K2-18b

James-Webb Telescope in Relative Repose

Are we alone in the universe? Whether alone or in a universe teeming with life, both prospects are equally terrifying. To date, we have zero definitive evidence for the existence of life outside our terrestrial bubble. SETI (Search for Extraterrestrial Intelligence) is an international research institution set up specifically to search for life in the universe and has been running for the past 41 years. Currently, it is estimated that over 50 billion US dollars have been spent on what many folk would contend is a fruitless and ultimately useless programme. The money would have been better spent on practical projects helping to sort out, or at least contributing to and ameliorating, the many problems affecting humanity. I will not become mired in the controversy here; suffice it to say that, in my opinion, it is money well spent.

Various techniques are employed in the search for extraterrestrial life. My primary interest concerns 'life forms' outside our own solar system. In contrast, a sister research programme focuses on the possibility of life within the solar system. Of course, in this instance, the emphasis is on detecting lower life forms, akin to our Earth-bound bacteria. Indeed, our companion planets and our moon appear essentially inhospitable to life. An absolute requirement for the formation and propagation of life is the presence of liquid water. Liquid water only forms under restrictive physical conditions, and water, in liquid form, has not been unequivocally observed on planets within our solar system. However, there are indications that liquid water may exist on Jupiter's moons. Interest has centred upon Jupiter's satellite, Phobos, and the possibility exists that water may be present under occluding bodies of surface ice. Furthermore, other celestial bodies within our solar system might be viable candidates for the presence of life. I won't be discussing these possibilities today, as the purpose of this post is a tad more ambitious.

Today, I want to concentrate on the search for life outside our solar system. It is sobering to think that only within the last 30 years have we been able to detect planets belonging to stars within our Milky Way galaxy. Planets are commonplace, and most stars have their own collection of orbiting planets.   

Two methodologies are employed in the search for extraterrestrial life. The most ambitious involves looking for advanced technological signs that would indicate the presence of highly advanced organisms at least as advanced as ourselves. There is also a more modest sister programme. In this instance, the researchers are looking for general biomarkers. It is possible to detect the presence of chemicals in a planet's atmosphere using spectroscopy. I'll not discuss spectroscopy here, although its discovery and methodology are worthy of a separate post. For our purposes, it is sufficient to state that the device analyses electromagnetic emission spectra. By examining the spectra for specific absorption profiles, it is possible to determine the chemical composition of the source of the electromagnetic radiation. This is a powerful tool with applications in astronomy, biology, chemistry and physics.

There are a variety of organic compounds associated with life, albeit simple life. These chemicals usually cannot be formed by known inorganic, physical processes; therefore, their presence can be used as a general marker for the presence of life. A number of planets outside our solar system have recently provided promising results using spectroscopy. Although the results are indicative, they are often equivocal. However, in a recent paper, researchers claim they have detected a planet with a very strong set of biomarkers. Again, it needs to be stressed that these markers do not indicate advanced life and, if confirmed, are likely to be associated with primitive non-complex life forms. A Cambridge University team examined the atmosphere of a planet aptly named K2-18b and discovered a set of chemicals usually associated with simple bacterial organisms. This is another great discovery for the James Webb telescope. To date, it represents the strongest evidence for extraterrestrial life. However, it needs to be stressed that additional work is required for validation. 

The planet in question lies 124 light years from Earth and is roughly 2.5 times the size. In addition, the planet lies in the so-called 'Goldilocks Zone '.  This implies that temperatures on the planet are compatible with the formation of life and indicate the presence of liquid water. The atmosphere is analysed as the planet passes across its small red dwarf sun. Two chemicals have been detected, dimethylsulfide (DMS) and dimethyl disulfide (DMDS). Both of these chemicals are associated with phytoplankton found in the seas on Earth. If the results are confirmed,  the implications are profound. Intriguingly, the analysis suggests that the chemicals are in very high concentrations. These high concentrations are impossible to replicate in our terrestrial setting by non-biological means. From the report, the latest measurements provide a three-sigma certainty, which is tech jargon for a 99.7% certainty. Although this result provides strong evidence for DMS and DMDS, further confirmatory work is required before everyone gets too excited.

So what does all this mean? Even if the work is confirmed, this does not necessarily prove that life exists on K2-18b. While it is true that DMS and DMDS have previously only been linked to biological processes in a terrestrial setting, this does not rule out the possibility of a non-organic origin. There may be unknown physical forces responsible for this unusual chemical profile. Current research is focused on the possibility of producing DMS and DMDS under laboratory conditions without recourse to biology- watch this space. 

Although the results from this study provide the best evidence of extraterrestrial life, so far, extreme caution needs to be exercised. DMS has been detected on comets, which may suggest production by physical and chemical means alone, although past biological processes can not be ruled out. Let us assume that after the Astrobiologists have performed their due diligence, the results can only be interpreted as indicating the presence of life, albeit simple bacterial life. Of course, in reality, there is never absolute certainty in science, only a level of statistical validity.  With that caveat out of the way, let's speculate. The presence of extraterrestrial life would have fantastical implications. It would confirm our strong suspicion that independent life is possible elsewhere in the universe and suggest that the formation of life is inevitable given the right physical conditions. Our Milky Way Galaxy contains at least 100 billion stars, most of which contain orbiting planets. From a 2015 study, scientists have estimated that, on average, 1 to 3 planets exist within the habitable zone of their host star. Each of these planets is a possible incubator for life. Therefore, our galaxy should be teeming with life. Again, it must be emphasised that this does not necessarily mean we are inevitably dealing with the presence of sophisticated, technologically advanced life. I suspect that the probability of advanced life is extremely rare for reasons I will expound in a future post.   

References     

Nikku Madhusudhan et al. ‘New Constraints on DMS and DMDS in the Atmosphere of K2-18b from JWST MIRI.’ The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/adc1c8 

 Monthly Notices of the Royal Astronomical Society on March 18, 2015.