The search for life beyond Earth reached a fever pitch this month with a dramatic, and immediately contested, announcement. A team of astronomers, led by the University of Cambridge, reports the tentative detection of a potential biosignature in the atmosphere of the exoplanet K2-18b. Using NASA’s James Webb Space Telescope (JWST), the researchers claim to have found dimethyl sulfide (DMS), a molecule that, on Earth, is overwhelmingly produced by life.
This detection, published on 17 April 2025 in The Astrophysical Journal Letters, has been labelled by its authors as the “strongest evidence yet” of biological activity outside our solar system. However, this extraordinary claim was met with immediate and significant scepticism from the wider scientific community, which points to the signal’s statistical weakness, flawed analysis, and plausible non-biological explanations. This developing story highlights the immense power of the JWST and the rigorous, contentious nature of the scientific process when faced with claims of such magnitude.
The World at the Centre of the Debate
To understand the debate, one must first understand the planet. K2-18b, located 124 light-years away in the constellation Leo, is a “sub-Neptune” exoplanet, approximately 2.6 times the radius and 8.6 times the mass of Earth. It orbits a cool red dwarf star, K2-18, completing a full year in just 33 days.
Crucially, K2-18b orbits within its star’s habitable zone, the region where it receives a similar amount of stellar radiation as Earth receives from the Sun, allowing for temperatures that could potentially support liquid water. Discovered in 2015 by the Kepler space telescope, it has become a prime target for atmospheric study.
A ‘Hycean’ World or a Mini-Neptune?
The core mystery of K2-18b is its composition. Observations in 2023 from the JWST provided a breakthrough, detecting abundant methane and carbon dioxide in its hydrogen-rich atmosphere. These carbon-based molecules confirmed it was not a barren, rocky world, but the findings also deepened a central question: what lies beneath that atmosphere?
The leading theory, championed by the Cambridge team, posits that K2-18b is a “Hycean” world—a novel class of planet defined by a global liquid water ocean lying beneath a thick hydrogen envelope. This theory is supported by another key piece of JWST data: the robust absence of detectable ammonia. Proponents argue a vast water ocean would naturally absorb and dissolve ammonia from the atmosphere, explaining its apparent scarcity.
However, this interpretation is not universally accepted. Other models suggest K2-18b could be a gas-rich mini-Neptune with no habitable surface at all. This intense debate over the planet’s fundamental nature forms the precarious backdrop for the new claims of biological activity.
The Contentious Signal: Dimethyl Sulfide
The new research adds a tantalising, and volatile, element to this mix. The Cambridge team, led by Professor Nikku Madhusudhan, used JWST’s MIRI (Mid-Infrared Instrument) to re-examine the planet. Their analysis reports the chemical fingerprints of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS).
On Earth, the vast majority of DMS is produced by microbial life, particularly marine phytoplankton in oceans. Its presence is considered a strong potential biosignature. The team estimates the abundance of this gas in K2-18b’s atmosphere to be “thousands of times higher” than on Earth. Prof. Madhusudhan stated that if this association with life is real, the planet “will be teeming with life.”
Furthermore, the researchers argue this detection strengthens the Hycean world model. They contend that the lack of other sulphur-based compounds, like hydrogen sulphide, makes it harder to explain the presence of DMS through non-biological geological processes, leaving a biological origin as a strong possibility.
A Cascade of Scientific Scepticism
The scientific community, however, has urged extreme caution, immediately challenging the paper’s conclusions on several fronts.
The most significant issue is the statistical weakness of the detection. The finding is reported at a three-sigma level of significance, which translates to a 99.7% probability that the signal is real. While this sounds high, it is far below the five-sigma (99.99994%) “gold standard” required in physics and astronomy to claim a definitive discovery. A three-sigma result carries a 0.3%, or 1-in-370, chance of being a random statistical fluke.
Criticism of the methodology has also been sharp. Dr. Ethan Siegel, writing in Big Think, labelled the analysis “flimsy, at best,” pointing to potential flaws in how the data was modelled.
Abiotic Alternatives and Unstable Foundations
Even if the DMS signal is real, its origin is far from settled. Critics quickly noted that DMS can be produced without life. NASA planetary scientist Dr. Sarah Hörst highlighted laboratory experiments that successfully create DMS and other organosulfur gases through abiotic (non-living) photochemical processes. Astronomer Dr. Sun Kwok also pointed out that DMS has been detected on lifeless comets in our own solar system.
This ambiguity strikes at the heart of the biosignature claim. Before life can be invoked, all plausible non-biological pathways must be ruled out—a step that has clearly not yet been achieved.
Finally, the claim rests on the assumption that K2-18b is a habitable water world, a fact that is itself heavily disputed. Other scientists offer compelling, lifeless alternatives that also fit the JWST data. Professor Oliver Shorttle of Cambridge University has suggested that a magma ocean, rather than a water one, could also effectively absorb atmospheric ammonia, explaining its absence. Dr. Nicolas Wogan of NASA’s Ames Research Center has published research suggesting K2-18b is a mini-gas giant with no surface at all.
Conclusion: An Extraordinary Claim, Awaiting Extraordinary Evidence
The detection of dimethyl sulfide on K2-18b, if confirmed, would be a “tipping point,” as Prof. Madhusudhan suggests, in the human quest to find life elsewhere. The Cambridge team hopes to secure a five-sigma result within one to two years of follow-up observations.
However, as of today, the scientific consensus is one of profound scepticism. The signal is statistically weak, the analysis is under fire, and plausible abiotic mechanisms for producing DMS exist. Furthermore, the claim is layered upon an already uncertain foundation, as we remain unsure if K2-18b is a habitable ocean world, a magma-covered rock, or a small gas giant.
The true story, for now, is the extraordinary power of the JWST to probe these distant atmospheres with enough precision to even start this debate. We are witnessing the scientific method working in real-time—an extraordinary claim has been made, and the rigorous, global process of verification and refutation begins.
In my view, however, this specific case now carries a profound existential weight. K2-18b is presented as an almost perfect candidate for a “Hycean” world. If this signal ultimately vanishes—if this ideal candidate, and others like it, are confirmed as sterile—it forces us to confront two deeply uncomfortable possibilities. The first is that our methods are still primitive; that life is abundant but we are simply not yet equipped to “see” it. The second is a far more sobering thought, one that touches on the ‘Great Filter’ hypothesis: that the universe possesses a “firewall” for life, making the jump from a merely habitable planet to a living one an impossibly rare, or fleeting, event.
Whether this signal solidifies into a discovery or fades into a statistical ghost, the stakes for the search for life have been irrevocably raised.
FAQ
Astronomers using the James Webb Space Telescope (JWST) have reported the tentative detection of dimethyl sulfide (DMS) in its atmosphere, a gas that is primarily produced by life on Earth.
On Earth, DMS is overwhelmingly produced by marine life like phytoplankton. Its presence on a habitable-zone planet is considered a strong potential “biosignature,” or sign of life.
It is a “sub-Neptune” exoplanet, 8.6 times the mass of Earth, located 124 light-years away. It orbits its red dwarf star within the habitable zone, meaning it could have temperatures that allow for liquid water.
The evidence is considered statistically weak. It is a “three-sigma” detection, which falls short of the “five-sigma” standard required to claim an official scientific discovery.
A Hycean world is a theoretical type of planet with a global liquid water ocean existing beneath a thick, hydrogen-rich atmosphere. K2-18b is a leading candidate for this class of planet.
