Features Physics World  October 2021

Scanning the cosmos for signs of technology

Ever since planets beyond our solar system were first discovered, astronomers have been hunting life beyond our world. While biological signatures are crucial, the idea of scouring the skies for signs of technosignatures from advanced civilizations is gaining momentum, as David Appell discovers 

Probing pollution One indication of advanced alien life could be industrial pollution. Therefore, the presence of gases such as nitrogen dioxide might serve as a technosignature that we could detect on exoplanets. (Courtesy: NASA/Jay Freidlander)

In 1802 the young German mathematician Carl Friedrich Gauss suggested a way to make our presence known to would-be Martians – by clearing a huge area in the Siberian forest, planting it with wheat, and creating a pattern indicative of the Pythagorean theorem. Some 80 years later, astronomer Percival Lowell – founder of the Lowell Observatory at Flagstaff, Arizona, and proponent of the idea that astronomers had spotted canals on Mars – suggested digging our own canals in the Sahara desert. His plan was to fill the canals with oil and set them alight, thereby attracting the attention of residents of the red planet. 

While neither idea was ever implemented, both were examples of a “technosignature” or a “technomarker” – a telltale indication of past or present technological activity, pointing out the existence of an advanced planetary civilization. Such searches might sound like science fiction, but over the last couple of years, astronomers have been drawing up plans for the technosignatures that might be viewable with the next generation of space telescopes, such as the James Webb Space Telescope (JWST), due to be launched in December.

Astronomers have been drawing up plans for the technosignatures that might be viewable with the next generation of space telescopes

Searching the skies

For about 60 years now, radio astronomers have trained their increasingly sophisticated telescopes at the sky, looking for signals that might have come from extraterrestrial intelligence. While no definitive technosignature has been captured to date, we have picked up a few intriguing signals over the years. These include the famous 72-second-long “WOW!” signal, detected on 15 August 1977 by Ohio State University’s Big Ear radio telescope. There was also the Breakthrough Listen Candidate 1 (BLC1) radio signal observed in April and May 2019 by the privately funded Breakthrough Listen project at the Berkeley Search for Extraterrestrial Intelligence (SETI) Research Center at the University of California. While the WOW! signal bore many of the expected hallmarks of extraterrestrial origin, it was never detected again. BLC1, which came from the direction of our next closest star Proxima Centauri, is still being analysed.

Another huge upheaval for the field in recent decades has been the exoplanet revolution. Since the discovery of the first two exoplanets in 1992, astronomers have made breathtaking use of the Kepler Space Telescope and others to discover more than 4400 confirmed exoplanets around distant stars, with about as many candidates awaiting closer study. Exoplanets come in many sizes and types, from terrestrial rocky worlds to super-Earths, Neptune-like exoplanets, hot Jupiters and more (see “Planets galore” April 2014). On average each star has one planet, but many host a family similar to our solar system. 

With the discovery of all these exoplanets, astrobiologists have been studying “biosignatures”. Analogous to technosignatures, these are the signs of life on alien worlds, intelligent or otherwise. They do this by analysing the electromagnetic absorption spectra of an exoplanet’s atmosphere as it transits its sun, seeking the presence of gases such as oxygen, methane, water vapour and ozone (a proxy for oxygen). The atmospheres of a few Jupiter-sized exoplanets have already been scoured in this way, and the JWST should allow similar searches for smaller, Earth-like exoplanets.

As the science of biosignatures progressed, astronomers realized that similar searches in various wavebands could be carried out for technosignatures too. These don’t have to be dedicated searches; they could piggyback on the same data used to search for biosignatures – so-called commensal observing – or even exploit archived astronomical data going back decades. “Data is king in this part of science,” says planetary scientist Ravi Kopparapu at NASA’s Goddard Space Flight Center. NASA and the National Science Foundation in the US have for the first time funded at least three projects in the technosignature field, and issued two grants, for a workshop and a symposium. 


Exoplanetary investigator The Large UV/Optical/IR Surveyor (LUVOIR) is a multi-wavelength space observatory proposed by NASA that could be ideally placed to spot technosignatures. It is one of the projects under consideration in NASA’s Astro2020 decadal survey. (Courtesy: NASA)

Who’s out there, and how do we spot it?

As we hunt for signs of intelligent life, is anybody really out there? Today, we know that 22% of Sun-like stars harbour an Earth-size planet in their “habitable zone”, where the surface temperature allows for liquid water to exist. With 100 to 400 billion stars in the Milky Way alone, there are tens of billions of planets where life may have developed and evolved. 

But can we see them? Assuming that any detectable technosignature must be within the bounds of our past “light cone” (the path that light from any single event would take through space–time), theoretical astrophysicist and astrobiologist Amedeo Balbi of the University of Rome Tor Vergata in Italy has derived some simple but strong conclusions. To be able to pick up a signal, a civilization must have been established in our past, and the light from it had time to travel to us. So long as there is no preferred epoch for the appearance of exo-civilizations over the history of our galaxy (i.e. they appear uniformly over the history of the galaxy), Balbi reasons that a key factor in detecting a signal from such a civilization is that the technosignature must be as long-lived as possible – preferably on the scale of 100 million to a billion years (AJ 161 222).

“We shouldn’t think in terms of civilization or species longevity,” said Balbi at TechnoClimes 2020, a NASA-sponsored online workshop, “but in terms of technosignature persistence.” As for an observing strategy, it’s better to focus the search on a few long-lived technosignatures than on lots of short-lived technosignatures, he says.

Over the years, there have been many suggestions about the kind of technosignatures we might see from Earth. A brief list includes nightside city lights; atmospheric industrial pollution; solar-energy collectors like silicon-based photovoltaic arrays that would leave an imprint on a planet’s reflected light. We may spot artificial surface constituents; dense orbiting satellite constellations; waste heat from megastructures such as Dyson spheres; and even odd or fading objects observed as they transit their star. Another extreme possibility is “stellar engineering”, where an advanced civilization may go as far as to alter the appearance of stars and other celestial objects in otherwise inexplicable ways. 

Other indicators include electromagnetic beacons such as radio-waves or laser pulses; or even a space probe sent out by an advanced civilization, making its way to our solar system. We’ve already done this ourselves, with the Pioneer 10 and 11 and Voyager 1 and 2 space craft, which are currently in interstellar space. This so-called artefact SETI is a legitimate part of the technosignature field, despite being abused by ridiculous claims, such as that a face on Mars had been spotted when it was in fact just a rock. 

Another possibility is that there could be artificial objects in our solar system or outside it, in the Kuiper Belt or Oort Cloud, that we might recognize by the way they reflect light. While sunlight reflected from a natural, moving object would fade as 1/r4, where r is the object’s longitudinal distance from us; a moving, artificially illuminated source would fade only as 1/r2, a detectable difference.

A technological civilization could well produce artificial lighting, and the aliens may live in dense settings that approximate our urban areas. One intriguing technosignature that’s been suggested is to look for city lights on the nightside of a planet as it passes alongside its star, using a coronagraph to block out the star’s image. To work out if such lights could be detected, astronomer Thomas Beatty at Steward Observatory at the University of Arizona, investigated the possibility using direct imaging of Earth-like commercially available high-power lights on generic Earth-like exoplanets around stars in our galactic neighbourhood (arXiv:2105.09990). We know of about a dozen potentially habitable terrestrial planets orbiting stars within 10 parsecs of the Sun.

Noting that only 0.05 % of the Earth can be considered “heavily” urbanized (with peak night-time illumination of cities such as New York or Tokyo), Beatty calculates that planets around nearby M-dwarf (red dwarf) stars (which are cooler, and therefore dimmer, than the Sun) could be detected by two space-based telescopes that could be funded, as NASA gears up to publish its Decadal Survey in Astronomy and Astrophysics (Astro2020). These are the Large Ultraviolet Optical Infrared Surveyor (LUVOIR) observatory and the Habitable Exoplanet Imaging Mission (HabEx) telescope, which includes a coronagraph and starshade occulter spacecraft to directly image Earth-like planets. 

Beatty found that by using coronagraphs, the lights could be detected using 100 hours of observing time, with urbanizations levels of 0.4% (eight times Earth’s) to 3% of the planetary surface. Planets around Sun-like stars would be detectable with urbanization levels of 10% or higher (since their brighter star makes detection more difficult). Beatty also considers the idea of an “ecumenopolis”, a planet-wide city. Assuming a typical cloud cover for Earth, and that most light seen from space is reflected off concrete and roads, he found that such a planet would be detectable around roughly 50 nearby stars, by both potential telescopes.


Bright lights, big alien city One visible sign of humanity’s presence on Earth is city lights. We can look for similar artificial light signatures on the night side of exoplanets. (Courtesy: Joshua Stevens, NASA Earth Observatory/Miguel Román, NASA GSFC)

Possible signal?

In 2015 citizen scientists from the Planet Hunter project noticed odd fluctuations in the light curve from an F-type main-sequence star, some 450 parsecs from Earth. It drew the attention of professional astronomers, including Tabetha Boyajian of Yale University, who detected an irregular dimming of the star’s brightness of up to 22%. These unexpected observations of “Tabby’s star”, as it is now known, led to conjectures of everything from a planetary debris field, to an extraterrestrial megastructure, to freed exomoons, before finally concluding the most likely cause was space dust. 

Despite the ultimately mundane explanation, the incident motivated Ann Marie Cody, an astronomer from NASA’s Ames Research Center, to survey data from the Transiting Exoplanet Survey Satellite (TESS), which has been monitoring the brightness of tens of millions of stars on 30-minute timescales since 2018. Cody is currently working to automate searches through TESS light curves for rare events such as substantial fading or brightening. 

By using more than 100 different statistical measures, she hopes to differentiate between ordinary anomalies – such as eclipsing binary-star systems – and rarities including occulting materials around the star, solar panels, orbiting megastructures that are harvesting energy, or other unknown technosignatures. Such anomalies will ultimately be referred to ground-based telescopes for more dedicated searches, including radio SETI searches. 

Picking up pollution

Another technosignature could be the pollution that aliens in the early stages of technological development are pumping into the atmosphere of the planets they inhabit. Indeed, atmospheric chemical pollutants could be identified in the same way as biosignatures like oxygen and methane – by looking at the spectral data. 

In 2014 astronomer Henry Lin and colleagues from Harvard identified certain chemical pollutants – such as chlorofluorocarbons – in the Earth’s atmosphere that have significant absorption features in the spectral range covered by the JWST. They found that these chemicals could be detected by the telescope, in the atmospheres of transiting Earth-like planets around white dwarfs, over roughly a day and a half of observing time, if these compounds are present at 10 times the current Earth level (ApJL 792 L7).

Another suggested technosignature pollutant is nitrogen dioxide, NO2, which is found here on Earth as a byproduct of combustion from vehicles and fossil-fuelled power plants. In a study published this year (ApJ 908 164), Kopparapu and colleagues examined whether NO2 could be detected in the atmospheres of exoplanets within 10 parsecs. If the planet was cloud-free, Earth-like NO2 levels – which can reach 5 parts per billion in urban areas – could be detected in the infrared with about 400 hours of observing time using the proposed LUVOIR telescope. Some 40 years ago, NO2 levels in the US were about three times higher than today, so a nascent alien industrial civilization might be detectable with less viewing time.

Spectral templates will come from running climate models that depend on a planet’s features, its host star and the viewing instrument

Characterizing the signatures of NO2 and chlorofluorocarbons is half of the first NASA non-radio technosignature grant ever awarded, in June 2020, to Adam Frank of the University of Rochester in New York. Called Characterizing Atmospheric Technosignatures (CATS), the $287,000 grant is being used to build an online library of spectral lines. Frank is looking at what kind of signature a planet’s technology might leave in its spectra. The spectral templates will come from running climate models that depend on a planet’s features, its host star and the viewing instrument, all of which can be compared with what astronomers actually observe.

The second half of the CATS project is to build similar templates for potential ground-based solar panels, which would leave an imprint in the exoplanet’s reflected light from the minerals in the panels, after the light passes through the atmosphere’s constituents. “The hard core of CATS is running climate models,” says Frank. 


Energy harvester The classic Dyson sphere is a “shell” (top left) that completely surrounds a star. This would be mechanically unstable, but other Dyson megastructures that are more likely to work include the ring (top right), bubble (bottom left) and swarm (bottom right). (Courtesy: iStock/dottedhippo; CC BY/Vedexent; CC BY/Kevin Gill; CC BY/PNG crusade bot)

Alien megastructures 

A notable technosignature would be the detection of a Dyson sphere – a hypothetical megastructure first proposed by Freeman Dyson in Science magazine in 1960. Originally conceived as a hollow shell that an advanced exocivilization might construct surrounding its host star, the sphere would capture all of the star’s energy – in our case, two billion times more energy than falls on the Earth’s upper atmosphere. The US theorist John Wheeler later extended the idea to a similar shell around a spinning black hole. 

Recently Tiger Yu-Yang Hsiao from National Tsing Hua University in Taiwan and co-authors found more favourable energy-extraction possibilities from a black hole’s accretion disc, corona or relativistic plasma jets (MNRAS 10.1093/mnras/stab1832). A super advanced civilization might even go as far as to surround a galaxy to capture the total electromagnetic energy emitted by its stars and black holes. For the Milky Way galaxy, that’s at least 400 billion stellar luminosities, or ≳1038 W. 

An actual shell, perhaps constructed out of a system’s planets and asteroids, would be mechanically unstable (as Dyson knew) but other megaconstructions, such as a spherical “cage”, “swarm”, “bubble” or “ring” would work. Solar collectors on these structures could beam microwaves down to the planet’s surface for power, which could drastically modify the star’s spectrum, creating an infrared blackbody. In the case of our Sun, which is a G-type main-sequence star, a shell at the Earth’s orbital distance would glow with waste heat with a blackbody spectrum at a temperature of about 300 K, radiating at a maximum wavelength of 10 μm. The researchers also found that a hot Dyson sphere around a stellar-mass black hole in the Milky Way within 10 kiloparsec of us would be detectable in the ultraviolet, optical, near-infrared and mid-infrared, making it viewable by current instruments such as the Wide Field Camera 3 on the Hubble Space Telescope. 

Several searches for Dyson structures have been carried out over the years, both within our galaxy and beyond, starting with data from the Infrared Astronomical Satellite launched in 1983. “Today, the Gaia mission by the European Space Agency is measuring precise distances to hundreds of millions of stars, which will greatly improve the efficiency of searches for Dyson spheres in the Milky Way,” says astronomer Jason Wright, from the Center for Exoplanets and Habitable Worlds at Pennsylvania State University. “No longer will our searches be confused by the millions of quasars and other objects that confuse searches using the WISE survey. We are also honing our detailed models for what the observational characteristics of Dyson spheres would be, which helps us know exactly what to look for.”

Ultimately, finding proof of extraterrestrial life will come from collecting vast amounts of data on technosignatures and biosignatures – unless the aliens pay us a visit. “We are unanimous in our conviction that the only significant test of the existence of extraterrestrial intelligence is an experimental one,” wrote Carl Sagan in a “petition” to Science in 1982 (218 426) that included more than 60 other signatories. “No a priori arguments on this subject can be compelling or should be used as a substitute for an observational programme,” he wrote, urging a “co-ordinated, worldwide and systematic search”. Hopefully, the coming century will see Sagan’s vision of detecting some sign of extraterrestrial intelligence fulfilled.