No event is free of controversy these days — not even the discovery of seven habitable planets that hit the news late last month. For a few days, drawings of those tranquil spheres loomed above the tumult of earthly affairs — the presidential tweets, the protests, the botched Oscars. But then a friend, who happens to be trained as a biologist, wrote to me complaining that by declaring these worlds “habitable,” NASA’s PR people were promoting a space-exploration delusion.

He has a point. Like many people who follow astronomy, I’d started to take for granted that when astronomers say the word “habitable” they don’t actually mean habitable — not in the sense most people use the word, to describe a place where you can go and not die instantly.

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