What is a “Habitable” Planet, Anyway?

We are living at the greatest moment in history for finding life beyond Earth. We have robots poking and sniffing around on Mars. We have a space probe in the works that will take a close look at Europa, to examine the likelihood of life on an ice-covered ocean world. Above all, we no longer have to wonder if our solar system is unique: We know of thousands of planets around other stars, and it’s clear that the total number of planets in our galaxy alone must number in the billions, if not t

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