An illustration of the Earth with the continents in their current form, but with the entire planet … [+]
Man-made greenhouse gases have long been a hot topic here on earth because, as pollutants, they are largely responsible for anthropogenic climate change. But what about extraterrestrial civilizations that deliberately use greenhouse gases to save their planet from a snowball from Earth situation; that is, a completely ice-covered planet? Or terraform a frozen desert-like planet not unlike Mars? Or even to prevent the effects of a long period of planetary glaciation?
In a document in which to appear Journal of Astrophysicsthe authors outline the rationale for searching for the technosignatures of such artificial greenhouse gases in the atmospheres of distant planets.
Previous papers have advocated looking for atmospheric pollution on such exoplanets in the process of dealing with the kind of chlorofluorocarbon pollution that peaked a few decades ago.
Unlike the passive accidental byproducts of industrial processes, man-made greenhouse gases would represent a deliberate effort to alter a planet’s climate with long-lived, low-toxic gases, the authors of this new paper write.
We outline two scenarios where artificial greenhouse gases could produce a detectable signature, Edward Schwieterman, lead author of the paper and an astrobiologist at the University of California, Riverside, told me by email. The first would be if an alien civilization terraformed an otherwise uninhabited planet in their planetary system, as humans have proposed for Mars, he says.
The second scenario would be if a civilization used these gases to stop an ice age on their planet, says Schwieterman. For us, the advantage of such a technosignature would be that it could be long-lasting and would not require a deliberate effort on the part of the alien society to communicate, he says.
Most importantly, we’ll be looking for these anomalies as we characterize the planets to answer other science questions, says Schwieterman. So we might be on the lookout for these signatures as we analyze planetary spectra to learn about rocky exoplanets in general, he says.
Trappist-1 is a red dwarf star, the most common variety, located about 40 light-years away in … [+]
The team used computer models to simulate realistic planetary spectra of Earth-like planets with varying concentrations of these climate-changing gases. Several compounds are highly absorbent in the mid-infrared spectrum, making them excellent greenhouse gases.
C2F6 (hexafluoroethane) has a heating potential 10,000 times that of carbon dioxide over a 100-year time frame, says Schwieterman. SF6 (sulfur hexafluoride) has a warming potential 23,500 times greater than CO2 over a 100-year time frame, he says. These gases are also chemically inert and don’t have the harmful effects of ozone depletion, says Schwieterman.
And they are also long-lived, with lives of a thousand years or more. The first place to look might be a planet in the relatively nearby Trappist-1 system, located about 40 light-years away.
We simulated a hypothetical version of TRAPPIST-1f with a terraformed atmosphere and investigated whether these gases could be detected in its atmosphere, says Schwietermann. TRAPPIST-1f was chosen because it lies in the outer habitable zone of TRAPPIST-1, receiving only 35% of the amount of stellar radiation that Earth receives from the sun, he says. Depending on the gas or combination of gases, a concentration of 1 to 100 parts per million of man-made greenhouse gases can be detected with the James Webb Space Telescope, says Schwieterman.
A terraformed planet would stand out as odd
A terraformed planet would look “strange” in the infrared, says Schwieterman. It would appear larger than it should in mid-infrared transmission spectroscopy and cooler than it should in emitted light, he says.
As for the time required to detect such terraforming?
If a planet is terraformed using artificial greenhouse gases, those greenhouse gas levels will need to be maintained over time, says Schwieterman. This could be the entire remaining life of civilization or more (if such maintenance were automated), he says. So it’s not the time it takes for a planet to terraform that’s important to detectability, but the length of time that terraformed state will be maintained, says Schwieterman.
How many such terraformed planets can be found a thousand light years from Earth?
The answer may be zero, but we have a better chance of fingerprinting a terraformed planet than detecting signs of industrial pollution from an alien civilization, says Schwieterman. We are more likely to “catch” a planet with a deliberately modified climate than a planet going through a short-lived era of high pollution, he says.
