A terraformed planet is one that has been artificially altered to be suitable for life. According to the study, these gases could be detected even at low concentrations in the atmospheres of planets outside our solar system using current technology, including the James Webb Space Telescope or a future European-led space telescope concept.
Although these pollutant gases must be controlled on Earth to avoid negative climate effects, they might be intentionally used on an exoplanet.
"For us, these gases are bad because we don't want to increase warming. But they'd be good for a civilization that perhaps wanted to forestall 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 do not naturally occur in significant amounts, their presence would indicate intelligent, technology-using life forms. These indicators are known as technosignatures.
The five gases identified by the researchers are used on Earth in industrial processes, such as manufacturing computer chips. They include fluorinated versions of methane, ethane, and propane, along with gases composed of nitrogen and fluorine or sulfur and fluorine. A new paper in the Astrophysical Journal outlines their effectiveness as terraforming gases.
One key advantage is their potency as greenhouse gases. For instance, sulfur hexafluoride has 23,500 times the warming effect of carbon dioxide. Even a small amount could warm a freezing planet enough to sustain liquid water.
Additionally, these gases are exceptionally long-lasting, persisting in an Earth-like atmosphere for up to 50,000 years. "They wouldn't need to be replenished too often for a hospitable climate to be maintained," Schwieterman said.
While others have suggested refrigerant chemicals like CFCs as technosignatures because they are almost entirely artificial, CFCs may not be ideal since they destroy the ozone layer. The fully fluorinated gases discussed in the new paper are chemically inert and do not have this issue.
"If another civilization had an oxygen-rich atmosphere, they'd also have an ozone layer they'd want to protect," Schwieterman said. "CFCs would be broken apart in the ozone layer even as they catalyzed its destruction."
Furthermore, CFCs are short-lived and more easily broken apart, making them harder to detect.
The fluorinated gases must absorb infrared radiation to impact the climate, producing an infrared signature detectable by space-based telescopes. Current or planned technology could identify these chemicals in certain nearby exoplanetary systems.
"With an atmosphere like Earth's, only one out of every million molecules could be one of these gases, and it would be potentially detectable," Schwieterman said. "That gas concentration would also be sufficient to modify the climate."
To reach this conclusion, the researchers simulated a planet in the TRAPPIST-1 system, located about 40 light-years from Earth. This system, which contains seven known rocky planets, is one of the most studied planetary systems beyond our own and a realistic target for existing space-based telescopes.
The researchers also considered the European LIFE mission's capability to detect these fluorinated gases. The LIFE mission could directly image planets using infrared light, allowing it to target more exoplanets than the Webb telescope, which observes planets as they transit in front of their stars.
This research involved collaboration with Daniel Angerhausen at Swiss Federal Institute of Technology/PlanetS, and with scientists at NASA's Goddard Space Flight Center, the Blue Marble Space Institute of Science, and Paris University.
While the researchers cannot yet estimate the probability of discovering these gases, they are confident that current missions aimed at characterizing planetary atmospheres could detect them if they exist.
"You wouldn't need extra effort to look for these technosignatures if your telescope is already characterizing the planet for other reasons," said Schwieterman. "And it would be jaw-droppingly amazing to find them."
Other researchers share the excitement not only for the potential of detecting signs of intelligent life but also for the advancements in technology that bring us closer to this goal.
"Our thought experiment shows how powerful our next-generation telescopes will be. We are the first generation in history that has the technology to systematically look for life and intelligence in our galactic neighborhood," added Angerhausen.
Research Report:Artificial Greenhouse Gases as Exoplanet Technosignatures
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