Astrobiologists identify five greenhouse gases that could be indicative of a terraformed exoplanet

The five man-made greenhouse gases identified by Riverside astrobiologist Edward Schwieterman and his colleagues at the University of California would be detectable even at relatively low concentrations in the atmospheres of exoplanets using the NASA James Webb Space Telescope /ESA/CSA and future space telescopes.

An illustration of various planetary technosignatures, including artificial atmospheric gases. Image credit: Sohail Wasif / UC Riverside.

“For us, these gases are bad because we don't want to increase warming,” Dr. Schwieterman said.

“But they would be good for a civilization that might want to prevent an impending ice age or terraform an otherwise uninhabitable planet in its system, as humans have proposed for Mars.”

“As these gases are not known to occur in significant quantities in nature, they must be manufactured.”

“Finding them would therefore be a sign of intelligent life forms using technology. Such signs are called technosignatures.

The five gases proposed by the authors are used on Earth in industrial applications such as the manufacture 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.

One of their advantages is that they are extremely efficient greenhouse gases. Sulfur hexafluoride, for example, has a warming power 23,500 times greater than carbon dioxide. A relatively small amount could warm a frozen planet to the point that liquid water could persist on its surface.

Another advantage of the proposed gases – at least from an extraterrestrial perspective – is that they are exceptionally long-lived and would persist in an Earth-like atmosphere for 50,000 years.

“They wouldn't need to be restocked too often to maintain a hospitable climate,” Dr. Schwieterman said.

Others have proposed refrigerant chemicals, such as CFCs, as technosignature gases because they are almost exclusively man-made and visible in Earth's atmosphere.

However, CFCs may not be beneficial because they destroy the ozone layer, unlike the fully fluorinated gases discussed in the new paper, 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,” Dr. Schwieterman said.

“CFCs would be broken down in the ozone layer even as they catalyze its destruction. »

“Because they break down more easily, CFCs also have a short lifespan, making them harder to detect.”

Finally, fluorinated gases must absorb infrared radiation to have an impact on the climate.

This absorption produces a corresponding infrared signature that could be detectable with space telescopes.

With current or planned technology, scientists could detect these chemicals in some nearby exoplanetary systems.

“In an atmosphere like Earth's, only one molecule in a million could be one of these gases, and it would be potentially detectable. This concentration of gas would also be enough to change the climate,” said Dr. Schwieterman.

To arrive at this calculation, astrobiologists simulated a planet in the TRAPPIST-1 system, located approximately 40 light years from Earth.

They chose this system, which contains at least seven rocky planets, because it is one of the most studied planetary systems after ours.

Although they cannot quantify the likelihood of finding artificial greenhouse gases in the near future, they are confident that – if they are present – ​​it is entirely possible to detect them during missions currently planned for characterize the planetary atmosphere.

“You wouldn't need extra effort to look for these technosignatures if your telescope is already characterizing the planet for other reasons,” Dr. Schwieterman said.

“And it would be incredibly amazing to have them back.”

The team's work was published in the Astrophysics Journal.

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Edward W. Schwieterman and others. 2024. Artificial greenhouse gases as technosignatures of exoplanets. ApJ 969, 20 ; do I: 10.3847/1538-4357/ad4ce8