K2-18B: “Three big mistakes” behind the announcement of a possible extraterrestrial life!

From last week And the announcement by astrophyscians of Cambridge of the discovery of a molecule considered as potential signature of living organisms on an exoplanet to 124 light years of us, the media controversy continues to swell. Clearly, the discovery of an extraordinary proof of life would be extraordinary, undoubtedly the most beautiful scientific discovery of all time.

For Carl Sagan, “life is the hypothesis of last resort”

But precisely, as Marcello Truzzi said, “An extraordinary discovery requires extraordinary evidence“: Do we have these extraordinary evidence regarding K2-18B? The famous Carl Sagan, sometimes considered one of the first exobiologists, said”Life must be the hypothesis of last resort“.

Let’s take the thread of these announcements. The Cambridge team published on April 17 on Astrophysical Journal Letters an article Affirming the discovery of a chemical compound, the dimethyl sulfide (DMS) as well as its close relative, the dimethyledisulfur (DMDS). On earth this sulfur compound is well come from Phytoplankton decomposition in the ocean. It is therefore actually linked to the living on earth.

The DMS detected: It is not a “serious” work according to many researchers

In the scientific world, for a discovery to be announced, it is generally a reliability index of 5 SIGMA, the team led by Nikku Madhusudhan evokes here 3 SIGMA, which is not enough, but would correspond to 99.7% of confidence. We would be tempted to think that it is already substantial. Except that in reality, the “3 sigma” do not exist, it is a hazardous transposition of the Bayes factor, an index which allows to know the reliability of a spectral signature, and the latter is at 2.68, which corrects “no evidence”. For “strong evidence” it takes 150.

And precisely, Nikku Madhusudhan would have the reputation of arranging as he wants with physical models and statistics …

As a reminder, K2-18B is an exoplanet considered as a mini neptune about 8 times more massive than the earth, and there is chances that it be rocky. It has an atmosphere that James-Webb has been studying for several years by analyzing the light of its star filtered by the gases on the planet.

“A stupid error beast”

The same team had already announced that it has a signal corresponding to DMS in 2023. Problem: this detection had been invalidated later. Nikku Madhusudhan and his colleagues return with the same announcement and, according to them, 99.7% of confidence.

According to Franck Selsis, director of research at the CNRS at the Bordeaux astrophysics laboratory, this is “of a beast statistical error“Because we do not see any trace of the famous DMS in the spectrum of K2-18B! Methane, yes, DMS, no.

And it is not the only one to say that. “”Conlure that the DMS has been detected seems premature“, declares in diplomatic terms Manasvi Lingam, astrobiologist at the Florida Institute of Technology (USA).

On the light spectrum published in the scientific article, we see too large error bars, but above all, it is the very constuction of their detection model which poses a serious problem. For Dr. Ryan Macdonald, it is done so that it cannot explain this spectrum other than by the presence of DMS and DMDS. In other words, N. Mathudusan finds in the atmosphere of K2-18b what he had come to exclude anything else.

The French astrophysicist, Dr. Eric Lagadec does not have another opinion and he shares it on Bluesky:

K2-18B, however, remains a very interesting planet

In a TV5 Monde interviewFranck Selsis talks about three important errors in the study in question (from 8 “20).

The first, we saw it. The second is that the DMS, even if it was detected, would in any way be a certain way: if on earth this compound is indeed of biological origin, we know abiotic origins in space, with for example the interaction of the light of a star and the atmosphere of a planet, which could well be the case here. The comet Tchouri also had traces.

The third error is quite enormous: K2-18B really does not present the characteristics of a habitable planet in the sense that we hear it because it orbits so close to its star (in 33 days) that its temperature must be around 1000 ° C, which is not compatible with an ocean of liquid water. However, this does not prohibit water in its atmosphere, of course.

Cepdendant, K2-18B remains a planet with sufficiently intriguing characteristics to deserve the observations of the James-Webb.

In conclusion:

• DMS, a compound linked to life on earth, was not detected with confidence on K2-18B.
• The DMS is not a reliable biosignature.
• K2-18B probably does not have a liquid water ocean (even if it is considered in “habitable zone of its star” because probably burning).

This study therefore presents far too many defects and breaches of the rigor of the scientific method to allow the enthusiasm that one could have legitimately felt after such an announcement …