Sign Up for CNN’s Wonder Theory Science Newsletter to delve into the universe with updates on remarkable discoveries, scientific advancements, and more exciting news. Recently, a team of astronomers has reported what they call the most promising signs yet of a potential biosignature, indicating the possibility of past or present life linked to biological activity on an exoplanet known as K2-18b. However, the study's authors and other experts remain cautious and have not confirmed a definitive discovery of life beyond Earth.
Using the James Webb Space Telescope, researchers detected chemical fingerprints in the atmosphere of K2-18b that suggest the presence of dimethyl sulfide (DMS) and potentially dimethyl disulfide (DMDS). On our planet, both of these molecules are primarily produced by microbial life, specifically marine phytoplankton. The findings were detailed in a study published in The Astrophysical Journal Letters on Thursday.
K2-18b, located 124 light-years from Earth, is theorized to be a Hycean world, which indicates it could be a habitable planet entirely engulfed in liquid water and surrounded by a hydrogen-rich atmosphere. Lead study author Nikku Madhusudhan, a professor of astrophysics and exoplanetary science at the University of Cambridge’s Institute of Astronomy, explained that the team first proposed the concept of Hycean worlds in 2021 after finding evidence of liquid water oceans on K2-18b.
The planet resides within the habitable zone of its star, meaning it is at an optimal temperature and distance to support liquid water on its surface. Madhusudhan pointed out that earlier theoretical work had predicted the presence of high levels of sulfur-based gases like DMS and DMDS on Hycean worlds, and their observations align with these predictions. “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,” he stated.
Despite these promising findings, there remains the possibility that these molecules were produced through unknown chemical processes that do not involve life. This recent discovery builds on previous research conducted by the same group of astronomers, who detected carbon dioxide and methane in K2-18b's atmosphere using Webb’s Near-Infrared Imager and Slitless Spectrograph, as well as the Near-Infrared Spectrograph instruments. The new detection utilized Webb’s Mid-Infrared Instrument.
“This is an independent line of evidence, using a different instrument and a different wavelength range of light, where there is no overlap with our previous observations,” Madhusudhan explained. “The signal came through strong and clear.” However, he emphasized that more data is necessary before making any claims about direct evidence of life on another planet. The team believes that between 16 and 24 hours of follow-up observations using Webb will be crucial to solidifying their findings.
Other experts express caution, suggesting that while the results are promising, confirming the existence of life beyond Earth—and even determining the exact nature of K2-18b—will require more time and data. Sara Seager, an astrophysicist and professor at the Massachusetts Institute of Technology, highlighted that independent teams have differing interpretations of K2-18b. “Some propose it as a Hycean world, others suggest a hot magma ocean, and still others see it as a mini-Neptune,” she explained. For context, K2-18b is 8.6 times as massive and 2.6 times as large as Earth.
Seager believes that the findings indicating a potential biosignature will remain in the candidate category for an indefinite period. She noted that for nearly a century, astronomers have grappled with the idea that certain gases in a planet's atmosphere “don’t belong,” suggesting they could only exist if replenished, possibly by life.
Madhusudhan and his team previously detected a weak signal from K2-18b that hinted at the possibility of it being a Hycean world. “We didn’t know for sure if 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 remarked. The James Webb Space Telescope is known for its capability to observe the atmospheres of exoplanets, detecting chemical signatures as these planets transit in front of their host stars.
Both DMS and DMDS belong to the same chemical group and exhibit overlapping characteristics, which complicates the ability to definitively differentiate between the two based on current data. The team estimates that the presence of these molecules on K2-18b could be thousands of times stronger than what is typically found on Earth, where concentrations are often below one part per billion in volume.
While some scientists, like Eddie Schwieterman, an assistant professor of astrobiology at the University of California, Riverside, express intrigue about the findings, there’s skepticism surrounding the necessity to confirm the presence of DMS in K2-18b's atmosphere. Schwieterman’s own research indicates that sustaining the abundance of dimethyl sulfide would require a production rate significantly higher than that found on Earth. “This is a high bar, but plausible,” he noted, suggesting that local areas in Earth's oceans could be more productive than the global average.
Schwieterman also mentioned the need for validation from multiple independent teams analyzing the same data for the chemical signatures of these molecules. “We need to see additional Webb observations with a higher level of statistical significance,” he stated, adding that searching for similar molecules in the atmospheres of other planets within habitable zones will also be essential.
The research team claims their observations have reached a three-sigma level of significance, indicating a 0.3% probability that the detections occurred by chance. However, to establish a scientific discovery, a five-sigma threshold must be met, representing a probability of 0.00006% for chance occurrences. While these findings do not yet constitute a clear detection of DMS or DMDS, Dr. David Clements, an astrophysicist at Imperial College London, remarked, “It is a step in the right direction.”
Madhusudhan remains optimistic, viewing his team’s finding as a major milestone in the quest for extraterrestrial life. “In my mind, it is no longer a question of whether we will find life if it exists. We have demonstrated that we have the capability to do so,” he concluded, urging society to reflect on what constitutes life beyond our planet.
