The True Story Of How Humans Are Searching For Intelligent Alien Life

These 20-foot-wide instruments are part of the Allen Telescope Array. They are designed and operated to consistently and systematically scan the skies for radio signals possibly sent by advanced civilizations from distant star systems and planets. Marc Kaufman/Washington Post via Getty Images

Natural sources of radio waves have distinct fingerprints. Namely: Those sources broadcast across a wide range of frequencies, explains Jason Wright, an astronomer at Penn State University. They emit signals that can be picked up on many stations of an astronomer’s (jacked-up) radio dial.

An artificial source of radio waves (i.e., an alien transmitter beaming out a message) would look very different. Think about humanity’s own radio communications. When you want to listen to a particular radio station, you must tune into a very specific frequency. That is essentially what radio SETI research is: a hunt for coherent transmissions broadcast on an extremely narrow range of frequencies (dubbed “narrowband”). “Nature just cannot do that,” says Jean-Luc Margot, a radio astronomer and technosignature researcher at the University of California Los Angeles. Narrowing down a broadcast to a particular frequency, or a few frequencies, requires machinery — with essentially no exceptions.

That means this quest is fundamentally straightforward. Scientists are looking for “stuff you don’t see normally coming from stars and galaxies,” says Michael Garrett, the director of the Jodrell Bank Centre for Astrophysics. “Anything that you don’t expect nature to produce.”

But that’s easier said than done. Nature has temporarily hoodwinked astrophysicists in the past. Take pulsars. Today, scientists know that they are the rapidly spinning, hyperdense corpses of stars, emitting beams of radiation from two poles like a deity’s lighthouse. But that wasn’t always the case.

The flamboyant behavior of pulsars was first theorized about in 1967. In 1968, a different group of scientists discovered the signal from a pulsar for the very first time, but they didn’t know exactly what it was; the regularity of the radiation bursts seemed so nonrandom that, for a moment, astronomers could not entirely rule out an artificially generated signal as a possibility, even dubbing the source LGM1 — “little green men 1”. But later that year, another scientist connected the regular rhythm of LGM1 with the pre-existing star carcass lighthouse theory, and LGM1 was understood to be a natural phenomenon, not a beacon of an alien design.

There are always caveats, and nature is always capable of surprises. But a nonrandom narrowband radio signal coming from space is an excellent place to start. Siemion likens it to seeing the Great Pyramid of Giza amid a vast field of rocks. Sure, it’s technically possible some natural process could have resulted in a pyramid, but the likelihood of crafting this sort of order out of such chaos is infinitesimally small — and even if you find one, you can check to make sure it isn’t a fluke of nature.

The technosignature checklist

Theoretically, anyone doing regular radio astronomy — gawping at supermassive black holes, for instance — could stumble upon one of these seemingly nonnatural radio signals. But it’s very unlikely. This sort of radio astronomy requires opening your instrument’s mechanical ears to a huge range of frequencies.

Looking for a radio technosignature means very carefully searching for signals amid the storm of natural noise. Fortunately, contemporary radio SETI searches levy the power of supercomputers and, increasingly, machine learning programs to simultaneously twiddle the dials of many different radio receivers, scouring for narrowband signals of interest.

But lest we forget, space is gargantuan. Even if signal searches are more efficient these days, there’s a lot of space to peruse. And most SETI researchers won’t always do targeted research — for example, listening in on tranquil stars known to have potentially habitable rocky worlds orbiting them. “You don’t know what an advanced civilization would do,” says Margot. They might build a radio beacon far from their homeworld, perhaps around a more hostile star, one they monitor remotely. “I think the best approach is to look over the entire celestial sphere,” he says.

If we are lucky, there may be a way to cut some corners. In their 1959 paper, Cocconi and Morrison noted that the most abundant stuff in the universe is hydrogen. Hydrogen gas is everywhere, and it naturally produces radio emissions at a specific frequency: 1,420 MHz. Other astronomy-practicing civilizations would almost certainly know this, and realize that other civilizations would also know this — so why not broadcast a transmission at that exact frequency?

“If you have to look for somebody you don’t know at the airport, you go to the rendezvous point,” says Margot. “1,420 MHz may be the rendezvous point for civilizations trying to advertise their presence.”

Even if that turns out to be the case, humanity’s radio transmissions are obfuscating our efforts to look for the alien equivalents. Earth itself, and the many satellites orbiting it, generate a hurricane of radio waves. Sometimes, SETI researchers can pick up signals from space that are our own robotic spacecraft, or terrestrial signals bounced back from the moon.

“It’s really annoying,” says Wright.

That’s why some researchers are (ambitiously) calling for a SETI-focused radio observatory on the far side of the Moon, which would avoid much of this noise — at least while the soon-to-be-permanent human presence on the lunar surface remains small.

But let’s say you can rule out humanity’s own radio interference. Scientists must then determine it’s coming from somewhere else in space. One or a handful of radio observatories could be used to get a decent idea of where that signal was coming from. But if you use a huge number of radio dishes at once — the 64-dish MeerKAT radio telescope in South Africa, for example, or even combining multiple observatories and arrays across the planet — you get a major boost in sensitivity that could allow you to locate the planet around a star from which the signal is broadcasting.

Tick all these boxes and you have something extremely promising on your hands. But it can get even better: If the signal changed its structure during the transmission — something known as modulation — then that would blow everyone’s socks off. “If there was some obvious modulation there, then we would know that it was also conveying information,” says Garrett. This is the difference between hearing a constant dial tone on the phone versus hearing hold music or someone speaking.

“It’s not clear you’d ever understand them any more than cavemen would understand the London newspapers,” says Shostak, of the SETI Institute. But we could recognize the modulations are representative information of some sort — anything from mathematical sequences to sounds that represent a spoken language — without being able to translate the content.

At this point, so long as multiple research groups all came to the same conclusion independently, it would be difficult to doubt that a technosignature (a.k.a. intelligent life) has been identified. As you’ve probably guessed, this is yet to transpire, although there have been several moments in which, for a second, things looked mighty promising.

In 2020, for example, a seemingly nonrandom narrowband radio signal was detected by the Breakthrough Listen project. Dubbed BLC1, it was especially tantalizing as it was thought to be emanating from the closest star to the sun, Proxima Centauri, a system thought to have rocky (and potentially habitable) worlds in its orbit. But multiple studies eventually concluded the signal was Earth-based interference of some kind, nothing alien.

“It was a roller coaster, I guess you could say,” says Siemion. “It was really exciting.” And by interrogating the signal so rigorously, it proved to be a great educational experience. “We’re looking forward to BLC2,” he adds.

Video killed the radio star

As is often said, seeing is believing, and there is a chance that our first technosignature will come about not by listening to the universe’s radio stations but by peering down the sights of a telescope.

One way astronomers search for other planets is the transit method. If a star’s brightness temporarily dips, then something probably passed in front of it as seen from Earth. If it dips in regular intervals and by the same amount each time, that’s usually caused by a planet.

If, during a transit, a star’s light passes through a world’s skies, then it carries information about the chemical makeup of that planet’s atmosphere — information that, with the right instruments, astronomers can decode. This is useful for all sorts of reasons, including working out if a world is potentially habitable to biology of any variety, or even to search for hints of possible biosignatures themselves — chemicals that can be produced (sometimes exclusively) by life.

This technique could also be used to find the pollution from an alien civilization. Nitrogen dioxide, for example, is made by forest fires, volcanoes, lightning, and other natural sources. But much of Earth’s nitrogen dioxide comes from the burning of fossil fuels, particularly from road vehicles. Detecting that on an exoplanet may hint at the presence of a fossil fuel-burning civilization that has yet to move exclusively onto sources of futuristic clean energy, like nuclear fusion.

Like many biosignatures with both natural and artificial sources, the detection of plenty of nitrogen dioxide wouldn’t be a slam-dunk confirmation of an alien intelligence.

Astronomers can tell a lot about the size and composition of a planet just by observing the light it blocks as it orbits its star. NASA Ames

Other chemicals would sound a clearer alarm, such as chlorofluorocarbons (CFCs). These are found in aerosol sprays, packing materials, solvents, refrigerants, and more; they ate away at the ozone layer before being broadly banned across the globe by the Montreal Protocol. “There is no natural process that can produce CFCs,” says Kopparapu, the NASA planetary habitability researcher. It is not inconceivable that, as the James Webb Space Telescope is examining an exoplanet for biosignatures, it detects the presence of CFCs.

SETI scientists are not just interested in the information carried by that starlight; they are also curious about the total amount of starlight they are receiving. When an object passes in front of the star, its apparent brightness dips. And it’s possible the dip could be caused by something other than a planet — something much more implausible but considerably more fantastic.

Science fiction is full of alien megastructures, unfathomably giant objects like world-sized space stations or colossal orbs surrounding stars to siphon off an almost endless supply of solar energy. There is always a chance that a transit reveals the existence of something decidedly nonnatural around a distant star — a detection that could be followed up by targeted radio SETI work.

Some transits have already raised astronomers’ eyebrows. The chaotic, sporadic dimming around Tabby’s Star (named after an American astronomer who led the team that discovered the star’s weird light fluctuations), for example, cannot be explained by the periodic orbit of a planet. Nobody can confidently explain the cause of these shenanigans, but various hypotheses have been suggested, including the shattered remnants of a planet, swarms of comets, and, yes, an alien megastructure — a type of optical technosignature. Although few scientists are betting on an extraterrestrial intelligence explanation, ongoing work has still to conclusively rule it out.

Interstellar interlopers

It is possible that the first confirmation of ET intelligence will be from a detection much closer to home, possibly within our own solar system. Astronomers are looking out for strange objects near our sun’s orbit. And sometimes they find them.

On October 19, 2017, astronomers detected something deeply unusual soaring through our solar system: A pancake or cigar-shaped, extremely reflective body that was accelerating as it was leaving the solar system, a speed uptick that its gravitational slingshot around the sun alone could not apparently explain.

The object was dubbed ‘Oumuamua, and it was the first confirmed sighting of an interstellar object, something that originated from another star system.

Its exotic disposition was initially inexplicable, but the idea that it was a natural entity with unusual characteristics was quickly accepted by astronomers — with one notable exception.

Avi Loeb, a Harvard astronomer known for making provocative (and unsubstantiated) claims about alien technology that much of the scientific community finds both exhausting and loathsome, concluded that the most plausible explanation for ‘Oumuamua is that it’s the product of an extraterrestrial intelligence. Perhaps it was a type of reconnaissance craft, a vessel propelled by the wind-like push of starlight on its reflective sail. He wrote a bestselling book about it titled Extraterrestrial: The First Sign of Intelligent Life Beyond Earth.

Six years on, there is still considerable debate as to the true nature of the object. But astronomers are in lockstep about one thing: Loeb’s theory about ‘Oumuamua doesn’t pass the smell test.

It has been thoroughly debunked by multiple researchers, including Wright, whose co-authored breakdown of the key claims ends with a more rational conclusion, one that the wider community shares: It’s a comet (or asteroid) whose shape is no weirder than many of the objects found in the outer solar system and whose odd acceleration can, in fact, be explained by several different natural processes, including the vaporization of ices acting like an ephemeral rocket booster. (Researchers at the SETI Institute and members of Breakthrough Listen also found no radio signals coming from ‘Oumuamua.)

The sun sets behind Rubin Observatory, which is under construction on Cerro Pachón in Chile. Rubin Obs./NSF/AURA/W. O’Mullane

Like those UAP hearings in Congress, this sort of breathless hype is distracting from the real work that SETI researchers conduct.

Optical SETI is an increasingly popular field of research — and it’s about to get even easier to do, thanks to the under-construction Vera C. Rubin Observatory in Chile, a machine equipped with a next-generation telescopic eye that can see vast swaths of space while also spying faint objects very far away. Most observatories can only do one or the other. But Rubin will find millions of new objects in the solar system every single year, including comets, asteroids, and even interstellar objects visiting our galactic backwater.

Its forensic 10-year census will provide astronomers with a visual encyclopedia of the solar system’s menagerie, from the regular, round-ish asteroids and comets to the odder ones that look like dog bones, cigars, or snowmen, from those close to Earth to those at the fringes of the solar system.

From then on, scientists would be able to quickly spot an object that, compared to the hundreds of millions of others on record, looks genuinely unnatural. “If the Death Star was sitting out at 200 au [200 times the Earth-sun distance], probably we would see it,” says Meg Schwamb, an astronomer at Queen’s University Belfast. Not only that, but if an object isn’t orbiting the sun in a way that can be explained by conventional physics, the Rubin Observatory would be able to spy it acting weirdly. “It’s a reasonable question to ask: Did anything move in the wrong direction?” says Schwamb.

If you know, you know

SETI research is grueling work. No matter how scientists do it, it takes time, effort, and heapings of healthy skepticism. Those looking for the tangible gratification of a classic UFO are almost certainly going to be very disappointed.

Of course the US military is hiding things, says David Spergel, an astrophysicist at Princeton. But that doesn’t mean it’s hiding aliens.

The way Spergel sees things, an alien intelligence visiting Earth either wants to be seen — in which case it would be rather flamboyant about it, showing itself to governments and citizens alike — or it doesn’t, in which case it will remain inscrutable to everyone, even spies. They would not attach blinking lights to their interplanetary spacecraft, just as Ukraine’s drones do not announce their existence to Russian forces with flashing underside lights.

Spergel quips that the only way that bright lights on an alien reconnaissance spacecraft make even the tiniest bit of sense is if an extraterrestrial intelligence is pranking us. “What if the aliens we see are basically teenagers cow tipping — and we’re cows?” he says, smirking.

Spergel is also the chair of a recently inaugurated NASA committee on UAPs, one whose members have been discussing ways in which the space agency and its commercial partners can (unlike the US military) transparently gather and share data to assist the American government’s analyses of potential UAP sightings.

That transparency is emblematic of SETI research as a whole. Those conducting it want to be public about it, to share their excitement, to nix the daydreams of conspiracy theorists — and to underscore that it is tough work that comes with no guarantees.

In that 1959 paper, Cocconi and Morrison noted that “the probability of success is difficult to estimate; but if we never search, the chance of success is zero.” More than half a century later, that feeling is still commonplace among SETI researchers.

“Yes, you have to be patient. Yes, it’s hard and frustrating,” says Siemion. “But at the end of the day, you’re uncovering the greatest mystery in the universe.”

To him, the most interesting part of the search for alien intelligence is as philosophical as it is scientific. Channeling the late Carl Sagan, he says the “most interesting property of the universe by a wide margin is that somehow it has evolved a capacity to know itself, to ask questions about itself.”

In other words, billions of years of unconscious physics and chemistry created biology, and another few billion years have resulted in at least one species (i.e., us humans) that wonders aloud how everything came to be, effectively giving the cosmos self-awareness.

Wouldn’t it be nice to know that we aren’t the only ones capable of that? Everyone needs some alone time, but nobody likes to be truly, permanently alone. SETI scientists are simply applying that notion on a species-wide scale. We could potentially join other intelligent species on a quest for self-understanding. They might not know the answers to the Big Questions — the “why are we here” category of queries — any more than we do, but we can join them in figuring it out.