April 17, 2025

expert reaction to study on a three-sigma probability event of biological activity outside our solar system

A study published in The Astrophysical Journal Letters looks at the probability of biological activity on the K2-18 b exoplanet.

Prof Jeffrey Kargel, Senior Scientist, Planetary Science Institute, said:

“The detection by Nikku Madhusudhan and colleagues of dimethyl sulfide (DMS) and possible dimethyl disulfide (DMDS) in an exoplanetary atmosphere is an exciting next step of an earlier tentative, weak detection of dimethyl sulfide in the atmosphere of the same exoplanet, K2-18 b, where methane and carbon dioxide also were detected. The possible biological origins of these organic sulfur-bearing gases stems from the fact that on Earth, atmospheric and oceanic traces of these gases are due almost entirely from phytoplankton in the sea. The relationship is so strong that DMS and DMDS are considered biomarkers.

“The authors recognize that these gases also are produced by the effects of ultraviolet radiation in the Earth’s stratosphere, but they point out that minuscule amounts can be produced and accumulated this way, because the gases are also effectively destroyed by ultraviolet light. The only way that larger accumulations can form in Earth’s atmosphere is by continuous massive emissions by life, mainly by phytoplankton in the sea.

“The authors also note that some comets contain DMS. Since comets are an unlikely abode of life, an abiotic source of K2-18 b’s DMS and DMDS cannot be ruled out. A further abiotic occurrence of DMS was reported just two months ago by Miguel Sanz-Novo and colleagues–in the interstellar galactic clouds, which hardly can be considered life friendly! I include that reference below.

“Dimethyl sulfide is also produced industrially by the reaction of hydrogen sulfide with methanol. Since both of those substances are present in comets and could occur in K2-18 b’s atmosphere, it is certainly worth considering further the possibility that somewhere in the expected varied environments of K2-18 b’s atmosphere, surface, and deeper interior, there could exist some means by which a natural but abiotic process–maybe similar to the industrial one–generates DMD and DMDS. Nonetheless, in the one place we know really well–Earth–these gases are generated overwhelmingly by life. The connection is so strong that consideration of these gases as signposts of life in K2-18 b is intriguing, and I’d say compelling, though not totally persuasive.”

Dr David Clements, Astrophysicist, Imperial College London, said:

“This is really interesting stuff and, while it does not yet represent a clear detection of Dimethyl Sulfide and Dimethyl Disulfide, it is a step in the right direction.

“To get to a solid claim for the existence of life on this planet we’d need to have a solid detection, >5 sigma, a clear demonstration that this is a biomarker and not some other molecular species masquerading as a biomarker, and then a clear understanding that there is no non-biological way of producing the biomarker molecule in the amount seen. Planetary atmospheres are complicated and difficult to understand, especially with the limited information we get from a planet 124 light years away, so there will almost always be some provisos and uncertainties about interpretation, but more and better data will help, and the first step is getting a detection to >5 sigma so that we can be sure that something interesting is there.”

Dr Stephen Burgess, group leader at the University of Cambridge, said:

“Most scientific experiments have some element of uncertainty. This could be sampling uncertainty – maybe we only have a small number of observations. Or it could be measurement error – maybe our measurements are noisy. If we picked 5 random men and 5 random women from the street, sometimes we will find that the men are taller on average than the women, but occasionally we will find that the women are taller on average than the men. If we want to conclude that men are typically taller than women, we need to collect enough data to be confident that the differences we observe are genuine differences, and not just chance fluctuations. The more data that we collect, the more certain we can be of this. “Three-sigma” is a threshold saying that differences observed in the experiment are sufficiently notable that we can exclude the possibility of a chance finding except in rare cases – equivalent in rarity to tossing a coin 10 times and getting the same result each time. “Five-sigma” is a stricter threshold – equivalent to tossing a coin 20 times in a row and getting the same result each time. It’s still possible that we were simply lucky – and the more data that we look at, the greater the chances of making an observation that is purely a chance finding. But a five-sigma finding is one that would only arise purely by chance exceptionally rarely, and so we can be very confident that this observation isn’t just a chance finding. A separate question to uncertainty is bias – it is possible that there is some flaw with the experiment. This is not something that can be ruled out by statistics. A “five-sigma” finding is therefore exceptionally unlikely to arise due to chance alone: it is either a true result or an experimental error.”

‘New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI’ by Nikku Madhusudhan et al. has been published in The Astrophysical Journal Letters

Declared interests

Dr Stephen Burgess: I am employed at the same university as the lead author of this paper. However, I do not know them personally or professionally.

For all other experts, no reply to our request for DOIs was received.