone means we’re unlikely ever to do that unless they exist on the moons of planets within our own solar system. We may detect certain tell-tale signs of life (methane, oxygen) but those won’t enable us to distinguish between archaea-equivalents and moss-equivalents, nor between bacteria-equivalents and worm-equivalents.

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