An illustration of various planetary technosignatures, including artificial atmospheric gases. Credits: Sohail Wasif/ UC Riverside
If aliens modified a planet in their solar system to make it hotter, we would be able to understand it. This is the conclusion of a new UC Riverside study published in Astrophysical Journal, which identifies i artificial greenhouse gases which would reveal the presence of a “terraformed” planet and suggests the times needed for the James Webb Space Telescope (Jwst) to detect different concentrations of such gases.
Terraforming is a hypothetical artificial process that serves to make a planet habitable by intervening on its atmosphere – creating it or modifying its chemical composition – so as to make it similar to that of the Earth and capable of supporting an ecosystem. Currently, studies on terraforming are entirely speculative. However, the gases described in the study would be detectable even at relatively low concentrations in the atmospheres of planets outside the Solar System using existing technology. These could include Jwst or future experiments such as Life, the Large Interferometer For Exoplanets of the Federal Institute of Technology in Zurich (ETH).
While these polluting gases need to be controlled on Earth to avoid damaging climate effects, there are reasons why they could be used intentionally on an exoplanet. “For us, these gases are bad because we don’t want to increase global warming. But they would be great for a civilization that wants to prevent an impending ice age or terraform an otherwise uninhabitable planet in its system, as humans have proposed for Mars,” he says. Edward Schwietermanastrobiologist at UC Riverside and first author of the study.
Since these gases are not present in significant quantities in nature, they must be manufactured. Finding them, therefore, would be a sign of the presence of intelligent and technological life formsthe so-called technofirms. The five gases proposed by researchers – CF4 (tetrafluoromethane), C2F6 (hexafluoroethane), C3F8 (octafluoropropane), SF6 (sulfur hexafluoride) and NF3 (nitrogen trifluoride) – are used on Earth in industrial applications such as the production of computer chips. They include fluorinated versions of methane, ethane And propane, as well as gases composed of nitrogen and fluorine or sulfur and fluorine. In particular, the authors analyzed the potential of these gases to generate detectable atmospheric signatures.
Unlike accidental passive byproducts of industrial processes, artificial greenhouse gases would represent an intentional effort to modify a planet’s climate with long-lived, low-toxic gases, and would have a low potential for false positives. As Schwieterman points out, an extraterrestrial civilization might be motivated to undertake such an effort to halt the cooling of its own world or to terraform an otherwise uninhabitable terrestrial planet within its system.
One advantage is that they are incredibly effective greenhouse gases. Sulfur hexafluoride, for example, has a heating power 23,500 times greater than that of carbon dioxide. A relatively small amount could warm a freezing planet to the point where liquid water could persist on its surface.
Another advantage of the proposed gases – at least from an alien perspective – is that they are exceptionally long-lived and would persist in an Earth-like atmosphere. up to 50 thousand years. “They wouldn’t need to be refueled too often to maintain a hospitable climate,” Schwieterman explains.
Qualitative mid-infrared transmission and emission spectra of a hypothetical Earth-like planet whose climate has been modified with artificial greenhouse gases. Credits: Sohail Wasif/ UC Riverside
Some have proposed refrigerant chemicals such as Chlorofluorocarbons (CFCs) as tracers of technosignatures because they are almost exclusively artificial and visible in the Earth’s atmosphere. However, CFCs may not be beneficial because they destroy the ozone layer, unlike the fully fluorinated gases discussed in the study, which are chemically inert. “If another civilization had an oxygen-rich atmosphere, it would also have an ozone layer that it would want to protect,” Schwieterman says. «CFCs would be broken up in the ozone layer even if they catalyze its destruction». Being more easily broken down, CFCs also have a short life, which makes them more difficult to detect.
Finally, fluorinated gases must absorb infrared radiation to have an impact on the climate. Absorption produces a corresponding infrared signature which could be detected with space telescopes. With current or future technology, scientists could detect these chemicals in some nearby exoplanetary systems. “With an atmosphere like Earth’s, only one molecule in a million could be one of these gases and would be potentially detectable,” adds Schwieterman. “That concentration of gas would also be sufficient to modify the climate.”
To reach their conclusions, the researchers simulated a planet in the Trappist-1 system, about 40 light-years from Earth. They chose this system, which contains seven known rocky planets, because it is one of the most studied planetary systems, besides our own. It is also a realistic target for existing space telescopes to examine.
As mentioned above, because artificial greenhouse gases absorb strongly in the mid-infrared thermal window of temperate atmospheres, a terraformed planet will possess strong absorption characteristics of these gases at mid-infrared wavelengths (∼8-12 μm), possibly accompanied from near-infrared features. The authors calculated the observation time needed to detect 1[10](100) ppm of C2F6/C3F8/SF6 on Trappist-1 f with the Miri Low Resolution Spectrometer (Lrs) and with NirSpec, on board Jwst. They found that a combination of 1[10](100) ppm of C2F6C3F8 and SF6 can be detected with a signal-to-noise ratio greater than 5 in just 25[10](5) transits with Miri/Lrs.
The group also considered the European Life mission’s ability to detect fluorinated gases. The Life mission would be able to directly photograph planets using infrared light, allowing it to detect more exoplanets than the Webb telescope, which observes planets as they pass in front of their stars.
While we cannot quantify the likelihood of finding these gases in the near future, researchers are confident that – if present – it is entirely possible to detect them during currently planned missions to characterize planetary atmospheres. “There would be no need for additional effort to look for these technosignatures if the telescope is already characterizing the planet for other reasons,” concludes Schwieterman. “And it would be surprising to find them.”
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