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The theory of the purple planet is really intriguing, and it's intriguing because it's important for us to think about the fact that we might not be looking for life that looks like life on the modern Earth, we might be looking for life that looks like Earth in the Archean (era of early life), or we might be looking for life that looks like Earth life that lives way at the bottom of the ocean. And we have to think about what that's going to look like. If it uses retinal pigments, it'll be purple.