Astronomers have detected the most promising signs yet of a possible biosignature outside the solar system, although they remain cautious. Using data from the James Webb Space Telescope (JWST), the astronomers, led by the University of Cambridge, have detected the chemical fingerprints of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS), in the atmosphere of the exoplanet K2-18b, which orbits its star in the habitable zone. (Image Credit: A. Smith, N. Madhusudhan (University of Cambridge))

Astronomers have spotted a small, but potentially significant biosignature outside the solar system — but so far remain cautious about declaring it a conclusive sign of life, because there is a slight, but important, statistical probability that the observance occurred by chance.

The biosignature contains what appears to be traces of one or two chemicals that, on Earth, are only produced by living microorganisms, such as marine phytoplankton. Astronomers using the James Webb Space Telescope (JWST) detected dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS), in the atmosphere of the exoplanet K2-18b, they report in The Astrophysical Journal Letters.

Exoplanet Demonstrating Signs of Life

That exoplanet orbits a star in what is considered the habitable zone. That zone is a sort of sweet spot for potential life — a distance from a star not too close where heat would evaporate water and not too far where cold would freeze it.

This is not the first time that this exoplanet demonstrated — if not signs of life, then at least precursors for it. Earlier observations found methane and carbon dioxide in K2-18b’s atmosphere — the first instance of carbon-based molecules discovered in the atmosphere of an exoplanet in the habitable zone.

K2-18b is about 2.6 times as large as Earth, and lies 124 light years away, in the constellation Leo. Those earlier observations were consistent with what’s called a Hycean planet: an ocean-covered world beneath a hydrogen-rich atmosphere.

Read More: How Different Stars Affect the Habitability of Their Planets

Second Signal from JWST

The signal the astronomers report in the paper is actually the second such detection of the chemical or chemicals on the exoplanet. An “earlier, tentative inference of DMS” was made with one set of instruments covering a particular range of wavelengths, according to a press release.

The new observation was made using a different instrument covering another range of wavelengths. The astronomers essentially doublechecked their original measurements with different tools to confirm what they thought they were seeing.

“We didn’t know for sure whether the signal we saw last time was due to DMS, but just the hint of it was exciting enough for us to have another look with JWST using a different instrument,” Nikku Madhusudhan, a Cambridge University astronomer who led the research, said in a press release. “This is an independent line of evidence, using a different instrument than we did before and a different wavelength range of light, where there is no overlap with the previous observations. The signal came through strong and clear.”

Still Need More Evidence for Signs of Life

While Madhusudhan called the finding “exciting” he emphasized that it is not conclusive. Other unknown or misunderstood chemical processes on the exoplanet might be responsible for the biosignature. Further imaging efforts and experimental data will be required before the scientists can definitively say that what they’ve found is, indeed, a sign of life.

“It’s important that we’re deeply skeptical of our own results, because it’s only by testing and testing again that we will be able to reach the point where we’re confident in them,” Madhusudhan said in the release. “That’s how science has to work.”

Read More: 6 Exoplanets in our Universe That Could Support Life Other Than Earth

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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.