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. The core of the search is to find out what kind of life we want. Life exists in a variety of forms on Earth and possibly beyond. It can be divided into two categories for purposes of finding it in the universe: dumb life and smart life. Dumb life includes microbes, plants, and other organisms that are capable of proliferating on a given planet, but not humans, who can think technologically and can make decisions. 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. Even the most sophisticated tools to survey faraway worlds will not be able to deliver on-the ground pictures of alien pine tree or anteaters due to the vast distances between them. 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 at discovering life, it would be ideal to search a planet looking for both signatures. Astronomers are limited by their time and resources. There is a lot to discover. Scientists must be careful when choosing their projects, as they can take decades before bearing fruit. (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, i.e., SETI. In 1960, the astrophysicist Frank Drake launched Project Ozma, an experiment using radio telescopes to search for signals from chatty high-tech civilizations. No one had ever imagined that there was a way to find trees, insects, or microbes on faraway planets. Although for decades SETI remained the only game in town in the search for life, it always suffered from a giggle factor. Many times, Congressmen used the “search for little green men” to criticize NASA for spending tax dollars. Radio SETI funding was affected. 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. It was the first known planet to orbit another sunlike star. Astronomers discovered that starlight can be detected directly in exoplanets’ atmospheres, which allowed them to detect biosignatures. This technique is called “atmospheric characterisation” and has been a major success 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 an 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. Earth’s history is our only guideline on the evolution of technology. It’s evident that life has existed for much longer than technology. Earth was sporting biosignatures for all the universe to see more than 3 billion years ago. Technosignatures have only been introduced in the last century. That means technosignatures have been on Earth for less than 0. 00001 have been on Earth for less than 0. From this perspective, technosignatures might seem like mere icing on the cake of detectable life.
There’s a deeper dimension to this question than evolutionary progress can recognize. A new study, led by Jason Wright of Penn State University and to which I contributed as part of a NASA-funded technosignature-research group, has 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–their planets. Biosignatures cannot leave the biosphere from which they originated. For that matter, many biosignatures on Earth would also disappear if life completely disappeared from Earth. The planet’s existence is responsible for the oxygen we breathe. The atmosphere would become rock-like and quickly disappear on the time scale of very deep time if this life were to end.
To detect a biosignature we need to locate 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 biosphere might die, including the loss of the planet’s atmosphere from solar winds or a really big asteroid impact. The biosignatures will likely follow the biosphere.
Technosignatures have no such constraint. You should consider the fact that there are already many of Earth’s technologicalsignatures in the solar system. More than 10 spacecraft are orbiting Mars or on its surface right now. This is just one planet. There are hundreds of spacecraft out there exploring the spaceways. We have even blasted five craft entirely out of the solar system and into the interstellar domain. Each of the machines that we have sent to space is a technological signature, an artifact. All of these machines are active and sending out radio signals to space. Although these signals may be weak, they still represent a technological signature that could possibly be detected by other species.
Unlike biosignatures, technosignatures move and endure. 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 panels and then succumb to some civilization-crashing accident, those panels might still be visible to alien observers long after we disappeared. Imagine an interplanetary civilisation that uses freighters to move between planets. If the alien civilization burns its trash, engine-exhaust plumes and tight-beam laser communications could be detected on Earth as a sign–a signature. All of these technosignatures could be transmitted far from the alien civilization’s home world (let’s call it a “technosphere”). Uninhabitable planets within the solar system could be used by an alien civilization to house its energy generation or industry. 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. One civilization could also produce billions or more of objects, each with its own technosphere. Each object could be capable of creating detectable technosignatures. Think of a civilization thousands to millions years old than we are. Not only might it routinely create legions of artifacts that emit technosignatures; it might also create more technospheres. Technospheres are able to reproduce themselves through intentional space settlement, unlike biospheres. 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 by an asteroid impact and then lands on another fertile world. Calculations show that 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. Extraterrestrial life has yet to be discovered, and so far we don’t know the true proportion of intelligent to stupid life in this universe. Advanced civilizations may well be so exceptionally rare that the odds are still in dumb life’s favor. This is something I would not bet on, but it’s possible that so much more remains to discover.
One misunderstood conclusion from the research of our group could be that technosignatures should take priority over life hunting. That is not, however, what we concluded. We came to view biosignatures, technosignatures, and their past biases as part of a continuum by reviewing them. Scientists have so far designed life detection tools that target smart life or dull life. However, exoplanet discovery has led to the development of the same telescopes as well as the same detectors to be used to find both types 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 arguing over the question of life in the universe, we are finally capable of searching for answers. Any kind of life, smart or dumb, would be a fundamental reconfiguration of the place we have in the universe. Let’s search for it all.
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