We tend to assume that our mistakes as a species flag us as immature, a young civilization blundering about with tools it is misusing on a course that may lead to extinction. But assume for a moment that an intelligent extraterrestrial civilization goes through phases more or less like our own. If we’re sifting through radio signals and looking for optical flashes to find them, shouldn’t we consider other ways such a civilization announces itself? What if we’re not the only polluters in the universe, for example, and other cultures are making the same mistakes?
In a 2010 paper, Jean Schneider (Observatoire de Paris-Meudon) and colleagues noted the possibility of using pollutants as a way of moving beyond biosignatures to find ETI. Let me quote from the paper:
…another type of far from equilibrium signals can be seen as techno-signatures, i.e., spectral features not explained by complex organic chemistry, like laser emissions. In the present state of our knowledge one cannot eliminate them a priori, although we have no guiding lines to search for them. For instance, in the present Earth atmosphere, CFC (Carbon Fluoro Compounds) gases are the result of technological chemical synthesis. Observed over interstellar distances, they would reveal to the observer the presence of a technology on our planet.
Lisa Kaltenegger (Harvard-Smithsonian Center for Astrophysics) and James Kasting (Pennsylvania State) have been looking at the CFC idea for some time. As Kasting told New Scientist in 2009, “There’s a whole host of things we make industrially as solvents, cleaners and refrigerants – they certainly have absorption lines. If you had a big enough telescope, you could detect them.” CFCs themselves absorb infrared light at specific wavelengths and are detectable at very low concentrations, as low as a few parts in a trillion. Moreover, they do not form naturally, and though a detection would be tricky, Kaltenegger has suggested a future array of space telescopes working at infrared wavelengths should be able to do the job. For more on this, see To Spot an Alien, Follow the Pollution Trail, the original article in New Scientist.
Image credit: CfA.
But would it take such a futuristic flotilla of telescopes to spot pollution? New work out of the Harvard-Smithsonian Center for Astrophysics suggests even the James Webb Space Telescope may be up to the challenge. The paper argues that atmospheric levels of CFCs about ten times greater than we have produced here on Earth could be traced by JWST. The focus here is on tetrafluoromethane (CF4) and trichlorofluoromethane (CCl3F), described as the easiest CFCs to detect. Henry Lin and team point out that the lifetimes of CFCs range from 10 to 105 years, meaning that a polluting civilization existing any time in the past 105 years would be theoretically detectable.
But the strategy only works in detecting pollutants on Earth-like planets circling a white dwarf star, a scenario that maximizes the atmospheric signal. Recent work has shown that white dwarfs can have long-lived habitable zones, and the similarity in size between the planet and star offers the best contrast between the planet’s atmospheric transmission spectrum and the star it is transiting. The paper considers white dwarfs that have cooled to surface temperatures around 6000 K, the same surface temperature as the Sun, so that the habitable zone is close in, at about 0.01 AU, which greatly increases the chance of a transit.
From the paper:
…a recent study by Worton et al. (2007) estimates the atmospheric concentration of CF4 at ~75 parts per trillion (ppt), whereas CF4 levels were at ~40 ppt around ~1950. Assuming a constant production rate…we expect as a very crude estimate that in roughly ~1000 years, the concentration of CF4 will reach 10 times its present levels. Coupled with the fact that the half-life of CF4 in the atmosphere is ~50,000 years, it is not inconceivable that an alien civilization which industrialized many millennia ago might have detectable levels of CF4. A more optimistic possibility is that the alien civilization is deliberately emitting molecules with high GWP [global warming potential] to terraform a planet on the outer edge of the habitable zone, or to keep their planet warm as the white dwarf slowly cools.
The JWST should be able to detect CF4 and CCl3F signatures in the atmospheres of transiting Earths around white dwarfs as long as their concentrations are on the order of ten times that of the Earth — CF4 detection demands 1.7 days of exposure time on the instrument and CCl3F 1.2 days. The exposure time is already built into biosignature study times, given that these will take on the order of an entire day to detect. Looking for pollutants, then, adds little in terms of additional observing costs.
Bear in mind that while some CFCs last for tens of thousands of years in the atmosphere, others persist no more than ten. Harvard’s Avi Loeb, one of the trio of researchers in this project, points out that finding a short-lived CFC on an exoplanet would signal an active civilization. The other scenario may be starker. If we detect molecules from long-lived CFCs but none of the short-lived variety, we could be witnessing a changing civilization. “In that case,” says Loeb, “we could speculate that the aliens wised up and cleaned up their act. Or in a darker scenario, it would serve as a warning sign of the dangers of not being good stewards of our own planet.”
The paper is Lin et al., “Detecting Industrial Pollution in the Atmospheres of Earth-like Exoplanets,” accepted by The Astrophysical Journal and available as a preprint. The Jean Schneider paper is “The Far Future of Exoplanet Direct Characterization,” Astrobiology 10 (2010), p. 121.