It would be possible to detect if aliens made a planet in their solar system warmer. According to a new study from UC Riverside, specific artificial greenhouse gases could indicate that a planet has been terraformed.
Even at low concentrations, these gases could be identified in the atmospheres of exoplanets using current technology. The James Webb Space Telescope or a potential European-led space telescope concept could be used for this purpose.
While such pollutant gases must be controlled on Earth to prevent harmful climate effects, they might be intentionally used on an exoplanet for specific reasons.
“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.
The manufacture of these gases is necessary as they do not naturally occur in significant amounts. Therefore, the presence of these gases would indicate the existence of technologically advanced life forms. Such indications are referred to as technosignatures.
The group of researchers has proposed five gases that are utilized in industrial processes on Earth, such as the production of computer chips. These gases consist of fluorinated versions of methane, ethane, and propane, as well as gases composed of nitrogen and fluorine or sulfur and fluorine. A recent publication in the Astrophysical Journal outlines their suitability as terraforming gases.
One of their advantages is their exceptional effectiveness as greenhouse gases. For instance, sulfur hexafluoride has a warming potential 23,500 times greater than that of carbon dioxide. A relatively small quantity of these gases could raise the temperature of a frozen planet to a level where liquid water could exist on its surface.
The proposed gases have the advantage of being exceptionally long-lived, which could be beneficial from an extraterrestrial perspective, as they could persist 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.
Some individuals have suggested refrigerant chemicals such as CFCs as potential technosignature gases because they are predominantly artificial and observable in Earth’s atmosphere. However, CFCs may not be favorable due to their ozone-depleting effects, unlike the chemically inert fully fluorinated gases discussed in the new paper.
“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.”
CFCs are more easily broken down, which also makes them shorter-lived and, therefore, more challenging to detect.
In order to have an impact on the climate, fluorinated gases need to absorb infrared radiation. This absorption results in a detectable infrared signature that could be observed using space-based telescopes. With current or planned technology, these chemicals could be identified in specific 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.”
The researchers conducted simulations on a planet within the TRAPPIST-1 system, located approximately 40 light-years away from Earth, to reach this conclusion. They selected this system, which hosts seven known rocky planets, as it is one of the most thoroughly studied planetary systems apart from our own. Additionally, it is a feasible target for existing space-based telescopes to investigate.
The ability of the European LIFE mission to detect fluorinated gases was also a topic of discussion. With its capacity to directly capture images of planets using infrared light, the LIFE mission can target a larger number of exoplanets compared to the Webb telescope, which observes planets as they transit in front of their host stars.
This collaborative effort involved Daniel Angerhausen from the Swiss Federal Institute of Technology/PlanetS, as well as researchers from NASA’s Goddard Space Flight Center, the Blue Marble Space Institute of Science, and Paris University.
Although the researchers cannot provide a specific probability of discovering these gases in the near future, they are confident that if these gases are indeed present, they could be detected during the ongoing missions designed to analyze the atmospheres of planets.
“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.”
The potential of discovering signs of intelligent life is a source of enthusiasm not only for the researchers but also for other members of the team. They are also excited about how much progress current technology has made toward achieving 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.
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