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What exactly do we mean by alone when we ask whether we’re alone in the universe?

The search for extraterrestrial life is one of astronomy’s grandest projects. But the search is more multifaceted than anyone casually intrigued by aliens might realize. At its core lies the question of what version of life we are seeking. On Earth, and presumably beyond, life exists on a spectrum of forms and capacities. But for the purposes of tracking it down in the cosmos, it can be lumped into two somewhat crude categories: “dumb life” and “smart life.” Dumb life consists of things such as microbes and plants that can proliferate across a planet but are unlike humans as self-conscious, technological thinkers. Smart life consists of creatures like us that build planet-spanning technologies.

With deep apologies to microbes, plants, and even elephants for the ham-fisted nomenclature, this distinction between dumb and smart life matters because each can be detected in a different way. Given the mind-wilting distances between stars, even our most advanced tools for surveying far-off worlds won’t be delivering on-the-ground pictures of alien pine trees or anteaters anytime soon. Instead, we must look for indirect signatures of life when surveying a planet. First, there are biosignatures, such as the presence of oxygen and methane in the atmosphere. These are gases that might only be found together because a biosphere—the collective activity of all life on a planet—keeps them there. Second, there are technosignatures. The presence of complex industrial chemicals in the atmosphere or the reflected glint of massive solar-panel deployment would tell astronomers that a technologically capable species like us inhabits that distant world.

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