Illustration of the Earth, with the continents in their current form, but with the planet complete … [+]
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 alien civilizations deliberately using 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, similar to Mars? Or even to prevent the consequences of a long period of planetary glaciation?
In a newspaper to appear in The Astrophysics Journalthe authors outline the rationale for searching for the technosignatures of such artificial greenhouse gases in the atmospheres of remote planets.
Previous articles have advocated looking for atmospheric pollution of such exoplanets in the process of tackling the kind of chlorofluorocarbon pollution that peaked several decades ago.
Unlike passive incidental byproducts of industrial processes, artificial greenhouse gases would represent a deliberate attempt to alter a planet’s climate with long-lived, low-toxicity gases, the authors of this new paper write.
We outline two scenarios in which 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 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 deployed these gases to stop an ice age on their planet, Schwieterman says. For us, the advantage of such a technosignature would be that it could be long-lived and would not require a deliberate effort from ET society to communicate, he says.
Most importantly, we should look for these anomalies as we characterize planets to answer other scientific questions, Schwieterman says. So we might as well look for these features when we analyze planetary spectra to learn more about rocky exoplanets in general, he says.
Trappist-1 is a red dwarf star, the most common type, located about 40 light-years away. … [+]
The team used computer models to simulate realistic planetary spectra of Earth-like planets with different concentrations of these gases relevant to climate change. A number of compounds are highly absorbent in the mid-infrared spectrum, making them excellent greenhouse gases.
C2F6 (hexafluoroethane) has a heating potential 10,000 times greater than that of carbon dioxide over 100 years, says Schwieterman. SF6 (sulfur hexafluoride) has a warming potential of 23,500 times that of CO2 over 100 years, he says. These gases are also chemically inert and do not have the harmful effects of destroying the ozone layer, Schwieterman says.
And they are also long-lived, with a lifespan of a thousand years or more. The first place to look might be a planet in the relatively nearby Trappist-1 system, about 40 light-years away.
We simulated a hypothetical version of TRAPPIST-1f with a terraformed atmosphere and investigated whether these gases would be detectable in that atmosphere, says Schwieterman. TRAPPIST-1f was chosen because it is in the outer habitable zone of TRAPPIST-1 and receives 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 artificial greenhouse gases could be detectable with the James Webb Space Telescope, Schwieterman says.
A terraformed planet would stand out as strange
A terraformed planet would look “weird” in the infrared, Schwieterman says. It would appear larger than it should be under mid-infrared transmission spectroscopy and colder than it should be under 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 should be maintained over time, Schwieterman says. This could be the entire remaining lifespan of civilization, or longer (if such maintenance were automated), he says. So it’s not the time it takes to terraform a planet that matters for detectability, but the amount of time the terraform is maintained, Schwieterman says.
How many such terraformed planets could there be within a thousand light-years of Earth?
The answer may be zero, but we have a better chance of fingerprinting a terraformed planet than of finding signs of an alien civilization’s industrial pollution, says Schwieterman. We’re more likely to “capture” a planet with a deliberately modified climate than one experiencing a short-lived era of high pollution, he says.