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LOOKING for alien intelligence? One way might be to keep an eye out for an exoplanet that has been messed up with chemical pollution or is spilling light into space (see “To spot an alien, follow the pollution trail”). If we do discover an alien civilisation this way, we’ll know we aren’t the only dirty denizens of the galaxy. But there’s a flip side to this idea: our own pollution could reveal our presence to extraterrestrials. That provides yet another good reason why we should mend our polluting ways. What’s bad for Earth’s environment could also invite an alien invasion.…