There are tens of billions of Sun-like stars in the Milky Way, an immense tally that encourages the possibility that extraterrestrial life peppers the universe. Finding evidence for aliens — even microscopic ones — would confirm the widespread belief that life is not an unlikely event, constrained to a small number of planets and moons. It could be commonplace, a ubiquitous cosmic infection.
This idea is not new. Since the time of the classical Greeks, many scholars have assumed that life was everywhere, even though they lacked the technical means to find it. By the 19th century, European novelists were writing stories about sophisticated inhabitants of our Moon, and a century later both scientists and the public were convinced that Mars was teeming with clever beings who had built massive irrigation structures visible from earthly telescopes.
Even though the idea of lunar life is now considered foolish and the belief in canals on Mars has evaporated, the hope for discovering life beyond Earth has intensified. Much of this contemporary interest is powered by our successful discovery of more than nearly 6,000 planets around other stars, and that is surely just a small sample of what’s out there. With all that opportunity for life, the idea of hunting for aliens by looking for their infrastructure — artifacts, in SETI parlance — whether irrigation or power systems, surely deserves another look.
Nearby neighbors?
Exoplanets, it turns out, are as common as garden ants. However, it might not be necessary to search even that far afield to find some cosmic company. In our solar system, astronomers recognize at least seven worlds that could have reservoirs of liquid water — places where alien microbes could exist. And in a shift of thinking that would have surprised those Victorian-era scientists so focused on arid Mars, we now consider some of the moons of the outer solar system to be our best bets for finding life within rocket range. To 19th-century astronomers, these moons appeared only as luminous dots. But we now know these modest points of light are swathed in water and ice, and may be where we’ll first find life beyond our own planet.
Our searches also are less direct now than they were when the Victorians turned their telescopes on the Red Planet. Instead, we try to find secondary clues to the presence of biology, such as using spectroscopy to search for atmospheric gases that are the byproduct of metabolism. This approach can work for moons and planets both near and far, for teeming ecosystems or scant microbiology.
There’s little doubt that finding any extraterrestrial life would be a stunning achievement. But the most interesting discovery would be to find intelligent life, which is to say organisms that can rival humans in their cognitive abilities, beings that are like the aliens in our movies and TV shows. Detecting thinking organisms would surely be one of the biggest news stories of all time, if not the biggest.
how’s it going?
In the absence of direct contact, of course, we are limited to looking for other clues. By the time of World War II, most scientists had discounted the century-long claims of engineering projects — specifically, canals — built by thinking beings on Mars, ascribing them to optical illusions. As the Space Age got underway, flybys of the Red Planet not only failed to find such features, but revealed a desolate and seemingly sterile world.
Astronomers, undaunted, swung their instruments to more distant targets. In 1960, Frank Drake made the first modern attempt to pick up deliberate radio signals from extrasolar societies. He sequentially pointed an 85-foot-diameter (26 meters) radio telescope in West Virginia toward two nearby star systems in an endeavor known as SETI, the Search for Extraterrestrial Intelligence. He whimsically dubbed his two-week effort Project Ozma in a nod to Frank Baum’s Wizard of Oz book series.
Drake didn’t detect any extraterrestrial transmissions. Nonetheless, more than six decades later, radio searches remain the go-to method for trying to prove that someone is out there. Contemporary experiments are similar to Drake’s and continue to use the highly sensitive equipment of radio astronomy to search for the kind of narrowband signal that only an artificial transmitter could make.
We’ve still not found aliens, even though a majority of the scientific community, as well as the general public, believe they exist. Mind you, radio waves from the cosmos are commonplace, sustaining a substantial cohort of radio astronomers who study these emissions to learn about the universe. But while we’ve cataloged several thousand pulsars (spinning neutron stars) and more than a million quasars (feeding supermassive black holes), no signal picked up by our radio telescopes has yet had the characteristics of a deliberate broadcast.
Radio is only one technology communicating aliens might use, however. There are others: for example, flashing signals into space with lasers. Lasers can send many more bits of information per second than a radio transmitter, and consequently some advanced aliens might be using them for interstellar communication. To that end, the LaserSETI network, currently operated from roughly a dozen observatories worldwide, is designed to continually search the entire night sky for flashes of optical laser light originating beyond our solar system.
A classic idea
Signals are not the only kind of evidence for aliens we might find. In particular, we could look for massive artifacts, constructions large enough — or bright enough — to be detectable by our telescopes.
This is hardly a new idea; it goes back at least a half-millennium, when polymath and bishop Nicholas of Cusa suggested that all heavenly bodies were populated by plants and animals. During the Victorian era, several reputable astronomers claimed to have seen signs of intelligent beings in our own solar system. Italian astronomer Giovanni Schiaparelli and American astronomer Percival Lowell both claimed to espy straight-line markings on Mars. Lowell explained these as irrigation ditches constructed to support martian agriculture. The astronomers weren’t seeing the martians themselves, of course — just their civil engineering efforts. The so-called martian canals eventually became a trope, if not a fact.
In 1960, physicist Freeman Dyson suggested another approach. He noted that technically sophisticated aliens might surround their home stars with a constellation of satellites outfitted with solar panels. This phalanx of orbiting light collectors, now termed a Dyson sphere, could capture a large fraction of the star’s radiative output. This energy would then be beamed back to the aliens’ home planet, powering their high-tech lifestyles with a scheme that offered an endless supply of energy and had no environmental impact. Crucially, scientists understand that these satellites would inevitably become warm, and consequently radiate a weak infrared glow that would make them visible to our telescopes.
The idea of Dyson spheres marking the location of sophisticated societies was not only ingenious, but also made a prediction that could be experimentally verified. The prediction in this case was that the satellites would produce infrared radiation. This meant that astronomers could find a Dyson sphere by noting any unexpected infrared emission from a star system.
For several decades, researchers have tried to do exactly that. Most recently, astronomers at Sweden’s Uppsala Observatory surveyed 5 million stars, looking for the telltale glow of a Dyson sphere. They report finding 60 candidates that have more-than-expected infrared radiation, and are following up with additional observations.
The advantage of artifacts
Searching for artifacts offers a straightforward advantage over the way SETI has historically been pursued. The traditional approach pioneered by Drake assumes that our telescopes are pointed in the aliens’ direction at the same time their signal arrives at Earth.
This synchronicity requirement clearly reduces the chances of successfully eavesdropping on ET. By how much? Alas, we can’t say without knowing how many societies are out there. We also need to have an idea of their typical lifetimes — how long do they signal for before they either die or are snuffed out? We don’t know either of these things.
But if we hunt for artifacts instead of signals, we can largely avoid such uncertainties. Artifacts can be found at any time subsequent to their construction. The Egyptian pyramids and the American interstate highway system are testaments to human existence that could plausibly outlive humans themselves. Even after we are long gone, these engineering projects will tell anyone who finds them that we were once here. And, as a further advantage, a hunt for artifacts doesn’t depend on guessing the aliens’ choice of frequency or transmitter power.
What sort of artifacts might we expect? While Dyson spheres are certainly one possibility, we can’t confidently describe how an alien construction would appear any more than the first humans could accurately anticipate the appearance of a 21st-century cityscape. A fundamental challenge to any artifact hunt is that we can’t be sure what we’re looking for.
But it’s not necessarily an insurmountable problem. We might still find these artifacts simply because they would be novel, and not already members of the known cosmic bestiary. For example, consider the aforementioned quasars. No one predicted the existence of these objects, let alone what they would look like. Indeed, both quasars and pulsars were initially suspected of being alien handiwork. We now have natural explanations for both, but they were discovered because they were different from other objects we knew about at the time.
Only the best
Despite the fact that we can’t say much about the appearance of alien artifacts, there’s little doubt that any we find will be subject to selection effects. The easily detectable ones will surely be the biggest and brightest. This is akin to searching for dogs by listening for barks. You won’t find all the dogs, and not necessarily the most interesting ones. Just the ones that make a lot of noise.
Similarly, the hunt for alien engineering efforts — if it turns up anything — will only show us the societies that build things easily visible from light-years away. Such constructions may not be typical, nor even of recent vintage. But no matter how singular such artifacts might be, their discovery would answer a long-standing question: Is anyone out there?
For nearly two centuries, we’ve searched for clever company in our cosmic neighborhood. Once, that search meant examining our solar system for artifacts, until we later switched strategies, hoping to pick up radio signals or other transmissions as the indication of another technological civilization. The latter is an approach that we’ve followed for a long time now.
Maybe we should revive the strategy of looking for artifacts. Space is vast, and it’s conceivable that we’ll never meet the aliens themselves. But we might uncover some of their public works.
