by Corey S. Powell
of the European Southern Observatory
at La Silla, Chile.
by Serge Brunier/ESA
to find extraterrestrial civilizations
by the way they live.
But if we can see them,
can they see us...?
Are we alone in the Universe?
These questions are as immediate as the latest Netflix hit and as primal as the ancient myths that associated the planets with spirits and gods.
In 1686, Bernard le Bovier de Fontenelle, the long-term secretary to the French Academy of Sciences, put an Enlightenment stamp on speculations about alien life with his book Entretiens sur la pluralité des mondes (Conversations on the Plurality of Worlds).
In a series of spirited
philosophical conversations, he declared that 'it would be very
strange for the Earth to be so well inhabited, and the other planets
perfectly solitary', and argued that alien beings might attempt to
communicate with us or even visit us using some advanced form of
In 1818, the German mathematician Carl Friedrich Gauss proposed communicating with aliens using a heliotrope, a system of mirrors that he devised to send coded signals using reflected sunlight.
After the development of early electric lights, the French inventor Charles Cros suggested that such lamps could be amplified to beam messages to Venus or Mars.
Nikola Tesla wrote in 1900 that,
A year later, he reported
that he had detected likely signals broadcast from another world.
A full century after Tesla, researchers engaged in the search for extraterrestrial intelligence (commonly shortened to SETI) were still scanning the heavens with antennas and listening for artificial radio transmissions incoming from other worlds.
The efforts led to
ever-tightening statistical upper limits and a handful of briefly
exciting false alarms, but mostly a whole lot of nothing.
Adam Frank is one of the leading researchers embracing a different approach, one that focuses on the hunt for 'techno-signatures':
The shift from SETI to techno-signature is an intellectual sea change in thinking about what extraterrestrials could be, and about how they might reveal themselves to us.
The emerging science of techno-signatures has also reopened a high-stakes, long-dormant debate.
To make Contact,
SETI sprung from the Space Age mindset of the 1950s, when rocket scientists were drafting plans to send the first spacecraft to other planets and astronomers were beginning to conduct detailed studies of radio emissions from distant cosmic objects.
Frank Drake, a
radio astronomer at the National Radio Astronomy Observatory (NRAO)
in West Virginia merged those twin themes in his head and began
wondering whether intelligent creatures on those other worlds might
similarly be looking outward and attempting to contact us via radio
waves, signals that could be detectable using radio dishes such as
NRAO's 85-foot antenna.
His small-scale project drew significant media and academic attention.
A year later, Drake convened the first SETI meeting, drawing a dozen leading researchers to discuss the prospects for expanding and formalizing the search.
One of the young astronomers swept into this strange new field of study was Jill Tarter, who rose to become the project scientist for the biggest SETI initiative to date:
NASA's effort launched with shiny hopes in 1992.
Senator Richard Bryan from Nevada had singled it out as an egregious example of government waste, and launched a withering assault on what he called 'The Great Martian Chase'.
Congress not only shuttered the High Resolution Microwave Survey, it effectively banned NASA from funding any further research along those lines.
Tarter soldiered on, co-founding the nonprofit SETI Institute to
pick up her research with private support, but the field retreated
to the margins of respectability.
The grand pause had set in.
What nobody realized at the time, though, was that a little-known pair of Swiss astronomers was already laying the groundwork for SETI's grand revival.
In 1993, just as Senator Bryan was sneering that 'not a single Martian has said "take me to your leader",' Michel Mayor and Didier Queloz at the Geneva Observatory had completed a sensitive new instrument for discovering exoplanets, worlds beyond our solar system.
In 1995, they reported spectacular success, finding the first known planet orbiting another star similar to the Sun. As soon as their colleagues understood where and how to search, other detections followed at an ever-accelerating cadence.
The current exoplanet tally stands at more than 4,500...
The first ever image of an exoplanet (bottom left),
approximately 200 light years from Earth.
The large planet is thought to be around five times the mass of Jupiter
and attached to the young brown dwarf star (centre).
Photo courtesy ESO
With the exoplanet boom, the possibility of life on other planets was no longer just a matter of philosophical speculation:
Researchers coined a new word, 'bio-signatures', to describe chemical evidence suggestive of biological activity on another world.
On Earth, for instance, oxygen is produced by photosynthesis and methane gas is created by microbes eating organic matter.
The two gases readily react with each other, so methane would quickly disappear from an oxygen-rich atmosphere unless it were steadily replenished.
Any observable signal of metabolic activity (even the huffing and puffing of slime mould) qualified as a bio-signature.
Humans alter Earth in a million ways without deliberately intending to tell the cosmos that we are here.
We spew pollutants, belch factory heat during the day, and light up our cities at night. We can't help it, any more than bacteria can help emitting methane. By extension, any advanced aliens could be expected to visibly alter their planet as an inevitable byproduct of creating a manufactured, industrial civilization.
In a presentation to the International Astronomical Union in 2006, Tarter outlined this new concept and argued that SETI needed to be defined more broadly.
It was time, she stated, to pursue any clues pointing to the existence of nonhuman technology out there:
The breakthrough moment in the rebranding of SETI came in the fall of 2018, when NASA convened a techno-signatures Workshop in Houston, Texas.
The meeting drew together both veteran alien-hunters but also many other scientists, such as Adam Frank, who had never given SETI a serious thought.
Frank develops detailed computer models of stellar evolution and had no interest in spending years sifting through radio noise in the hopes of hearing somebody say hello.
But the idea of techno-signatures piqued his interest because it shifted the focus away from one specific action that aliens might or might not perform - sending signals - to the general process of how intelligent aliens might live.
Along with fresh thinking, the techno-signatures Workshop also promised the magic elixir that SETI research had been missing for a quarter of a century:
The discovery of exoplanets had given the agency both scientific motivation and political cover to resume support for alien-hunting.
It was easy to deride SETI as a search for Little Green Men.
Bio-signatures research, in contrast, was a sober-sounding observational program for detecting the spectroscopic markers of specific compounds on other planets. (Try laughing at that, Senator Bryan.)
Bio-signature grants sailed through the approval process smoothly, easing the way for techno-signature grants to follow.
Two decades of exoplanet research had also established that the search for techno-signatures, while challenging, is clearly possible. Some exoplanets are aligned so that they appear to pass directly in front of their stars as seen from Earth.
When that happens, starlight streams through the planet's atmosphere and a small fraction of that light gets absorbed by the gases there, indicating activity and chemistry on the surface. In other cases, it is possible to pick up a smidgeon of starshine reflected directly off the planet's surface.
By examining these exceedingly subtle effects, researchers have already identified sodium, water, carbon dioxide and other molecules - even oxygen and methane, though not together - on these far-off worlds.
Artificial compounds might be rarer and harder to find than natural ones, but in principle the detection process is the same.
If aliens want to convert the light from their star into emission-free energy, solar panels are a logical way to do it
Last year, Frank and a group of collaborators were awarded NASA's first-ever grant for techno-signatures research, bringing SETI's great pause to an end.
The group zeroed in on energy-generation and manufacturing as two essential aspects of an industrial civilization, and then sought out plausible ways that we could see those things happening from many light years away.
Drawing on earlier papers by Avi Loeb at Harvard University, the researchers settled on two signals in particular: solar panels and chlorofluorocarbons, or CFCs.
If aliens want to convert the light from their star into emission-free energy, the reasoning goes, solar panels are a 'logical' way to do it.
And if they decide to go all-in and blanket their home world with those panels, they would significantly alter the appearance of their planet:
Better still, solar panels make the planet look different in predictable ways, regardless of the specific energy technology another civilization might develop.
His group is now modeling the appearance of various plausible types of solar installations and assessing what it would require for us to observe them on a world around another star.
Chlorofluorocarbons are a more human-specific technology, but a promising starting point for exploring what Frank calls 'atmospheric techno-signatures' - that is, any distinctive substance that an industrial civilization puts into the air.
CFCs are highly versatile compounds that are used as solvents, refrigerants, foaming agents and aerosol propellants. If we find these chemicals so appealing, maybe other species will, too.
One big caveat is that CFCs damage the ozone layer and are being phased out here on Earth; it's odd to imagine advanced aliens recklessly polluting their home world.
Then again, they might not need an ozone layer...
They might live underground, Frank notes, or might have relocated their factories to an uninhabited service world. CFCs also are effective at trapping heat. Maybe aliens use them for planetary climate control?
From a techno-signatures perspective, CFCs are especially interesting because they do not exist in nature and they are easy to detect even at low concentrations.
They are therefore a good starting point for learning how to model the effect of an artificial chemical in an exoplanet atmosphere, design the right observing technique to look for it, and then pick out its visible signature amid the vastly brighter glare of the planet's nearby star.
Think of CFCs as the gateway drugs to innumerable other, more 'bizarre' (from a terran human point of view) visions of possible extraterrestrial civilizations.
Therein lies one of the greatest challenges of the techno-signatures hunt. Alien technology could take so many forms that it is impossible for the human mind to consider or even imagine them all.
The best we can do is start with the technology we know and extrapolate outward as creatively as possible.
In a piecemeal manner, some researchers have already begun doing that.
Next-generation space telescopes should be able to detect Earthly levels of nitrogen dioxide on a planet 30 light-years away, he finds.
These ideas have not yet been so well quantified, however.
Techno-signatures also don't have to take the depressing form of pollutants.
Artificial illumination has a spectrum that is quite distinct from starlight, meaning that it might be possible for future telescopes to pick up the nighttime glow of majestic alien metropolises.
Scanning for unusual infrared thermal emissions would be another way to identify cities or industrial parks on exoplanets.
These signals would be even more subtle than the ones from CFCs or solar panels, unfortunately. We would be unlikely to spot them through current means of detection unless extraterrestrial civilizations operate on a scale vastly beyond anything humans have done.
Then again, why shouldn't they?
Once we break free from the shackles of our own limitations, we can imagine aliens performing wild feats of planetary or interplanetary engineering.
Jason Wright at Pennsylvania State University, a leading SETI researcher (and one of Frank's collaborators) has written extensively on the possibility of discovering such alien artifacts - what might be called mechanical techno-signatures.
For instance, an extraterrestrial civilization that has outgrown its planet might build a megastructure around its star, to create more living area or to capture all of the star's energy.
In this case, we might observe not the byproduct of the technology, but the technology itself:
Alien tech could be millions or billions of years old
Six years ago, it seemed like that might have happened.
The astronomer Tabetha Boyajian (then at Yale, now at Louisiana State University) helped identify a bizarre variable star that did not behave like anything seen before.
Even her sober-minded colleagues had to admit that they couldn't decisively rule out an alien megastructure.
The current astronomical consensus is that Boyajian's Star is something less exotic, like a young star surrounded by eccentric rings of dust from a shattered planet or moon.
Still, the controversy helped draw attention to the emerging field of techno-signatures, and it got Wright thinking more actively about how to investigate alien artifacts in a responsible and scientifically rigorous manner.
The issue of rigor becomes especially touchy when researchers like Wright consider the possibility that extraterrestrial civilizations might have sent artifacts here, into our own solar system, if not onto Earth itself.
Wright is acutely aware that scanning for nearby alien technology sounds suspiciously like hunting for UFOs, and he would like to stop you right there.
(He recently wrote an essay for Slate explaining the stark difference, titled 'I Look for Aliens for a Living, and No, I Don't Study UFOs'.)
From a techno-signatures perspective, it seems ludicrous that creatures capable of interstellar travel would be buzzing around Earth in flying saucers at this exact moment in our planet's 4.5-billion-year history, playing hide-and-seek with US fighter jets like a bunch of bored teenagers.
That is exactly the kind of blinkered, human-centric thinking that the researchers are trying to steer away from.
Wright has a much more expansive view of alien tech.
Only after you adjust to this more open-minded way of thinking can you begin to develop plausible methods for identifying the kinds of techno-signatures that might be lurking right in front of us.
For starters, looking for alien tech on Earth is probably a waste of time.
Better to search on the Moon, where any structures or equipment left behind by visitors could remain intact for eons.
Image searches or radar scans could expose unusual-looking structures with right angles or with out-of-place material properties. Aliens might also monitor our solar system for a long time by parking space probes in extremely stable orbits.
The stability requirement rules out a lot of real estate, Wright notes, but it leaves locations like the asteroid belt where an ancient craft could blend in with the surrounding small rocks.
Fortunately, there are a few possible giveaways.
An artificial object might stand out by its distinctive color and composition. It would probably be hollow, with a much lower density than a typical asteroid, which would cause its orbit to shift due to the pressure of sunlight. If it is currently active, we might even catch it while it is thrusting and adjusting its trajectory.
Starting in 2023, the Rubin Observatory in Chile will begin by far the most extensive survey of small objects in the solar system.
Wright is eager to build an alert system so that, any time the telescope detects something that doesn't look quite right, it will flag that object as a potential techno-signature deserving closer scrutiny.
In all of its permutations, the techno-signatures approach stays true to its central premise, that we might detect extraterrestrial civilizations whether or not they have any interest in being detected.
The principle of transparency cuts both ways, however. Just as aliens could be exposed to us simply by what they do, so too are we exposed to them by what we do.
A study (Past, Present and Future Stars that can see Earth as a Transiting Exoplanet) published earlier this year in Nature identified 1,715 nearby star systems that would see Earth passing in front of the Sun.
That alignment creates the ideal setup for studying our atmosphere and gleaning the secrets of our industrial society:
Science-fiction stories are full of warnings against broadcasting our existence to extraterrestrials, who might harm us if they know where we are; it's a central theme in Cixin Liu's recent The Three-Body Problem trilogy, for example.
Traditionally, most scientists have also been squeamish about signaling projects.
Stephen Hawking was especially outspoken against the idea. But the techno-signatures concept suggests that these concerns are rooted more in fear than in logic.
That is the view of Douglas Vakoch, a psychologist and astrobiologist who used to work for Tarter at the SETI Institute and has since founded his own organization, METI International.
The name of Vakoch's organization hints at his answer.
METI stands for Messaging Extraterrestrial Intelligence, essentially SETI in reverse. Vakoch and a small community of likeminded METI advocates are starting to think of ways to get out the interstellar messages that we want, rather than the ones that just happen to slip out by accident.
Our techno-signatures are already out there for all to see, they note, and nothing bad has come of it.
We probably don't have to worry, then, about the danger of announcing ourselves to a potential off-world audience.
'Creatures that communicate through a sense of smell… would have a different sense of time and space'
If we let go of that fear of speaking, we can dig into the rich and fascinating issue of what it is that we would want to say.
So far, there have been only a handful of attempts at beaming out interstellar messages. The most famous of these was the first one, a radio signal designed by SETI pioneer Frank Drake and directed toward the star cluster M13 in 1974.
The transmission contained coded images of DNA, a person and the radio telescope used to transmit the message.
Drake made a sincere effort to think universally, but still ended up incorporating a lot of assumptions about how aliens perceive reality.
A more essential limitation in the 1974 message (and in the few that followed, as well) is that Drake didn't seriously address why anyone at the other end would care enough to send a reply.
Statistically speaking, any alien civilization we are able to contact is likely to be far older than our own. It is likely to be far more technologically advanced as well.
Perhaps we could offer them a reminder of their own origins, he suggests, or a perspective on the other ways that intelligent beings can develop.
Maybe they would respond just out of kindness and concern for the upstart humans of Earth - but maybe not.
Pondering the potential motivations of extraterrestrial societies is a next-level task for techno-signatures research.
An absence of motivation might also explain the Fermi Paradox - the lack of any communication from alien civilizations, even though our galaxy seems to contain a multitude of potentially habitable planets.
If they know we're here, why don't they call?
It's possible, of course, that intelligent extraterrestrials do not exist. But it's also possible that we have not yet given them any reason to be interested in us.
The late Ronald Bracewell, an electrical engineer at Stanford University, described SETI as an attempt to join the Galactic Club.
If Vakoch is right, listening alone won't do the trick.
The only way to join the Galactic Club is to take control of our own techno-signature and announce loud and clear: