The recent announcement from Cambridge researchers that dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) have been detected in the atmosphere of the exoplanet K2-18b to a significance of three sigma has reignited intense speculation about alien life. This detection, derived from the James Webb Space Telescope’s (JWST) MIRI instrument, has made headlines across science news and popular media. Many outlets are once again declaring these sulphur compounds to be exclusive biosignatures, implying that life may exist on K2-18b. However, such excitement may be premature, and arguably, misleading.
In this article, we examine what was actually detected, what type of world K2-18b really is, and why a geological rather than biological explanation is more scientifically grounded.
What Was Detected?
The chemicals in question, DMS and DMDS, are sulphur-bearing organic molecules. On Earth, they are primarily produced by biological processes, such as the breakdown of organic matter by marine microbes. DMS, in particular, is a well-studied byproduct of phytoplankton activity and plays a role in cloud formation.
The Cambridge team’s results, which are based on atmospheric retrievals from the MIRI instrument aboard JWST, indicate the possible presence of these compounds in K2-18b’s atmosphere. The three-sigma level indicates a statistical confidence of around 99.7%, suggesting a real signal, but falling short of scientific certainty.
While the researchers themselves have been careful to stress the ambiguity of the result and to note that abiotic sources could explain the detections, many commentators and popular articles have pushed the narrative of potential life far more aggressively.
The Problem with Biosignature Bias
On Earth, the presence of DMS and DMDS is heavily associated with life, which makes it tempting to leap to conclusions when these molecules are detected elsewhere. However, this is a classic case of Earth-centric bias: assuming that life is the only explanation because that’s what we see in our own biosphere.
This bias is akin to the mindset of flat Earth belief—an unwillingness to confront uncomfortable complexities in favour of simplistic, comforting narratives. When readers and even writers of popular science are more interested in the thrill of a headline than the slow grind of evidence, this kind of speculative fiction can easily masquerade as science.
Understanding K2-18b
To appreciate why a geological explanation is more plausible, we must first understand the planet itself.
K2-18b is located about 120 light-years from Earth in the constellation Leo. It orbits a red dwarf star within what’s loosely termed the “habitable zone.” It is roughly 8.6 times the mass of Earth and 2.6 times Earth’s radius. These figures indicate that K2-18b is not a rocky Earth-like planet but likely a Hycean world, one with a substantial hydrogen-rich atmosphere overlaying a possible water ocean.
At this size and mass, the planet’s surface gravity is high, and it likely retains a thick envelope of volatile gases. The internal structure probably includes high-pressure ices, silicates, and a rocky core, with no guarantee of a solid surface. Such worlds, sometimes called mini-Neptunes, are not scaled-up Earths; they are fundamentally different in chemistry, structure, and behaviour.
Moreover, being much more massive than Earth, K2-18b is almost certainly more geologically active. The decay of radioactive elements, residual formation heat, and potential tidal heating from orbiting moons can contribute to extreme volcanism. This is especially plausible if, as speculated, the planet possesses one or more large moons—perhaps Titan- or even Mercury-sized—that gravitationally stir the planet’s interior.
Sulphur Chemistry in Reducing Atmospheres
In a reducing (hydrogen-rich) atmosphere, the chemical environment is markedly different from Earth’s oxygen-rich one. Oxidative destruction of sulphur compounds, which is common on Earth due to the abundance of hydroxyl (OH) radicals and oxygen, is far less efficient on K2-18b.
As a result, sulphur-bearing molecules such as DMS and DMDS can persist in the atmosphere for longer periods. If volcanic activity is producing them—either directly or via reactions involving H2S, CO2, and other volcanic gases—they will accumulate. In such a setting, even moderate volcanic output could maintain detectable levels of these molecules.
Furthermore, experimental and observational data from Earth demonstrate that hydrothermal vents and some volcanic emissions release both DMS and DMDS in small quantities. When scaled to the energy output of a planet eight times Earth’s mass, these emissions could become significant.
Occam’s Razor: The Case for Volcanoes over Life
Occam’s Razor tells us to favour the simplest explanation that accounts for all the facts. In this case, that explanation is not life but volcanism.
Here’s what we know:
- K2-18b is not an Earth twin but a Hycean world with a dense, reducing atmosphere.
- Sulphur compounds can be emitted from volcanic and hydrothermal sources.
- The chemical lifetime of these compounds is extended in hydrogen atmospheres.
- No independent lines of evidence point to biology.
While life remains one possible source for DMS and DMDS, it is neither the only nor the most likely one. Given the planet’s size, structure, and probable thermal dynamics, volcanism provides a much more parsimonious explanation.
The Role of Caution in Astrobiology
The excitement over biosignatures is understandable—we all want to know if we are alone. But science must proceed with caution, especially when interpreting ambiguous or borderline data.
Biosignatures should only be declared when multiple, independent lines of evidence converge spectral features, temporal variations, chemical imbalances, and a ruling out of known abiotic pathways. We are nowhere near that standard with K2-18b.
The detection of DMS and DMDS is interesting. It warrants further study. But to frame it as evidence for life is, at best, speculative and, at worst, misleading.
Conclusion: Science Over Sensationalism
K2-18b is a fascinating world, and the potential detection of sulphur-bearing organics in its atmosphere opens a new chapter in the study of planetary chemistry. But we must resist the urge to fill in the blanks with fantasies of alien life, especially when solid geochemical reasoning offers a more grounded explanation.
The presence of DMS and DMDS in a hydrogen-rich atmosphere on a highly volcanic super-Earth or mini-Neptune is not only plausible—it is expected. Until and unless further data rules out geological sources and indicates complex chemical disequilibria that can’t be explained abiotically, life should remain at the bottom of the candidate list.
In the end, the most extraordinary claims still require the most extraordinary evidence. And so far, K2-18b offers intriguing signals—but not the unicorns people are hoping for.