As teams of researchers begin to detect molecules that could indicate the presence of life in the atmospheres of exoplanets, controversies will emerge. In the early stages, the method will be transmission spectroscopy, in which light from the star passes through the planet’s atmosphere as it transits the host. From the resulting spectra various deductions may be drawn. Thus oxygen (O₂), ozone (O₃), methane (CH₄), or nitrous oxide (N₂O) would be interesting, particularly in out of equilibrium situations where a particular gas would need to be replenished to continue to exist.
While we continue with the painstaking work of identifying potential biological markers — and there will be many — new findings will invariably become provocations to find abiotic explanations for them. Thus the recent flurry over K2-18b, a large (2.6 times Earth’s radius) sub-Neptune that, if not entirely gaseous, may be an example of what we are learning to call ‘hycean’ worlds. The term stands for ‘hydrogen-ocean.’ Think of endless ocean under an atmosphere predominantly composed of hydrogen. Now put it in the habitable zone.
Thus the interest in K2-18b, which appears to orbit within the habitable zone of its red dwarf host. Astronomers have known about water vapor here for some time, while JWST results in 2023 further indicated carbon dioxide and methane. On its 33-day transiting orbit, this is a planet made to order for spectral analysis of its atmosphere. Now we have new work that leans in the direction of a biological explanation for a possible biosignature, one that is tantalizing but clearly demands further investigation.
The biosignature, deduced by researchers at the University of Cambridge led by Nikku Madhusudhan, involves dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS). These are molecules that, at least on Earth, are produced only by life, most commonly by marine phytoplankton, photosynthetic organisms that play a large role in producing oxygen for our atmosphere.. The detection, say the authors, is at the three-sigma level of statistical significance, which means a 0.3% probability that these results occurred by chance. Bear in mind that five-sigma is considered the threshold for scientific discovery (below a 0.00006% probability that the results are by chance). So as I say, we can call this intriguing but not definitive, a conclusion the authors support.
Image: Nikku Madhusudhan, who led the current work on K2-18b. Credit: University of Cambridge.
What excites the researchers about this work is that they first saw hints of DMS using the James Webb Space Telescope’s NIRISS (Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph) instruments, but found further evidence using the observatory’s MIRI (Mid-Infrared Instrument) in the mid-infrared (6-12 micron) range. That’s significant because it produces an independent line of evidence using different instruments and different wavelengths. And in the words of Madhusudhan, “The signal came through strong and clear.”
But Madhusudhan said something else that has excited commentators. Noting that the concentrations of DMS and DMDS in K2-18b’s atmosphere are thousands of times stronger than what we see on Earth, there is an implication that K2-18b may be a specific type of living planet:
“Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds. And now we’ve observed it, in line with what was predicted. Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have.”
Centauri Dreams readers will want to check Dave Moore’s Super-Earths/Hycean Worlds for more on this category. A hycean world is considered to be a water world with habitable surface conditions, and in earlier work, Madhusudhan and colleagues have noted that K2-18b could well maintain a habitable ocean beneath a hydrogen atmosphere. We have no analog to planets like this in our own system, but the category may be emerging as a place where conditions of temperature and atmospheric pressure may allow at least microbial life.
Image: Artist’s conception of the surface of a hycean planet. Credit: Amanda Smith, Nikku Madhusudhan.
So what is producing these chemical signatures? There may be reason for some optimism about a life detection but the possibility of unknown chemical processes remains alive, and thus will spawn further work both theoretical and experimental. And this is the problem for the entire landscape of remote biosignature detection. We’re going to be seeing a lot of interesting results as our instrumentation continues to improve, but at the level of uncertainty that will ensure debate and the need for further taking of data. This is going to be a long and I suspect frustrating process. Astrophysicists are going to be knocking heads at conferences for decades.
So this is an example of how the debate is going to be playing out at many levels. The evidence for biology will be sifted against possible abiotic processes. From the paper:
…both DMS and DMDS are highly reactive and have very short lifetimes in the above experiments (i.e., a few minutes) and in the Earth’s atmosphere (i.e., between a few hours to ∼1 day), due to various photochemical loss mechanisms (e.g. Seager et al. 2013b). Thus, the resulting DMS and DMDS mixing ratios in the current terrestrial atmosphere are quite small (typically ≲1 ppb), despite continual resupply by phytoplankton and other marine organisms…. sustaining DMS and/or DMDS at over 10-1000 ppm concentrations in steady state in the atmosphere of K2-18 b would be implausible without a significant biogenic flux. Moreover, the abiotic photochemical production of DMS in the above experiments requires an even greater abundance of H2S as the ultimate source of sulfur — a molecule that we do not detect — and requires relatively low levels of CO2 to curb DMS destruction (Reed et al. 2024), contrary to the high reported abundance of CO2 on K2-18 b (Madhusudhan et al. 2023b).
Image: The graph shows the observed transmission spectrum of the habitable zone exoplanet K2-18 b using the JWST MIRI spectrograph. The vertical shows the fraction of star light absorbed in the planet’s atmosphere due to molecules in the planet’s atmosphere. The data are shown in the yellow circles with the 1-sigma uncertainties. The curves show the model fits to the data, with the black curve showing the median fit and the cyan curves outlining the 1-sigma intervals of the model fits. The absorption features attributed to dimethyl sulphide and dimethyl disulphide are indicated by the horizontal lines and text. The image behind the graph is an illustration of a hycean planet orbiting a red dwarf star. Credit: A. Smith, N. Madhusudhan (University of Cambridge).
So there are reasons for optimism. We’ll keep taking such results apart, motivated by the unsparing self-criticism of the Cambridge team, which goes out of its way to scrutinize its findings for alternative explanations (a good lesson in scientific methodology here). Case in point: Madhusudhan and colleagues point out evidence for the presence of DMS on comet 67P/Churyumov-Gerasimenko, which could mean an abiotic source delivered by comets into the atmosphere. Because comets contain ices and gases that could be interpreted as biosignatures if found in a planet’s atmosphere, we’re again reminded of the need for caution. Even so, we can deflect this.
For at K2-18b, the atmosphere is massive compared to the trace gases that could be induced by cometary delivery, and the authors doubt that DMS and DMDS would survive in their present form during a hypervelocity comet impact. K2-18b just has too much DMS and DMDS, per these findings, to be accounted for by comets alone.
Detecting a biosignature will require accumulating more and more evidence to demonstrate first the actual presence of the detected molecules and second the possible abiotic photochemical ways of producing DMS and DMDS in an atmosphere like this. Madhusudhan cites this work as an opportunity for pursuing such investigations within a community of continuing research. No one is claiming we have detected life at K2-18b, but we’re getting a nudge in that direction that will be joined by quite a few other nudges as we probe alien atmospheres.
Not all these nudges point to the same things. For among papers discussing K2-18b, another is about to appear that questions whether it and another prominent sub-Neptune (TOI–270 d) are actually hycean worlds at all. This deep dive into sub-Neptune atmospherics, led by Christopher Glein at Southwest Research Institute, will be our subject next time. For before we can make the call on any hycean biosignature, we have to be sure that oceans are possible there in the first place.
The paper is Madhusudhan et al., “New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI,” accepted at Astrophysical Journal Letters (preprint).
#Biosignature #K218b
