What everyone here seems to be forgetting is one of the classic sci fi themes. That is to say, wholesale genetic alteration to fit extreme planetary conditions. Waterworld? Gillmen. Heavy gravity? Squatty musclebound fireplugs that make Arnold look like a "girly man". It is not implausible at all to me that an ancient advanced race with complete control over their genetic code could not design bodies to live in the most extreme of conditions, even on gas giants. Why limit yourselve to the current primitive world view? The universe is not only stranger than we imagine, it's stranger than we CAN imagine (hmmm, seems I've heard something like this before
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