What Kind of Extraterrestrial Life Should We Be Searching For?

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 grand 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, 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.

To maximize our chances of discovering life, the ideal would always be to scour a planet for both signatures. But astronomers have a big universe to explore and only so much time and money to do the exploring. With projects requiring decades to bear fruit, scientists must choose their shots carefully. (The James Webb Space Telescope, humanity’s newest and most powerful observatory, cost roughly $10 billion, which tells you something about the resources at play.) So far, in the search for extraterrestrial life, dumb life has won out. With healthy funding from NASA, astronomers have made astonishing progress over the past 20 years articulating what kinds of biosignatures might exist on alien worlds. This progress has been remarkably rewarding, but it could come with a cost. Could we be missing out on the promise of smart life?

It’s worth remembering that the first scientific search for extraterrestrial life was a search for extraterrestrial intelligence, ie, SETI. In 1960, the astrophysicist Frank Drake launched Project Ozma, an experiment using radio telescopes to search for signals from chatty high-tech civilizations. Back then, no one could even imagine a way to search for trees or insects or microbes on distant planets orbiting distant stars; no one even knew if such planets existed. Although for decades SETI remained the only game in town in the search for life, it always suffered from a giggle factor. More than once, congressional representatives used “the search for little green men” to whip NASA for wasting tax dollars. As a result, radio SETI’s funding suffered. The field has lived on life support for most of the past 40 years (though recent funding via the private Breakthrough Listen effort has helped).

Meanwhile, in 1995, the search-for-life-game changed forever. The first planet orbiting another sun-like star was discovered, and astronomers realized that they could directly detect biosignatures by observing starlight passing through an exoplanet’s atmosphere. The development of this technique, called “atmospheric characterization,” has been one of the major successes of NASA’s astrobiology program. Astronomers recently ranked a space-based “life-finder” telescope as one of their top funding priorities in a once-in-a-decade survey of the field. Amid the clamor of biosignatures, technosignatures have often seemed like afterthought, if they have come up at all.

The allure of biosignatures is clear. Many astronomers start out assuming that biosignatures will be more prevalent than technosignatures. After all, you can’t have a civilization-building species evolve on a planet before life does. And from Earth’s history—our only reference point on life’s path—it’s clear that basic forms of life have been around far longer than technology. Earth was sporting biosignatures for all the universe to see more than 3 billion years ago. Only over the past century or so have we begun dressing ourselves in technosignatures. That means there have been technosignatures on Earth for less than 0.00001 percent as long as there have been biosignatures. From this perspective, technosignatures might seem like mere icing on the cake of detectable life.

There is, however, another dimension to the question that a simple evolutionary progression fails to recognize. A new study, led by Jason Wright of Penn State University to which I contributed as part of a NASA-funded technosignature-research group, have laid out the argument that astronomy is overlooking the value of technosignatures. The problem with biosignatures is that they’re forever tied to their biospheres, ie, their planets. Biosignatures have no way to leave their biosphere of origin. And, for that matter, if all life were to disappear from Earth tomorrow, most of Earth’s biosignatures would disappear quickly too. For example, the oxygen in our atmosphere all comes from the planet’s life. If that life went extinct, atmospheric oxygen would react back into rocks and disappear quickly on the scale of deep time.

To detect a biosignature, in other words, we have to find a fully functional biosphere. But we don’t really know how long biospheres generally last. Ours has, thankfully, persisted for more than 3 billion years. But there are many ways a biospehere might die, including the loss of the planet’s atmosphere from solar winds or a really big asteroid impact. Once the biosphere goes, the biosignatures will likely go with it.

Technosignatures have no such constraint. Consider the fact that the solar system is already full of Earth’s technosignatures. There are more than 10 spacecraft orbiting Mars or on its surface right now. And that’s just one planet. Hundreds of other spacecraft are out there traversing the sun’s spaceways. We have even blasted five craft entirely out of the solar system and into the interstellar domain. Every one of these machines we’ve felt into space a material technosignature—in its own right. More important, all of the active ones are sending radio signals into space. These signals are weak, but each still a technosignature that some other species could conceivably detect.

Unlike biosignatures, technosignatures move and harden. The Apollo 11 moon lander will be sitting on the moon for millions of years because there’s no wind or water to erode it away. Projecting forward, if we were to cover a fraction of the moon with solar and then succumb to some civilization-crashing accident, those panels might still be visible to alien observers long after we disappeared. Meanwhile, in our own search for life, imagine an interplanetary civilization that has freighters routinely moving between worlds. Engine-exhaust plumes, tight-beam laser communications, even waste disposal, if the alien civilization burned its garbage, might show up as a signal—a signature—that we could detect on Earth. All of these technosignatures could be transmitted far from the alien civilization’s home world (let’s call it a “technosphere”). An alien civilization might even use uninhabitable worlds in its solar system to host its industry or energy generation. Such “service worlds,” as my colleagues and I call them, would only generate technosignatures, because no biosphere would be present.

Technosignatures could also be prolific. A single civilization and its technosphere could produce millions or even billions of individual objects that each could create detectable technosignatures. Imagine a civilization that is thousands or millions of years older than our own. Not only might it routinely create legions of artifacts that emit technosignatures; it might also create more technospheres. Unlike biospheres, technospheres can reproduce themselves via intentional space settlement. By these measures, imagining what distant civilizations might invent through advanced technologies, humanity so far might hardly even count as smart life.

There is plenty to argue about here. On a specific level, for instance, a critic might respond that biospheres can also reproduce via panspermia, the process when a chunk of microbe-bearing rock gets blown into space via an asteroid impact and then lands on another fertile world. Calculations show panspermia may be relatively rare occurrences even in the best of circumstances, however. Meanwhile, a single space-faring civilization could seed the entire galaxy with new technospheres as it settled ever more distant worlds. That said, this all is speculation. We have yet to discover extraterrestrial life, so we have no idea what true ratio of smart to dumb life exists out in the cosmos. Advanced civilizations may well be so exceptionally rare that the odds are still in dumb life’s favor. I wouldn’t bet on this; but then again, so much remains undiscovered.

One misconstrued takeaway from our group’s research could be that the priority in life hunting should now shift to technosignatures. That is not, however, what we have concluded. Instead, in reviewing the past biases and future possibilities, we came to see biosignatures and technosignatures as a continuum. So far, scientists have designed their life-detection tools to target either smart or dumb life, but in the wake of discovering exoplanets, the same kinds of telescopes and the same kinds of detectors attached to those telescopes can now be deployed for finding both kinds of life. These searches could even happen at the same time as astronomers look for signatures of biospheres and technology in the same parts of the electromagnetic spectrum while observing the same planet.

The decades-old biases against technospheres, including the giggle-factor tying them to little green men and UFO conspiracies, are no longer tenable. Astronomers will still have to make hard decisions based on limited resources, but those decisions should be made just on the strength of the specific search proposal rather than parsing the search for life into an artificial biosignature-versus-technosignature split. We are in truly a remarkable moment. After thousands of years of just arguing over the question of life in the universe, we are finally capable of searching for answers. Finding any kind of life—dumb or smart—would constitute the fundamental reframing of our place in the cosmos. Let’s look for all of it.

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