“Plenty of people believe humans could move to another planet,” says Adrienne Cool, a professor at San Francisco State University who studies globular clusters. “That narrative is out there, and part of our job as astronomers is to counter it, to make clear that there is no Planet B.” A few years ago, Cool was “struggling with the contrast of spending time and mental energy thinking about things that are so far away when what’s happening here on Earth is so urgent.” She, colleagues, and students resolved to work toward climate sustainability.

Meanwhile, in June 2019, astronomers in Europe gathered for a conference in Lyon, France, recalls Leo Burtscher, a staff astronomer at Leiden University. “There was a heat wave, and a group of us started talking about high temperatures being the new normal and how we are contributing to climate change,” he says. “I see it as irresponsible, ethically...

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