In mid-April, researchers announced they had identified potential signs of life on an exoplanet. This discovery made international headlines, as the authors of this work were convinced they had obtained particularly robust and representative data... but a new independent study strongly opposes this conclusion, which was already met with skepticism.
To provide context, the work in question focuses on the exoplanet K2-18b, located just under 125 light-years from Earth. Using the James Webb Space Telescope, a team from the prestigious University of Cambridge identified gases that they believed corresponded perfectly to biosignatures – direct signs of biological activity associated with living beings.
More specifically, astrobiologist Nikku Madhusudhan's team identified the spectral signature of methane, carbon dioxide, but also and especially dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) – two sulfur compounds that, on Earth, are produced almost exclusively by marine microorganisms.
In their press release, the authors presented their discovery as the biosignature “most promising to date” and suggested that it could be a major turning point in the history of astrobiology. They argued that their measurements pointed directly to the existence of a so-called “Hyceanic” world, that is, a planet covered in water and surrounded by a hydrogen-rich atmosphere that could support life forms.
Justified skepticism
Many researchers, however, have displayed cautious skepticism towards these claims. Nothing surprising in this context; This is even the most common reaction to this kind of claim.
Indeed, this is not the first time that a space telescope has found traces of potentially biogenic chemical compounds, that is, compounds likely to have been produced by living beings. However, until now, no one has ever been able to provide rigorous proof of this. The main obstacle is that the origin of the gases in question is never perfectly clear. Most often, this research is based on terrestrial life as we know it, and there is no guarantee that equivalents exist on other planets.
Moreover, in the vast majority of cases, these compounds could also have been produced by geological phenomena, without the slightest intervention of living beings. Methane, but also many nitrogen- and sulfur-based compounds, for example, have often been the source of false positives that have ultimately been refuted by other studies.
As usual, other researchers therefore embarked on independent analyses of the data presented by the Cambridge team. The objective: to verify whether they arrived at the same conclusions using a different methodology. This is what Jake Taylor, a planetary scientist at the University of Oxford, decided to do.
Confirmation bias?
To reach their conclusion, the Cambridge team started with a complex model in which they explicitly included sulfur compounds such as DMS and DMDS. The authors then allowed many so-called “free” parameters to vary, meaning their values are not predetermined by theory or measurements. Instead, all these values are gradually adjusted by algorithms to ensure that the model corresponds as closely as possible to the hypothesis being tested (in this case, the presence of these sulfur compounds). From there, all that remains is to analyze how well these models correspond to the spectral data reported by the JWST. The authors concluded that their model containing these sulfur compounds was strongly consistent with the observations.
This is a perfectly valid approach, but one that must be used with caution. When a model uses many free parameters, it is much easier to adjust them, intentionally or not, in a direction that supports the expected result. In other words, this can introduce what is called confirmation bias—a cognitive bias that leads us to favor hypotheses that reinforce a preconceived idea.
To avoid this, Taylor employed a more minimalist approach. Instead of assuming that the exoplanet's atmosphere contained these compounds and then verifying it later, he simply tried to spot raw statistical trends in the data, without cluttering his analysis with many interdependent parameters. Only after identifying these “peaks” did he seek to verify whether they could indeed correspond to DMS and DMDS. This is a subtle methodological difference, but a very important one in this case – not least because it led Taylor to a very different conclusion.
According to him, the data contain no evidence that the atmosphere of K2-18b actually contains DMS… or even any specific molecule. He believes that the sheer complexity of the Cambridge model has significantly muddied the waters, and that the statistical trends are in fact much less pronounced than the study suggests. He also raises another important point: if this gas were present and concentrated, as the Cambridge study suggests, other, easier-to-detect molecules like ethane should also appear—but that's not the case here.
In other words, Taylor suggests that Madhusudhan's team may have been wishful thinking.
The Hunt for Extraterrestrial Life Continues
It's important to note that neither study is definitive proof one way or the other. What is certain, however, is that new modeling and even a new JWST observation campaign will likely be needed to clarify the situation. And the study's authors remain acutely aware of this reality.
"It's important that we remain deeply skeptical of our own results, because only by testing again and again can we reach the point where we know for sure," said Madhusudhan. "That's how science should work."
It will be interesting to follow this additional work; it can both inform us about K2-18b's potential to host life forms, but also strengthen the methodological foundations to avoid false positives in future observations.
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