Telltale greenhouse gases can indicate extraterrestrial activity

If aliens modified a planet in their solar system to make it warmer, we could say so. A new study from UC Riverside identifies the artificial greenhouse gases that would be giveaways to a terraformed planet.

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

And while such pollutant gases must be controlled on Earth to avoid harmful climate effects, there are reasons why they could be used deliberately on an exoplanet.

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

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

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

One advantage is that they are incredibly effective greenhouse gases. For example, sulfur hexafluoride has 23,500 times the warming power of carbon dioxide. A relatively small amount could heat an icy planet to the point where liquid water could remain on its surface.

Another advantage of the proposed gases – at least from an extraterrestrial perspective – is that they are exceptionally long-lived and can persist in an Earth-like atmosphere for up to 50,000 years. “They don’t need to be refilled too often to maintain a hospitable climate,” Schwieterman said.

Others have proposed refrigerant chemicals, such as CFCs, as technosignature gases 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 new article, which are chemically inert.

“If any other civilization had an oxygen-rich atmosphere, they would also have an ozone layer that they would want to protect,” Schwieterman said. “CFCs would break down in the ozone layer even if they catalyzed its destruction.”

Because they are more easily broken apart, CFCs are also short-lived, making them more difficult to detect.

Finally, the fluorinated gases must absorb infrared radiation to have an impact on the climate. That absorption produces a corresponding infrared signature that could be detectable with telescopes in space. Current or planned technology could allow scientists to detect these chemicals in certain nearby exoplanetary systems.

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

To arrive at this calculation, the researchers simulated a planet in the TRAPPIST-1 system, about 40 light-years away 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 investigate.

The group also took into account the ability of the European LIFE mission to detect the fluorinated gases. The LIFE mission could image planets directly using infrared light, allowing it to target more exoplanets than the Webb telescope, which watches 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 Institute of Science and the University of Paris.

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

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

Other members of the research team are excited not only about the potential of finding signs of intelligent life, but also about 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,” said Angerhausen.