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If aliens have modified a planet in their solar system to make it warmer, we’ll be able to tell. A new study from the University of California, Riverside has identified man-made greenhouse gases that would be hallmarks of a terraformed planet.

A terraformed planet is artificially made hospitable to life. The gases described in the study could be detected even at relatively low concentrations in the atmospheres of planets outside our solar system using existing technology. This could include the James Webb Space Telescope or a future European-led space telescope concept.

And while such pollutants must be controlled on Earth to prevent harmful climate effects, there are reasons why they would be used intentionally on an exoplanet.

“For us, these gases are bad because we don’t want to increase the warming. But they would be good for a civilization that might have wanted to stave off an impending ice age or terraform an otherwise uninhabitable planet in their system, as humans have proposed for Mars,” said UCR astrobiologist and lead study author Edward Schwieterman.

Since these gases are not known to occur in significant quantities in nature, they must be produced. Finding them would therefore be a sign of intelligent life forms using technology. Such signs are called technosignatures.

The five gases proposed by the researchers are used on Earth in industrial applications such as the manufacture of computer chips. These include fluorinated versions of methane, ethane, and propane, along with gases made from nitrogen and fluorine or sulfur and fluorine. New Astrophysical Journal paper describes their advantages as terraforming gases.

One advantage is that they are incredibly efficient greenhouse gases. Sulfur hexafluoride, for example, has 23,500 times the warming power of carbon dioxide. A relatively small amount can heat a frozen planet to the point where liquid water can remain on its surface.

Another advantage of the proposed gases – at least from an extraterrestrial perspective – is that they are extremely long-lived and would persist in Earth’s atmosphere for up to 50,000 years. “They won’t need to be replenished very often to maintain a welcoming climate,” 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 deplete 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, they would also have an ozone layer that they would want to protect,” Schwieterman said. “CFCs will break down in the ozone layer even as they catalyze its destruction.”

Because they break down more easily, CFCs are also short-lived, making them harder to detect.

Finally, fluorinated gases must absorb infrared radiation to have an impact on climate. This absorption creates a corresponding infrared signature that can be detected with space telescopes. With current or planned technology, scientists could detect these chemicals in certain nearby exoplanet systems.

“With an atmosphere like Earth’s, only one in a million molecules could be one of these gases and would be potentially detectable,” Schwieterman said. “That concentration of gas would also be enough to change the climate.”

To arrive at this calculation, 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. This is also a realistic target for research with existing space telescopes.

The group also looked at the ability of Europe’s LIFE mission to detect fluorinated gases. The LIFE mission will be able to directly image planets using infrared light, allowing it to target more exoplanets than the Webb Telescope, which looks at planets as they pass in front of their stars.

This work was done in collaboration with Daniel Angerhausen of the Swiss Federal Institute of Technology/PlanetS and with researchers from NASA’s Goddard Space Flight Center, the Blue Marble Space Science Institute, and the University of Paris.

Although the researchers cannot quantify the probability of finding these gases in the near future, they are confident that – if they are present – it is quite possible to detect them during the currently planned missions to characterize the planetary atmospheres.

“You won’t need extra effort to look for these technosignatures if your telescope is already characterizing the planet for other reasons,” Schwieterman said. “And it would be incredibly amazing to find them.”

Other members of the research team express not only their enthusiasm for the potential to detect signs of intelligent life, but also how much closer current technology has brought us to that goal.

“Our thought experiment shows how powerful our next-generation telescopes will be. We are the first generation in history to have the technology to systematically search for life and intelligence in our galactic neighborhood,” added Angerhausen.