Searching for Life … and Death … in the Universe

exoplanet with dyson swarm

exoplanet with dyson swarm
Artist impression of an exoplanet and its dyson swarm.

By Douglas Vakoch, Director of Interstellar Message Composition, SETI Institute 

For over a half century astronomers have been seeking out signs of sentient life in the Search for Extraterrestrial Intelligence (SETI). Now a group of scientists wants to reverse this strategy. Instead of searching for signs of life beyond Earth, they want to hunt for signs of death in the cosmos.  

In a paper titled “Observational Signatures of Self-Destructive Civilisations” to appear in the International Journal of Astrobiology, Adam Stevens, Duncan Forgan, and Jack O’Malley-James argue that the next generation of astronomical instruments may be able to detect the remnants of global catastrophes that have wiped out life on distant planets.  

“Civilisations which initiate a nuclear catastrophe produce strong but relatively brief signatures of their destruction, which are partially masked by the dust thrown into the atmosphere by multiple nuclear detonations,” the researchers noted. “Victims of bioterrorism produce powerful atmospheric signatures of decaying organic matter, but these dissipate on timescales of a few decades.”  

With recent exoplanet discoveries, we are finding other worlds that are increasingly similar to Earth. But even if life evolves on such worlds, and then develops the ability to communicate at interstellar distances, the life span of such societies is also a factor to consider when trying to detect them.  

Answers may not be quick in coming. Although the extinction scenarios that Stevens and his colleagues explore vary from case to case in their detectability, the researchers conclude that “in most instances, the destruction of a technological civilization leaves atmospheric traces that persist for a short time, requiring observations to be relatively serendipitous.” 

What then are the implications for finding evidence of extinct extraterrestrials? The authors of this paper have suggested some things to look for, but the challenge is in identifying realistic ways we might be able to do so.  

“The odds of observing any of these clues – even aside from the technology problems – are statistically small, given their extraordinarily limited temporal extent,” notes Seth Shostak, Senior Astronomer at the SETI Institute, as he assesses the various scenarios presented in this new paper. “The exception is if a planet has been blown up, and contaminated the interplanetary medium.  That evidence would be around for a long time, but would be impossible for anyone to detect today.”

A Long History 

The scientific search for intelligence in the universe has long included strategies that would look for unintentional remnants of technological civilizations. Perhaps the most famous proposal is the Dyson sphere, suggested by Freeman Dyson in his 1960 Science paper. Dyson hypothesized that an advanced civilization with tremendous energy requirements might create a shell, or more realistically a swarm, of solar cell satellites to encircle its home star, capturing energy at visible wavelengths and in turn producing excess radiation in the infrared. The result would be an electromagnetic signature that stands out as distinct from ordinary stars.   

So hope remains of detecting extraterrestrial civilizations by searching for their byproducts. “One thing we could look for today is, for example, depleted deuterium in nebulae – the consequence of fusion activities to generate energy – for direct use by civilizations or for their rockets,” Shostak said. “That sort of thing could be done straightforwardly with optical telescopes.”  

And of course there’s always the “tombstone beacon” idea, an artifact of an expired civilization that might lend itself to detection with something like the Allen Telescope Array, the SETI Institute’s 42-antenna instrument in Northern California. The related idea that extraterrestrials might leave a “cosmic time capsule” was explored by the late Albert Harrison at an international workshop held at the SETI Institute in November 2014.  

To find such transient remnants of an alien apocalypse, humankind may need to sustain its search for generations—simply to be observing at the same time these changes are visible from Earth. And we will need to avoid the same cataclysmic events ourselves. Our own civilization has had the ability to detect other intelligence in the universe for about as long as we’ve had the capacity to destroy ourselves as a species. The current state of our evolution as a society provides a chilling reminder of the high stakes of this search, and the impact of what we might discover.