Recent claims involving signs that extraterrestrial life may exist on the exoplanet K2-18b are facing new challenges from astronomers using NASA’s James Webb Space Telescope, which casts doubt on the controversial past findings.
Potential biosignature detections, first announced in late 2023, had previously revealed the presence of what some astronomers interpreted as evidence of dimethyl sulfide (DMS), a gas associated with life on Earth. However, new findings by an international team led by NASA Jet Propulsion Laboratory scientist Renyu Hu pour cold water on that interpretation.
The new findings, which have not yet been peer-reviewed and were published on the preprint server arXiv.org, argue that observed levels of DMS from the previous study may actually have non-biological origins, resulting from natural processes occurring in K2-18 b’s hydrogen-rich atmosphere.
Reassessing a Rising Star in the Search for Life
K2-18 b, a temperate exoplanet astronomers classify as a sub-Neptune orbiting a red dwarf star 120 light-years away from Earth, captured the attention of the scientific community last fall after a study, led by Cambridge astrophysicist Nikku Madhusudhan, reported tentative signs of DMS in its atmosphere.
On Earth, this compound is almost solely associated with microbial life forms. Hence, its detection in the atmosphere of a promising candidate world in the search for alien life was significant, to say the least.
Still, the early detections were weak, and additional data from the Webb Telescope and other sources have been unsuccessful in confirming the discovery.
Now, in their latest observations using Webb’s NIRSpec instrument, Hu’s team has obtained four high-precision transit spectra of K2-18b, revealing strong methane and carbon dioxide signatures, both of which are consistent with a planet harboring water and potentially life. However, absent from their findings was any evidence of water vapor, ammonia, or carbon monoxide, which points to the existence of a “cold trap” or a layered atmosphere on the exoplanet.
Of greatest significance, Hu’s team found that further attempts to validate the DMS signal were marginal at best, falling under the 3-sigma confidence threshold that astronomers would normally rely on to help validate such a detection.
An additional complication arose from the photochemical modeling conducted by the team, which suggested that both DMS and its cousin, methyl mercaptan, could form through non-biological processes, particularly in atmospheres with high metallicity, such as K2-18 b.
A Lesson in Scientific Caution?
The newest findings, particularly once they are peer-reviewed, will represent only the latest in a growing number of papers that have cautioned against interpreting data collected from observations of K2-18b as evidence of simple life forms.
Earlier this year, another paper (also still awaiting peer review) similarly questioned whether the traces of DMS were sufficient to support the evidence of biological processes. Add to this the fact that NASA’s own guidelines for confirmation of extraterrestrial life dictate that such a confirmation—if ever proven—would only represent the first in a series of steps required to validate such a claim.
K2-18 b is still considered one of the most promising sub-Neptunes for studying water-rich worlds outside the solar system, and further exploration of its atmosphere and interior may yield important insights. But for now, the excitement surrounding potential biosignatures has been significantly tempered by this new round of data and analysis.
While the new findings complicate the notion that biosignature gases have been detected in such a planetary environment, they do not rule out K2-18b as a target for future inquiries.
“K2-18 b remains an intriguing world,” the team writes in their new paper, though adding that, “caution is warranted in interpreting these chemical traces as evidence of life.”
The new paper, “A water-rich interior in the temperate sub-Neptune K2-18 b revealed by JWST,” appeared on the arXiv preprint server on July 16, 2025.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.