Telltale greenhouse gases could signal an alien

If aliens modified a planet in their solar system to be warmer, we could tell. A new UC Riverside study identifies man-made greenhouse gases that could be a gift from a terraformed planet.

A terraformed planet was artificially created to be hospitable to life. The gases described in the study would be detectable with current technology 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 polluting gases must be controlled on Earth to prevent harmful climate effects, there are reasons why they can 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 want to forestall an impending ice age or terraform an otherwise uninhabitable planet in its 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 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 researchers are used on Earth in industrial applications such as the manufacture of computer chips. They include fluorinated versions of methane, ethane, and propane, along with gases made from nitrogen and fluorine or sulfur and fluorine. New Astrophysical Journal the paper details their merits as terraforming gases.

One benefit is that they are incredibly efficient greenhouse gases. For example, sulfur fluoride has 23,500 times the heating power of carbon dioxide. A relatively small amount could heat a frigid planet to the point where 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 for up to 50,000 years in an Earth-like atmosphere. “To maintain a welcoming climate, they wouldn’t need to be replenished very often,” Schwieterman said.

Others have proposed refrigerant chemicals such as CFCs as process gases because they are almost entirely 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,” Schwieterman said. “CFCs would break down in the ozone layer, even though they would catalyze its destruction.”

Because they break down more easily, CFCs also have a short lifetime, making them more difficult to detect.

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

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

To arrive at this calculation, the scientists 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 the investigation of existing space telescopes.

The group also considered the ability of Europe’s LIFE mission to detect fluorinated gases. The LIFE mission would 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 carried out 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 scientists cannot quantify the probability of finding these gases in the near future, they believe that – if they are present – it is entirely possible to detect them during currently planned missions to characterize planetary atmospheres.

“You wouldn’t need a special effort to look for these technological signatures if your telescope is already characterizing the planet for other reasons,” Schwieterman said. “And it would be wonderful to find them.

Other members of the research team reflect not only excitement about the potential to find 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.