Reacción a "Study finds three sigma probability of biological activity outside our solar system"

The exoplanet K2-18b orbits a red dwarf located 124 light-years away from us, in the constellation Leo. It has a diameter 2.6 times that of Earth and a mass 8.6 times greater. It is especially interesting to astronomers and astrobiologists because it is rocky, has an atmosphere, and its temperature could allow liquid water to exist on the surface. Therefore, this “super-Earth” is considered potentially habitable and has in fact become one of the best candidates in the search for life beyond the Solar System.  

In 2019, using NASA’s Hubble Space Telescope (HST), a very significant discovery was made: its atmosphere contains water vapor. Because of this, that gaseous layer surrounding K2-18b became the target of research by different scientific teams from 2023 onward, thanks to the extraordinary capabilities of the James Webb Space Telescope (JWST, from NASA, ESA, and the Canadian Space Agency).  

Thus, the presence of approximately 1% methane (CH₄) and 1% carbon dioxide (CO₂) was detected in the atmosphere of this planet, which is clearly dominated by hydrogen (H₂). In addition, the possible presence of traces of a molecule called dimethyl sulfide (DMS, with the formula CH₃-S-CH₃) was discovered. This attracted strong attention because on our planet this gas is produced by living beings, including some that make up marine phytoplankton. That molecule contributes to the characteristic “sea smell” enjoyed on the coast, but at higher concentrations it is responsible for the unpleasant odor of boiled cabbage.  

Quickly, on social media and in some news outlets, we began to read that “life had been found” on that distant planet. The JWST team themselves, of course, had to deny it.  

And now we are living through this situation again, because new data obtained with the JWST’s MIRI spectrograph and published today confirm the presence of DMS and dimethyldisulfide –DMDS, a molecule with one additional sulfur atom– in the atmosphere of K2-18b, where it would be at least 20 times more concentrated than in ours. Since this gas breaks down quickly, such a concentration could suggest that there is a source constantly producing it. As often happens, the scientists who authored this interesting paper are more cautious than those amplifying it outside the academic sphere. 

In fact, in October of last year it was published that DMS can also be synthesized abiotically in laboratories, through photochemical reactions and without the involvement of living beings. Moreover, ESA’s Rosetta mission had already found this molecule in the coma of comet 67P/Churyumov–Gerasimenko, and recently my colleagues at the Center for Astrobiology published its detection in interstellar space, specifically in the molecular cloud G+0.693-0.027 located near the center of our galaxy. 

Therefore, it is clear that DMS can be synthesized by cosmic chemistry, without any biological process being involved in its origin. That is to say, this compound is not a true biomarker, which in astrobiology we define as a molecule that can only be produced by the metabolism of living beings. Perhaps the atmosphere of the planet K2-18b smells like boiled cabbage, but that does not mean there is life on that distant world.  

It is evident that both scientists and the general public hope for news about the detection of other living beings beyond this blue marble. We would like not to be alone in the universe. But we must be cautious and patient. When –these days or in the future– we read headlines claiming that life has been found beyond Earth, let us remember what the philosopher David Hume and the physicist and mathematician Pierre-Simon Laplace already told us back in the 18th century: “extraordinary claims require extraordinary evidence.”