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A team of astronomers have detected what they call the most promising signs to date of a possible biosignature, or signs of past or present life linked to biological activity, on an exoplanet named K2-18b. But the study authors, and other experts, remain cautious and have not declared a definitive discovery of life beyond our planet.Related video above: Star swallows planet in one big gulpUsing the James Webb Space Telescope, the team detected chemical fingerprints within the atmosphere of K2-18b that suggest the presence of dimethyl sulfide or DMS, and potentially dimethyl disulfide or DMDS. On Earth, both molecules are only produced by microbial life, typically marine phytoplankton.A study detailing the findings was published Thursday in The Astrophysical Journal Letters.K2-18b, located 124 light-years from Earth, could be a Hycean world: a potentially habitable planet entirely covered in liquid water with a hydrogen-rich atmosphere, said lead study author Nikku Madhusudhan, professor of astrophysics and exoplanetary science at the University of Cambridge’s Institute of Astronomy.Madhusudhan and his colleagues first theorized the concept of Hycean worlds in 2021 after determining that there may be liquid water oceans on K2-18b.The planet is located within the habitable zone of its star, meaning that the world is at just the right temperature and distance from the star to host liquid water on its surface.“Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds,” Madhusudhan said in a statement. “And now we’ve observed it, in line with what was predicted. Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have.”Webb telescope detectionsIt’s possible that the molecules were produced by another unknown chemical process on the planet, which does not require life.The latest findings build on previous research by the same group of astronomers, who also detected carbon dioxide and methane within the planet’s atmosphere using Webb’s Near-Infrared Imager and Slitless Spectrograph and Near-Infrared Spectrograph instruments. The team made the new detection using Webb’s Mid-Infrared Instrument.“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,” Madhusudhan said. “The signal came through strong and clear.”But Madhusudhan and his coauthors acknowledge that more data is needed before claiming direct evidence of life on another world. The team believes that between 16 and 24 hours of follow-up observations using Webb will make that possible.“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. “That’s how science has to work.”However, other experts believe that while the results promise excitement, confirming the existence of life beyond Earth — and even deciding what type of exoplanet K2-18b is — will take much more time and data.Astrophysicist Sara Seager, a professor of physics, planetary science, aeronautics and astronautics at the Massachusetts Institute of Technology, said independent teams have completely different interpretations of the planet itself. Seager was not involved in the new research.“Some propose a Hycean world, others suggest a hot magma ocean — a planet with molten rock beneath a hydrogen-rich atmosphere, which is about as inhospitable as it gets — and still others see it as a mini-Neptune,” Seager said, referring to worlds that are larger than Earth but smaller than Neptune. For reference, K2-18b is 8.6 times as massive and 2.6 times as large as Earth.Seager believes this finding, which suggests a candidate for a biosignature, “will remain in the candidate category indefinitely.”“For nearly 100 years, astronomers have wrestled with the idea that certain gases in a planet’s atmosphere ‘don’t belong’ — that they’re so reactive they shouldn’t exist without being continually replenished, possibly by life,” Seager said via email. “This idea traces back to James Jeans in 1930, who first identified molecular oxygen in Earth’s atmosphere as a sign of life, and used similar logic to place limits on oxygen in Venus’ atmosphere. Now, with thousands of exoplanets in view, the temptation to overinterpret is strong — and some are jumping the gun. When it comes to K2-18 b, enthusiasm is outpacing evidence.”An ‘exciting’ hintWhen Madhusudhan and his team previously observed K2-18b, they detected a weak signal that might fit the hypothesis that the planet is a Hycean world.“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,” he said.Webb has the observational capability to peer through the atmosphere of exoplanets, or planets beyond our solar system. As exoplanets pass in front of their host star, from the perspective of Earth, light moves through their atmospheres, allowing Webb to detect the chemical signatures of gases within them.Both dimethyl sulfide and dimethyl disulfide, which are part of the same chemical group, have overlapping characteristics, which is why the results can’t definitively differentiate between the two molecules, but future observations might, the study authors said.On Earth, concentrations of dimethyl sulfide and dimethyl disulfide are typically below one part per billion in volume. However, the team estimates the presence of these molecules to be thousands of times stronger on K2-18b.“The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them,” said study coauthor Subhajit Sarkar, a lecturer in the astronomy group at Cardiff University in Wales, in a statement.Eddie Schwieterman, assistant professor of astrobiology at the University of California, Riverside, is “somewhat skeptical but very intrigued and excited about what’s next.” Schwieterman, who has studied K2-18b in the past, was not involved in the new research.His group’s research found that in order to sustain the abundance of dimethyl sulfide in K2-18b’s atmosphere, “a production rate of about 20 times that of Earth’s DMS production is required.”“This is a high bar, but plausible considering local areas in Earth’s oceans are far more productive than the global average,” he said. “This threshold also poses an obstacle to attributing potential DMS to abiotic (non-biological) sources, since those abiotic sources would have to be much larger than any known ones.”But Schwieterman said that first, scientists need to confirm that dimethyl sulfide is really present in the atmosphere of K2-18b, which will require validation from multiple independent groups who study the same data and analyze it for the chemical signature of the molecules. Madhusudhan said the data the study team analyzed will be released next week, so other astronomers can do just that.Next, Schwieterman wants to see additional Webb observations with a higher level of statistical significance to see whether the interpretation of dimethyl sulfide being present holds. Searching for the signatures of these molecules in atmospheres of other similarly sized planets within the habitable zones of their stars would also help, although it’s a process that will take years.“I do have at (least) one reason to be skeptical, which is that I’d anticipate the presence of ethane (C2H6) to accompany DMS/DMDS if those gases were present,” he said. “This is because UV rays from the star would break apart the DMS/DMDS into components we’d predict would react to form ethane. The absence of ethane makes me think we’ve missed something. Perhaps our models are wrong, or perhaps the DMS/DMDS isn’t there.”Schwieterman believes there is a lot of work to be done to validate the finding and assess the biosignature hypothesis presented by the paper.A threshold for lifeThe research team that authored the new paper says the observations have reached the three-sigma level of significance, or a 0.3% probability that the detections occurred by chance. For a scientific discovery, the observations must meet a five-sigma threshold, or below a 0.00006% that the observations occurred by chance.While the findings don’t represent a clear detection of dimethyl sulfide and dimethyl disulfide, “it is a step in the right direction,” said Dr. David Clements, an astrophysicist at Imperial College London. Clements was not involved in the research.Madhusudhan believes his team’s finding is a “major landmark in our search for life” that ushers in a new era.“In my mind, it is no longer a question of whether we will find life if such life exists,” he said. “We have demonstrated that we have the capability to do so with our initial current findings at reasonable significance. A bigger question in my mind is whether we as a species are prepared to find life as we don’t know it. We as a society, as a species, should come together and ask ourselves the question: What is it that constitutes life elsewhere?”