Did the JWST capture the signature of extra-terrestrial life? An update.

Graham writes …

In a blog post in October 2023, I discussed the potential discovery of possible extraterrestrial life, and I signed off by saying that the research findings were not the last word on the topic, and to expect an update at some point in the future. This brief blog is that update, in which I discuss recent developments in this intriguing story. If you would like to know the full details of what was going on in 2023, you can read my previous blog by just clicking on ‘October 2023’ on the archive list on the right-hand side of this page. But if you don’t have the time or the inclination to do that, then briefly the story goes like this … 

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In 2023 one of the spectroscopy instruments aboard the James Webb Space Telescope (JWST) was brought to bear on the atmosphere of an exoplanet called K2-18b located 124 light-years away in the constellation Leo. This planet is classified as a "Hycean" world – a type of exoplanet characterized by a hydrogen-rich atmosphere with a potential global ocean beneath. Previous JWST observations had already identified methane and carbon dioxide in its atmosphere, supporting the idea of a habitable environment. However, the thing that really sparked media interest was the detection of a molecule called dimethyl sulfide (DMS) in K2-18b’s atmosphere. Here on Earth, the only natural mechanism for producing this is its production by marine micro-organisms such as phytoplankton. Its presence in an exoplanet's atmosphere is considered a potential biosignature, indicating possible biological activity. Last week (week beginning 14 April 2025) the main stream media caught up with this story, and went completely bonkers, implying that the JWST had discovered ET, complete with clips from films like Close Encounters of the Third Kind! Which is the reason why I’m led to write this – to calm the situation a little and to say what we have actually found in these new results.

Artist's impression of exoplanet K2-18b. Credit: A Smith and N Mandhusudhan, University of Cambridge.

The previous JWST data were acquired using an instrument which used a particular band of the infra-red (IR) spectrum in which DMS was degenerate with other atmospheric species. In other words, the ‘bump’ in the spectrum produced by DMS was coincident with the spectral features of the other gases, so introducing a difficult ambiguity into the analysis. The results showed that there was a one in 60’ish chance that the detection of DMS was a statistical fluke. Hence, the conclusion that it was a marker for life was not secure.

James Webb Space Telescope in a clean room environment. Credit: NASA

​So what’s new? The recent results were reported in April 2025 by astronomers at Cambridge University, UK, led by Dr. Nikku Madhusudhan. These were acquired by an alternative spectroscopy instrument aboard the JWST which uses a different bandwidth, deeper in the IR spectrum. In this part of the spectrum the ambiguity in detecting DMS is less, and its signature persisted giving a result at around the 3-sigma level (in statistician’s speak). This means that there is approximately a one in 300’ish chance that the DMS detection is a statistical fluke. This is obviously a great improvement on the previous findings, but still not cast-iron proof. To acquire that kind of confidence, then more work needs to be done to reach a 5-sigma result – that is, the chance of a fluke is around one in a million. Finally, and very significantly, there is also the prospect that DMS may be produced through abiotic chemical pathways, with which we are presently unfamiliar. The story continues …
 
To tell the tale in a more measured and authoritative way, please see this short YouTube video presented by the lead scientist at Cambridge University, Dr. Nikku Madhusudhan.
 
Graham Swinerd
 
Southampton, UK
April 2025