Carbon Monoxide Detectors Could Warn of Alien Life

"With the launch of the James Webb Space Telescope two years from now, astronomers will be able to analyze the atmospheres of some rocky exoplanets," said Edward Schwieterman, the study's lead author and a NASA Postdoctoral Program fellow in UCR's Department of Earth Sciences.

"It would be a shame to overlook an inhabited world because we did not consider all the possibilities."

The researchers used computer models of chemistry in the biosphere to identify scenarios in which carbon monoxide accumulates in the atmospheres of planets.

In the first scenario, the team looked at Earth a billion years ago. Back then, the ocean was full of microbial life despite the atmosphere being devoid of oxygen. In fact, the team found Earth at the time could maintain carbon monoxide levels of roughly 100 parts per million (ppm).

"That means we could expect high carbon monoxide abundances in the atmospheres of inhabited but oxygen-poor exoplanets orbiting stars like our own sun," said Timothy Lyons, one of the study's co-authors, a professor of biogeochemistry in UCR's Department of Earth Science, and director of the UCR Alternative Earths Astrobiology Center.

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"This is a perfect example of our team's mission to use the Earth's past as a guide in the search for life elsewhere in the universe."

In a second scenario, the team looked at the photochemistry around red dwarf stars like Proxima Centauri. This star is the nearest to our sun at 4.2 light years away.

The team's models found that if a planet around such a star were to have extraterrestrial life forms and be rich in oxygen, then it would also be very rich in carbon monoxide.

"Given the different astrophysical context for these planets, we should not be surprised to find microbial biospheres promoting high levels of carbon monoxide," Schwieterman said. 

"However, these would certainly not be good places for human or animal life as we know it on Earth."

Preparing for a crucial mission

Planets in the habitable zone of Proxima Centauri are likely targets for exploration by the James Webb Space Telescope, scheduled for launch in March 2021. The current study is making sure that no planet goes unexplored due to the false assumption that high carbon monoxide leads to lifelessness.

"Although other studies have done exoplanet photochemical modeling that includes carbon monoxide, no one had focused on carbon monoxide on Earth-like exoplanets in such a systematic way," Schwieterman said. 

"Now we have a guidebook for determining what levels of carbon monoxide are compatible with a photosynthetic biosphere."

This project was funded by the NASA Astrobiology Institute and the study published in The Astrophysical Journal.