Astrobiologists are calling on NASA to use what geologists are learning about a new era sometimes called the “Anthropocene” to help develop space missions for the search for extraterrestrial intelligence.
The Anthropocene, although not yet officially recognised as a geological era, reflects the time when humans began influencing the Earth strongly enough to leave significant traces in the geological record. These include everything from ecological changes to planet-wide deposits of radioactive fallout from nuclear bomb tests. There are also materials such as plastics, concrete, and exotic minerals that are likely to find their way into the geological record.
“We are in an era now where human influence is not just starting to affect the climate, but enacting permanent changes that will be visible in the rock record for millions of years,” says Jacob Haqq-Misra, an astrobiologist at the Blue Marble Space Institute of Science, in Seattle, Washington, US.
Not that astronomers can peer into the rock strata of alien worlds in the hope of finding traces of existing (or vanished) civilisations. What they can do is look for changes in planetary atmospheres — changes that Haqq-Misra and colleagues refer to as “techno-signatures” in a policy paper submitted to the US National Academy of Sciences.
Astronomers are already starting to search exoplanet atmospheres for signs not just of habitability, but of life — revealed, perhaps, by gases such as methane, oxygen, and ozone. But those are merely bio-signatures. Techno-signatures would be more exotic.
“If you saw a planet like Earth, around a yellow star and you saw methane and ozone and carbon dioxide, that’s a classic bio-signature,” Haqq-Misra says. “But if you see a planet like Mars, just a little outside the traditional habitable zone, where you’d expect it to be cold, but you observe CFCs (chlorofluorocarbons) and PFCs (perfluorocarbons) and exotic greenhouse gases that seem to be raising the [planet’s temperature], that would be pretty good evidence for terraforming — like us terraforming Mars.”
Techno-signatures might also take the form of infrared light that might represent heat loss from large-scale engineering projects, such as orbiting solar collectors or planetary sun shades designed to offset global warming.
Such things, Haqq-Misra says, might lie in the future of any technologically advanced civilisation faced with the dual problems of population growth and increasing energy needs.{%recommended 606%}
In fact, he notes, a recent example of a star in which astronomers wondered if such things might be going on is KIC 8462852, better known as “Tabby’s Star” for its discoverer, Tabetha Boyajian, of Louisiana State University.
Since 2015, this star has undergone several mysterious dips in brightness —much as one might expect if it its light were periodically being blocked by orbiting alien megastructures. Sadly for astrobiologists, its mystery now appears to be better accounted for by dust clouds.
“But people got excited because it was an unknown signature,” Haqq-Misra says, “It got the astrobiological community thinking, okay, we have to acknowledge the possibility we could see something like this.”
To pursue such searches, he adds, what’s needed is the next generation of space telescopes, still in the planning stages. The most important of these, he says, are projects called HabEx, LUVIOR and the Origins Space Telescope (OST).
“All of these would have capabilities certainly to do bio-signatures, and at least the OST would be able to get into techno-signatures,” he says.
Such research would not only tell us how common life is on other planets, but might help determine the stages through which life-harboring planets go. “How common are inhabited planets?” Haqq-Misra asks. “How common are technological civilisations?”
The answers could be cause for both optimism and pessimism, he adds. Pessimism because we might learn that technological civilisations are rare and short-lived, rapidly outgrowing their resources. Optimism because, knowing that, we might be able to chart a better future for ourselves.
Chris McKay, an astrobiologist at NASA Ames Research Center in Moffett Field, California, says the paper doesn’t break new ground, but is “a nice connection of many different ideas.” He notes, however, in a comment that would side with Haqq-Misra’s pessimists, that the entire concept of talking in terms of an “Anthropocene” era makes it sound as though it’s just another step in long-continuing planetary processes.
“I would expect that with the emergence of humans and their technology … there is not a continuity, there is a profound break,” he says.
This, of course, simply adds to the urgency of Haqq-Misra’s astrobiological goals. Assuming life exists at all on other planets, he says, the big question is what happens if it then develops to intelligence.
“Is what we have really special?” he asks, “Or do lots of planets go through this phase where we’re at, but none of them make it off the planet because they grow too quickly and use too much energy? Are there hard limits to growth that apply to intelligent species in general?”
