This concept came out of a workshop we held at the UK Centre for Astrobiology in 2014, to attract young astrobiologists across lots of different disciplines to work on new projects. This project asked a very simple question: can dead civilisations be spotted at interstellar distances?
There is a train of thought that says the reason that we don’t see any signs of intelligent life is because civilisations have a short lifespan. If this theory is true, then there are ruins dotted all over the Galaxy. If we could figure out a way to detect these remains, not only could we prove that other civilisations have existed, but we could also say something about the typical civilisation lifespan. This knowledge is extremely relevant for understanding the future of human civilisation!
We were motivated by recent advances in exoplanet detection methods, in particular spectroscopy techniques which are allowing us to probe exoplanet atmospheres for the first time. As we get better at doing this, astrobiologists are hopeful that we will see signs of biological activity in exoplanet atmospheres, and perhaps even the first indications of technology (such as pollution). If we can begin to see chemicals such as CFCs in exoplanet atmospheres, could we see the signs of a civilisation’s end?
The three of us got together and discussed the possible ways that humanity could end it all, and what traces that would leave for alien astronomers to search for. We came up with several gruesome ends, and thought about how these would show themselves in exoplanet observations.
We started by thinking about nuclear war, a common source of dread for humans for decades. If we detonated all our nuclear weapons, how would that change the Earth as viewed by alien astronomers? They would be extremely fortunate to witness the actual detonation events, which would bathe the Earth’s atmosphere in gamma rays and other high energy particles, but these would be pretty weak and difficult to spot unless our alien observers were actually in the Solar System.
The fallout would spray the atmosphere with large amounts of dust and radioactive particles, which would change the planet’s spectrum significantly at infrared wavelengths. The ionisation of the upper atmosphere would also be quite obvious to astronomers with a particularly good ultraviolet telescope, i.e. one much better than we can currently build.
If a genetic experiment goes out of control and kills all life on Earth, then that might also be visible. Decaying organisms emit very specific chemical compounds that can only be produced by biological sources, and a global extinction event would release huge amounts into the atmosphere. However, our alien observers would have to be quick, as this would rapidly disappear from the Earth’s atmosphere over the course of a year or so.
Nanotechnology could be just as devastating. A self-replicating nanobot could rapidly turn the world’s carbon (i.e. all living organisms) into sand in a couple of months. That sand would enter the atmosphere as aerosol particles, and the world would be covered in deserts made of perfect sand grains. There might be some very odd signatures in such a planet’s atmosphere, especially as the planet moves into secondary transit (where the host star comes between us and the planet).
Pollution of course is another possibility. This could be pollution of the host star, where a civilisation uses their Sun as a hazardous waste trash can, which would show up in spectra, or pollution of the planet‘s atmosphere, or even pollution of the planet’s orbital environment, which humans have become quite good at doing.
Finally, for a bit of fun, we thought about what would happen if a planet was completely destroyed (such as the fate of Alderaan in Star Wars, when it became a target of the terrifying Death Star). This might sound a bit frivolous, but when you hear SETI scientists talk about Dyson spheres and megastructures around stars, what they neglect to mention is that the raw material required would probably mean the destruction of several terrestrial planets!
So what did we conclude about detecting dead civilisations? For almost every scenario we came up with, we found that current technology was still unable to observe the signatures we were looking for. It may not even be possible with the next generation of space telescopes and ground based surveys, unless the destruction is vast, and happens to occur within a few years of us looking at the system.
However, it certainly seems true that within the coming century, the trajectory of exoplanet detection science is good enough that we will be able to start looking for the remains of alien technology. When that happens, we’ll know in much better detail what the fate of our own civilisation could be.