Clara Sousa-Silva looks for biosignatures that researchers might search for in the atmospheres of exoplanets, as beacons of extraterrestrial life.
One upshot of the research is stay upwind during any ET encounter – aliens are very likely to smell like farts.
Photo credit: Melanie Gonick

Researchers have found that phosphine is produced by another, less abundant life form: anaerobic organisms, such as bacteria and microbes, that don’t require oxygen to thrive.

Therefore, if phosphine is detected from a rocky planet, it would be an unmistakable sign of extraterrestrial life, they report in a paper recently published in the journal Astrobiology. This gas would generate a signature pattern of light in a planet’s atmosphere. This pattern would be clear enough to detect from as far as 16 light years away by a telescope such as the soon-to-launch James Webb Space Telescope.

Artist’s view of the James Webb Space Telescope (JWST) in space, up and operating tackling a full agenda of space science conquests.
Credit: Northrop Grumman

However, smell test alert: Phosphine is among the most odiferous and toxic gases on Earth, found in some of the foulest of places, such as penguin dung heaps, the depths of swamps and bogs, and even in the bowels of some badgers and fish. This putrid “swamp gas” is also highly flammable and reactive with particles in our atmosphere.

Fingerprints for molecules

“Here on Earth, oxygen is a really impressive sign of life,” says lead author of the new research, Clara Sousa-Silva, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “But other things besides life make oxygen too. It’s important to consider stranger molecules that might not be made as often, but if you do find them on another planet, there’s only one explanation.”

According to an MIT press statement, Sousa-Silva and her colleagues are assembling a database of fingerprints for molecules that could be potential biosignatures.

“I wanted to like all molecules equally but I do have a favorite: phosphine,” Sousa-Silva explains on her website.

Image courtesy of NASA, edited by MIT News

Confident in the interpretation

The team has amassed more than 16,000 candidates, including phosphine.

The vast majority of these molecules have yet to be fully characterized, and if scientists were to spot any of them in an exoplanet’s atmosphere, they still wouldn’t know whether the molecules were a sign of life or something else.

But with Sousa-Silva’s new paper, scientists can be confident in the interpretation of at least one molecule: phosphine. The paper’s main conclusion is that, if phosphine is detected in a nearby, rocky planet, that planet must be harboring life of some kind, according to the MIT statement.

Process pipeline

Sousa-Silva notes that, aside from establishing phosphine as a viable biosignature in the search for extraterrestrial life, the group’s results provide a pipeline, or process for researchers to follow in characterizing any other of the other 16,000 biosignature candidates.

“I think the community needs to invest in filtering these candidates down into some kind of priority,” Sousa-Silva adds. “Even if some of these molecules are really dim beacons, if we can determine that only life can send out that signal, then I feel like that is a goldmine.”

For the Astrobiology paper – “Phosphine as a Biosignature Gas in Exoplanet Atmospheres” – go to:

 Note: Special thanks to Jennifer Chu of the MIT News Office

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