Man-made greenhouse gases have long been a hot button topic here on earth since as pollutants, they are largely responsible for anthropogenic climate change. But what about alien civilizations that intentionally use greenhouse gases to save their planet from the onset of a snowball earth type situation; that is, a completely ice-covered planet? Or to terraform a frozen desert-like planet, not unlike Mars? Or even to stave off the effects of a long period of planetary glaciation?
In a paper to appear in The Astrophysics Journal, the authors outline the rationale in looking for the technosignatures of such artificial greenhouse gases in the atmospheres of far-flung planets.
Previous papers have advocated looking for atmospheric pollution of such exoplanets in the process of dealing with the sort of chlorofluorocarbon pollution that reached its height several decades ago.
In contrast to passive incidental byproducts of industrial processes, artificial greenhouse gases would represent an intentional effort to change the climate of a planet with long-lived, low toxicity gases, the authors of this new paper write.
We outline two scenarios where artificial greenhouse gases could produce a detectable signature, Edward Schwieterman, the paper’s lead author and an astrobiologist at the University of California, Riverside, told me via email. The first would be if an alien civilization were to terraform an otherwise uninhabitable planet in their planetary system, as humans have proposed for Mars, he says.
The second scenario would be if a civilization were to deploy these gases to arrest an ice age on their planet, says Schwieterman. For us, the advantage of such a technosignature would be that it could be long-lived and would not require a deliberate effort by the E.T. society to communicate, he says.
Most importantly, we would look for these anomalies as we characterize planets in order to answer other scientific questions, says Schwieterman. So, we may as well be on the lookout for these signatures as we analyze planetary spectra to learn about rocky exoplanets in general, he says.
The team used computer models to simulate realistic planetary spectra of earth-like planets with various concentrations of these gases relevant to climate modification. A couple of compounds are highly absorptive in the mid-infrared spectrum which makes them excellent greenhouse gases.
C2F6 (hexafluoroethane) has a heating potential of 10,000 times that of carbon dioxide over a 100-year timescale, says Schwieterman. SF6 (sulfur hexafluoride) has a warming potential of 23,500 times that of CO2 over a 100-year timescale, he says. These gases are also chemically inert and don’t have the deleterious impact of destroying the ozone layer, says Schwieterman.
And they are also long-lived, with lifetimes of a thousand years or more. The first place to look might be a planet in the relatively nearby Trappist-1 system, located some 40 light years away.
We simulated a hypothetical version of TRAPPIST-1f with a terraformed atmosphere and examined whether these gases would be detectable in its atmosphere, says Schwieterman. TRAPPIST-1f was chosen because it is in the outer habitable zone of TRAPPIST-1, receiving only 35% of the amount of stellar radiation as earth does from the sun, he says. Depending on the gas or combination of gases, 1 to 100 parts-per-million concentration of artificial greenhouse gases could be detectable with the James Webb Space Telescope, says Schwieterman.
A Terraformed Planet Would Stand Out As Odd
A terraformed planet would look “odd” in the infrared, says Schwieterman. It would appear bigger than it should in mid-infrared transmission spectroscopy and colder than it should in emitted light, he says.
As for the time needed to detect such terraforming?
If a planet is terraformed using artificial greenhouse gases, those greenhouse gas levels would have to be maintained over time, says Schwieterman. This could be the entire remaining lifetime of the civilization or longer (if such maintenance were automated), he says. So, it is not the time it takes to terraform a planet that matters for detectability, but the amount of time that terraformed state would be maintained, says Schwieterman.
As to how many such terraformed planets might lie within a thousand light-years of earth?
The answer may be zero, but we have a better chance of fingerprinting a terraformed planet than finding signs of an alien civilization’s industrial pollution, says Schwieterman. We’re more likely to “catch” a planet with a deliberately modified climate than a planet undergoing a short-lived high pollution era, he says.
