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

Happy Holidays. In this special issue we are reprinting our top stories of the past year. This article first appeared online in our “Wonder” issue in February, 2021.

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.

If those hypothetical aliens continued to keep watch, what would they witness in the next century? Will a final spasm of activity be followed by silence due to climate change? Or will the planet’s ecology stabilize? Will there be massive terraforming? Will an armada of spacecraft launched from Earth spawn new oases of life elsewhere?

Let’s think specifically about the future of space exploration. Successful missions such as Viking, Cassini, New Horizons, Juno, and Rosetta were all done with last-century technology. We can realistically expect that during this century, the entire solar system—planets, moons, and asteroids—will be explored by flotillas