If Aliens Exist, Here’s How We’ll Find Them

Suppose aliens existed, and imagine that some of them had been watching our planet for its entire four and a half billion years. What would they have seen? Over most of that vast timespan, Earth’s appearance altered slowly and gradually. Continents drifted; ice cover waxed and waned; successive species emerged, evolved, with many of them becoming extinct.

But in just a tiny sliver of Earth’s history—the last hundred centuries—the patterns of vegetation altered much faster than before. This signaled the start of agriculture—and later urbanization. The changes accelerated as the human population increased.

Then came even faster changes. Within just a century, the amount of carbon dioxide in the atmosphere began to rise dangerously fast. Radio emissions that couldn’t be explained by natural processes appeared and something else unprecedented happened: Rockets launched from the planet’s surface escaped the biosphere completely. Some spacecraft were propelled into orbits around the Earth; others journeyed to the moon, Mars, Jupiter, and even Pluto.

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