Finding life on a planet other than Earth is one of humanity's greatest desires, but it is proving to be quite challenging. In the 30 years since the first exoplanet was discovered, nothing less than 5,885 worlds outside the Solar System have been found, but none can be considered habitable with certainty. That is why the announcement from a team at the University of Cambridge on Holy Thursday has caused as much anticipation as controversy: they claim to have found, through the James Webb telescope, the strongest evidence to date of biological activity - that is, produced by living beings - outside our planet. And they found it on a planet already known: K2-18b, discovered in 2015 and 124 light-years away from us.
This striking claim is based on the confirmation of a discovery made a year and a half ago on the same exoplanet: allegedly, they found a molecule called dimethyl sulfide or DMS. This is a gas produced on Earth by living beings, especially microbial life like phytoplankton, and contributes to the unmistakable smell of the sea when we approach the coast.
In the previous work, they had mentioned the possible presence of traces of dimethyl sulfide. Now, in addition to achieving a more robust detection of DMS, they have found another gas from the same family, dimethyl disulfide (DMDS). Furthermore, they claimed that in the atmosphere of K2-18b, it would be 20 times more concentrated than in ours. Since this gas decomposes rapidly, such a concentration would suggest that there is a source constantly producing it. In summary: some form of living organism.
For several years, the planet K2-18b has been considered one of the best candidates to search for life outside our solar system: it is rocky like Earth, has an atmosphere with water vapor, carbon dioxide, and methane, and its temperatures would allow for liquid water on its surface. It orbits a red dwarf star and is considered a super-Earth because it has a diameter 2.6 times larger and a mass 8.6 times greater than ours.
Scientists suggested it could have an ocean, and now they consider the presence of dimethyl sulfide as an indication of alien marine life: "The results are the most solid evidence to date that life may exist on a planet outside our solar system," say the authors of this research published in The Astrophysical Journal Letters and led by Nikku Madhusudhan, who introduced caution in their text: "An unknown chemical process could be the source of these molecules in the atmosphere of K2-18b."
The announcement quickly made headlines worldwide, but many scientists expressed doubts in two directions: that there is indeed dimethyl sulfide in its atmosphere, and in the case that there is, that this compound originates from living beings. "I am surprised that very prestigious media outlets have jumped the gun and made claims not based on facts," says Carlos Briones, a researcher at the Center for Astrobiology (CAB/CSIC-INTA). "They are very optimistic headlines."
Is Madhusudhan's research the most solid evidence of extraterrestrial life to date? Have the provided data been misinterpreted? Or have the authors exaggerated in presenting them?
"I think they are getting a bit ahead of themselves, but they are not particularly sensationalist," reflects Briones. "The big problem is that dimethyl sulfide is not in itself a biomarker: that is, a molecule that only living beings can form. And basing the entire proposal on a molecule that chemistry itself can generate without the need for biology is not definitive proof or the closest thing to finding life."
Nevertheless, this scientist considers the work "very interesting": "They have characterized an atmosphere much better, but to say that we have found life we would need to find a true biomarker. That is, a protein or a fragment of DNA or RNA, or if not, a very large and complex organic molecule like those only produced by living beings because there is metabolism. The day someone finds this somewhere, I will say, well maybe it is life."
Like methane (CH4), dimethyl sulfide or DMS (CH3-S-CH3) on Earth is fundamentally the result of metabolic reactions of living beings: "It is produced by the breakdown of a compound generated by marine phytoplankton, as well as plants and some bacteria," explains his colleague Víctor Parro, director of CAB/CSIC-INTA. "This means that it is mainly produced by eukaryotic organisms, which did not appear on Earth until at least 1,000 million years after the origin of life. That means time, adaptation, and evolution."
Until now, this molecule had been considered a biomarker or biofirma in the atmospheres of potentially habitable exoplanets: that is, an molecular indication of the presence of carbon-based life. By definition, biomarkers imply that they can only be generated through the metabolism of living beings. "Finding these compounds in the atmosphere of an extrasolar planet and associating them by analogy with a biological process is always tempting, but any other purely chemical origin must be ruled out due to the planet's own composition, its evolutionary state, and the degassing processes it may be experiencing," says Parro.
"I would not speak of signs of life at all," states emphatically Guillem Anglada-Escudé, an astrophysicist at the Institute of Space Sciences (CSIC). This researcher, who discovered the closest exoplanet to Earth, Proxima b, describes this result as follows: "It is a bit preliminary, but it sets an interesting path with great potential impact."
Víctor Parro, on the other hand, believes that it is most likely "abiotically originated" and emphasizes that the amounts of DMS found are very modest: "They border on the limits between a good signal and the equipment noise." In his opinion, the most relevant aspect is detecting chemical compounds in the atmospheres of planets tens of light-years away. "Although the James Webb still does not reach the resolution we would like to characterize their atmospheres, it will help us focus on many planets to deepen with other more sensitive instruments that will be part of future missions," he states.
The detection of an atmosphere on a planet orbiting another star is a "extraordinarily complex" process, highlighted David Barrado, also from the Center for Astrobiology. "The data analysis is even more so," he adds. "Therefore, although it is possible that these compounds exist, the interpretation with the connection to biological activity is more than doubtful. The simpler the molecules are, the more likely they can originate from natural chemical methods, without the intervention of life. And that is the case with DMS."
In fact, last week, researchers from the Center for Astrobiology published the first detection of this molecule in interstellar space, using radio astronomy observations. The discovery of DMS was made in a molecular cloud located in the center of our galaxy, called G+0.693-0.027, thanks to data obtained with two Spanish instruments: the 30-meter diameter IRAM radio telescope at Pico Veleta, in Granada, and the 40-meter telescope at the Yebes Observatory of the National Geographic Institute, in Guadalajara.
"Our work confirms that DMS is not a good biomarker, or at least not the only one nor completely reliable," assures Miguel Sanz Novo, the scientist from the Center for Astrobiology leading this study. "We detected it in a molecular cloud, a completely inert environment, where it is generated by the chemistry of the cosmos itself, which is quite rich. In summary, when we put it in the context of exoplanet science and the search for life, if there really is DMS or DMDS in the atmosphere of K2-18b, we must be extremely cautious, since we know it can form abiotically."
It is not the first time that it has been shown that DMS can form without the presence of living organisms. In late 2024, it was found in the tail of comet 67P/Churyumov-Gerasimenko, famous because it was visited by the ESA Rosetta space mission. And recently, it has been demonstrated that it can be formed in the laboratory through photochemical reactions, without living beings.
In fact, from the CAB, it is suggested that perhaps more than a biomarker, which would imply the presence of living organisms, DMS could be considered a bioseed, a molecule that, along with others, in a suitable environment, can generate a chemistry that could give rise to life. "We are increasingly ruling out biomarkers because we are finding more supposedly biological molecules in gas and dust clouds, or in asteroids, with in situ detection thanks to robotic missions bringing samples to Earth," says Carlos Briones.
Another aspect to consider is that K2-18b is a relatively large planet, slightly deviating from Earth-like types, so its chemical composition could follow different patterns. "I would be very surprised if alternative and abiotic explanations for the presence of these chemical compounds are not found, if their detection is ultimately confirmed," says David Barrado, author of the book Exoplanetas y astrobiología.
Because another doubt that has been raised is whether these chemical compounds really exist on this exoplanet. "It is not clear that DMS and DMDS have actually been detected in the atmosphere of K2-18b," says Sanz Novo. "Although the authors claim to have found some indications, they are still not sufficient to be sure that the signals observed in the James Webb spectra are really due to these two molecules and not others."
Specifically, scientists at Cambridge said they had found a three sigma probability of biological activity outside our solar system. What does this mean? It is a threshold indicating that the differences observed in the experiment are significant enough to exclude the possibility that they are due to chance, except in rare cases.
But in order to affirm the existence of life on this planet, experts believe that a more solid detection, exceeding 5 sigma, would be necessary, which is a stricter threshold. "It is not yet entirely robust. It's like publishing a work that is still in progress, but clearly more data is needed," says Anglada-Escudé, who calls for more laboratory work and independent studies like those already being conducted by other groups to confirm that it is not an abiotic molecule.
Any conclusion that is reached will always be from a distance: 124 light-years is an impossible distance to send any type of vehicle to verify if there is life. "Only larger or more sophisticated space telescopes will allow us to obtain results with a higher degree of confidence," says David Barrado.
Víctor Parro agrees: "Over the next 500 years, we will continue to study them from a distance, although concepts have already been proposed, albeit very underdeveloped, for probes to Proxima Centauri b, which is located less than four light-years away and is the closest potentially habitable exoplanet known. There should be a real revolution in propulsion systems to approach the speed of light and make these interstellar journeys possible."
Even before embarking on a mission to search for alien life, it will be necessary to deepen our own definition of what we consider life: "There is no consensus on what it means", reflects David Barrado. "What is clear is that to affirm that there is life beyond Earth, the evidence must be extraordinarily strong, and all possible alternatives must be ruled out. We must be very cautious and avoid sensational headlines that only cause confusion and, eventually, skepticism."
This is the same request made by his colleague Carlos Briones: "Extraordinary claims require extraordinary evidence. Until we have them, we cannot say that we have found life beyond Earth."