It’s generally assumed that we will eventually find signs of life in the galaxy. But rarely do we consider searching for advanced civilizations that have destroyed themselves. Here’s how we could do it—and what the search for dead aliens could tell us about our own future.

Typically, astrobiologists and SETI enthusiasts are in the business of searching for signs of active extraterrestrial civilizations. Proposed signatures that could be detected by astronomical instruments on Earth include radio and optical signals, megascale engineering objects (such as Dyson Spheres) radiating in the far infrared, artificial illumination, and abnormal levels of atmospheric carbon dioxide suggestive of a post-industrial age civilization.

But a new study by a research team from Cornell University’s Carl Sagan Institute suggests we should also look for signs of alien civilizations that have destroyed themselves. It would be a grim task, but the ability to detect extinct civilizations would not only tell us something about the prospect of intelligent life in the galaxy, it might also tell us something about our own fate as well.


Refining the Drake Equation

“In a way, it doesn’t really matter if we have evidence for living or extinct life,” says study co-author Jack O’Malley-James. “Finding evidence for a civilisation that has destroyed itself still gives us evidence that there is other life out there. Furthermore, looking for the kind of destructive events that could only be caused by intelligent life tells us something about whether intelligent life is out there.”


Indeed, the ability to quantify the number of self-destructive civilizations across the Milky Way would have a significant influence on the numbers we plug into the Drake Equation—the famous formula that seeks to predict how many communicable alien civilizations exist in the galaxy. In addition to estimating such variables as the number of stars and planets, the Drake Equation also requires us to quantify the fraction of planets with life that go on to develop intelligent life, the fraction of civilizations that emit detectable signs of their existence into space, and the length of time these civilizations release these signals.

Consequently, it’s crucial for astrobiologists to understand the frequency at which civilizations emerge and destroy themselves.

“It would tell us something about the civilisation lifetime term,” O’Malley-James told io9. “If a lot of self-destructive civilisations were found, it would suggest short civilisation lifetimes are common. If we don’t find any, it could mean that civilisation lifetimes are long, but it could also mean that the emergence of intelligent life is rare, or that intelligent life deliberately masks its presence.”

Signs of Death

Using our planet and our own civilization as a blueprint, O’Malley-James, along with colleagues Adam Stevens and Duncan Forgan, contemplated several scenarios in which humanity could plausibly destroy itself. Each of the cataclysmic end-games considered resulted in some form of signature that could be probed by astronomers here on Earth. The researchers also considered the extreme distances involved and the limited timeframes for detecting certain signatures.

Though there are many existential risks confronting humanity, the researchers limited their choice of scenarios to those that are self-inflicted, technologically plausible, and result in the total elimination of a civilization. The team came up with these four distinct scenarios:

  1. Complete nuclear, mutually ­assured destruction
  2. A biological or chemical agent designed to kill either the human species, all animals, all eukaryotes, or all living things
  3. A technological disaster such as the “grey goo” scenario, or
  4. Excessive pollution of the star, planet or interplanetary environment

Let’s take a quick look at each of these.

Mutually Assured Destruction

No doubt, the prospect of mutual annihilation from global nuclear war is a very real one indeed—one that’s likely confronted by alien civilizations as well.

(Credit: U.S. Army Photographic Signal Corps/public domain)

When ignited, nukes produce a short, intense burst of gamma radiation that could be detected from afar using similar techniques to detect gamma ray bursts. Given just how short these bursts are likely to be, we would have to be fortuitously lucky to observe such an event. Moreover, the total energy released would have to be at least nine orders of magnitude larger than expected models of nuclear war for it to be detectable via gamma rays outside a stellar system.

More plausibly, astronomers would have to look for spectral signatures indicative of fallout from nuclear payloads. Not only would they persist for longer timescales, they would be visible in the atmospheres of exoplanets. For example, a large amount of beta-radioactive material would have a pronounced effect on atmospheric chemistry, producing specific spectral signatures. In addition, a blast would throw up a tremendous amount of dust into the atmosphere; the resulting nuclear winter would significantly increase atmospheric opacity.

“Global nuclear war therefore potentially offers several spectral signatures that could be observed,” write the authors in the study, adding that “to confirm that a planet had been subject to a global nuclear catastrophe would require the observation of several independent signatures in short succession.”

Indeed, as O’Malley-James told io9, we can never be one-hundred-percent sure that what we’re seeing is a nuclear armageddon.

“A suddenly dusty atmosphere could equally be the result of an asteroid strike, but if we had other evidence, like gamma radiation bursts and the destruction of ozone by ionizing radiation, for example, that would lend evidence to the case for the destruction being artificial, rather than natural,” he says.

Biological and Chemical Apocalypse

A number of experts reasonably fear that we’ll eventually use human-engineered biological or chemical agents to do ourselves in. This could involve bacteria, viruses, or other agents to intentionally cause death on a massive scale. Hence the fears surrounding the decoding of naturally occurring viruses and the publication of these genomes for all to see.

(Credit: Stokkete/Shutterstock)

But could we ever detect something like this through the channel of space? O’Malley-James and his team believe it’s possible, though it depends on the particular scenario. For example, a sufficiently nasty and indiscriminate biological agent would not just destroy humanity, it would also destroy a good portion of the planet’s biosphere. The resulting decay of this biomass could be detected from earth via spectral imaging, including, for instance, the sudden and massive release of methanethiol.

“For something like this, the signs would be indirect,” he told io9. “It would be the mass dying-off of alien people and animals that would be detectable. If this happened suddenly, over a short period of time, it could be argued that we were witnessing some kind of advanced biowarfare, but we could never be completely certain.”

Scanning for Grey Goo

Similarly, nanotechnology experts, including Eric Drexler and Robert Freitas, warn about a grey goo scenario—a grim prospect in which microscopic, exponentially self-replicating robots consume most of the planet’s resources over a short period of time, converting the Earth’s surface into a useless pile of goo.

(Credit: The Day the Earth Stood Still (2008))

But could we actually detect this from Earth? Interestingly, it’s a scenario that’s similar to a nuclear winter.

“In the case of grey goo, we may expect there to be a substantially larger amount of ‘dust’, as well as a fixed grain size,” write the authors in the study. “This will be deposited as sand dunes or suspended in the atmosphere, with similar spectral signatures [to a nuclear winter].”

Even though they’re microscopic, O’Malley-James says that, in large number, their cumulative effects on a planet could very well be detectable.

“Planet wide changes, like surface brightness changes as the land becomes covered in ‘grey sand’, would provide the kind of evidence we could detect,” he says.

Pollution of Star and Planet

One of the more interesting end-game scenarios proposed in the paper includes the pollution of the host star and planet.

It’s conceivable, for example, that a sufficiently advanced civilization would opt to dispose of its hazardous waste in its host star. According to the researchers, it’s possible that, by over-polluting its own sun, a civilization could actually alter the star’s energy output, shifting the habitable zone boundaries of their system and freezing and/or frying their own planet.

“Atmospheric pollution of the host planet is an easier scenario to imagine and would potentially be detectable, but, as evidenced by humanity’s continued existence, this does not necessarily imply a civilisation has destroyed itself,” says O’Malley.

(Credit: Gravity (2013))

In addition, astronomers looking for signs of dead aliens could look for the remnants of a Kessler Syndrome. This scenario was recently portrayed in the film Gravity, where a growing cascade of debris destroyed satellites in Low Earth Orbit. A Kessler event is actually quite plausible, and, given our reliance on satellites, represents a significant risk to our ongoing society and technological development.

“If a civilisation is very dependent on orbiting satellite technology and/or if there is a large mass of stuff orbiting their planet, then a Kessler Syndrome event could harm or destroy that civilisation,” O’Malley-James told io9. “Detecting the ring of orbital debris this leaves behind would be incredibly challenging, but potentially possible to detect with a highly sensitive telescope for a transiting planet.”

The Search for Dead Extraterrestrial Intelligence

As depressing as this topic is, O’Malley-James and his colleagues are clearly correct to point this all out. As noted, it will be exceptionally difficult to detect these signatures, and even when they are detected, it’ll be very difficult to prove that what’s being observed is some sort of apocalyptic event. That said, the next generation of telescopes, including the James Webb Space Telescope, will allow us to better characterize the atmospheres of exoplanets. And as O’Malley-James told me, it will only be a few more decades before we have access to instruments that will be sensitive enough to give us clearer results.

(Credit: Long Pham/Concept Ships)

Looking ahead to that era of SETI exploration, such searches could paint either extremely optimistic or pessimistic visions of advanced life, and by consequence, the human future. It’s possible, for example, that none, or very few, of these signatures will ever be detected. Which would be good news. Alternately, an abundance of these signatures would be exceptionally alarming, particularly if they appear in the absence of extant signs of intelligent life. Sadly, these searches could ultimately prove the presence of, or at least strengthen, the idea of the Great Filter—an existential barrier that cannot be conquered by any technological civilization.

Read the entire study at the pre-print arXiv: “Observational Signatures of Self-Destructive Civilisations”.

Contact the author at and @dvorsky. Top image by Jonas de Ro/Warner Bros/Jupiter Ascending Concept Art/Concept Ships