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The small signs revealed in April seemed to change the universe as we know it.
Astronomers have detected faint shining hints from two molecules that swirl into the atmosphere of a distant planet called K2-18b. This is a molecule that is produced only by living things on Earth. It was an appetite-inducing prospect. This is the most promising evidence of extraterrestrial signatures or traces of life associated with biological activity.
However, just a few weeks later, new findings suggest that searches must continue.
“It was exciting, but we quickly raised some red flags because the potential biosignature claims are historical, but the importance or strength of the statistical evidence seemed too high for the data.”
Molecules identified for K2-18B in an April study (dimethyl sulfide, or DMS, and dimethyldisulfide, or DMD) are primarily associated with microbial organisms on Earth, but scientists have pointed out that compounds can also form without the presence of life. Three teams of astronomers who are not currently involved in the study, including Welbanks, have evaluated the models and data used in the discovery of the original biosignature and obtained results submitted for peer review.
Meanwhile, the lead author of Nikku Madhusudhan, a study from April, and his colleagues, have done additional research that says it will enhance previous discoveries about the planet. And there could be additional observations and research from multiple groups of scientists on the horizon.
A series of research papers rotating around the K2-18B provides a glimpse into the real-time development of scientific processes. It is a window into the complexity and nuance of how researchers search for evidence of life across the globe, demonstrating why the burden of proof is so high that it is difficult to reach.
Located 124 light years from Earth, K2-18B is generally considered a suitable target to wash signs of life. Previous research led by Madusdan, a professor of astrophysics and deplanetary science at the Cambridge Institute of Astronomy, is thought to be the world of Hycyan, a planet completely covered in liquid water with a hydrogen-rich atmosphere. As such, K2-18B is rapidly gaining attention as a potentially habitable location beyond our solar system.
Convinced of the promise of the K2-18B, Madhusdan and his Cambridge colleagues used the observation of the planet by the largest operating space telescope to further study the planet. However, two scientists at the University of Chicago – Dr. Rafael Luk, a postdoctoral scholar in the university’s Faculty of Astronomy and Astrophysics, and Michael Chan, a fellow at 51 Pegasy B/Burbidge Postdoctoral, found some issues with what they found.
Following the 2023 study, after reviewing a paper from Madhusudan and his team in April, Ruk and Chan noticed Webb’s data seemed “noisy.”
The noise caused by telescope defects and the speed at which different particles of light reach the telescope are just one challenge astronomers face when studying distant exoplanets. Noise can distort observations and introduce uncertainty into the data, Chan said.
More uncertainty arises when attempting to detect certain gases in the distant exoplanet atmosphere. Most notable Characteristics from gases such as dimethyl sulfide are derived from the bonding of hydrogen and carbon molecules. This is a connection that allows light to be extended and bent and absorbed at different wavelengths, making it difficult to clearly detect one type of molecule.
“The problem is that basically every organic molecule has carbon hydrogen bonds,” Zhang said. “These molecules are hundreds of millions of times, so these characteristics are not unique. If you have complete data, you can probably distinguish between different molecules. But if you don’t have perfect data, many molecules, especially organic molecules, appear to be very similar.”
Dripping further into the paper, Luk and Chan noticed that the planet’s perceived temperature appears to be increasing rapidly from 300 Kelvins (-9.67 f to 80.33 f or -23.15 c to 26.85 c) from the range of about 250 Kelvins. study.
Such harsh temperatures could change the way astronomers think about the potential habitat of a planet, Zhang said, as areas that Webb can detect, and the lower surface or ocean could have even higher temperatures, particularly as cool temperatures continue to the top of the atmosphere.
“This is merely an inference from the atmosphere, but it certainly affects our thinking about the planet in general,” Luk said.
Part of this issue is that the April analysis did not include data collected from all three Webb Instruments Madhusudhan teams used in the past few years. So, Luk, Chang, and her colleagues conducted a combined study of all available data to see if the same results could be achieved, and even if dimethyl sulfide could be found. They discovered “inadequate evidence” for both molecules in the planet’s atmosphere.
Instead, Luque and Zhang’s team discovered other molecules, such as ethane, which could fit the same profile. But ethane does not mean life.
Wellbanks of Arizona and his colleagues discovered what appears to be the fundamental problem in a K2-18b’s April dissertation, including Dr. Matt Nixon, a postdoctoral researcher at the Astronomy Department at the University of Maryland College Park.
The concern, according to Welbanks, was how Madhusudhan and his team would model it to show how molecules exist in the planet’s atmosphere.
“Each (molecules) are tested one at a time against the same minimal baseline. So every model has an artificial benefit. That’s the only explanation allowed,” Wellbanks said.
When Welbanks and his team conducted their own analysis, they expanded the model from Madhusudhan’s study.
“[Madhusudhan and his colleagues]have not allowed these small signals or other species that could potentially produce observations,” Nixon said. “So the main thing we wanted to do was assess whether other species could provide sufficient compatibility with the data.”
Once the model is expanded, evidence of dimethyl or dimethyl disulfide sulfide “just disappears,” Wellbanks said.
Madhusudhan thinks that the study published after the April paper was “very encouraged” and “enabled sound discussions about the interpretation of data on K2-18B.”
He reviewed Luk and Chan’s work and agreed that their findings did not show “strong detection of DMS or DMDS.” When Madhusudhan’s team published the paper in April, he said that observation reached the importance of three sections, or a 0.3% chance that detection could occur by chance.
Because of scientific discoveries that are very unlikely to occur by chance, observations must meet the threshold in Section 5 or below the 0.00006% chance that observations occur by chance. Meeting such a threshold requires many steps, such as repeat detection of the same molecule using multiple telescopes and eliminating potential nonbiological sources.
Such evidence can be seen in our lives, but it is unlikely to be an Eureka moment, a slow build that requires consensus among astronomers, physicists, biologists and chemists.
“We have never reached that level of evidence in any of our studies,” Madhusdan wrote in an email. “We found evidence of less than three sigmas in two previous studies (Madhusudhan etal. 2023 and 2025). We call this moderate evidence or hint, but not a strong detection. (Luk and Zhang’s) claims are consistent with the study, and we discussed the need for strong evidence for research and communication.”
In response to a study conducted by the Welbanks team, Madhusudhan and his Cambridge colleagues wrote another manuscript extending the search for K2-18B Includes 650 different molecules. They submitted a new analysis for peer review.
“This is the biggest search for exoplanet chemical features to date by searching for 650 molecules using all available data from K2-18B,” says Madhusudhan. “As we pointed out in previous studies, robust detection requires more observations, but DMS continues to be a promising candidate molecule on this planet.”
Wellbanks and Nixon were pleased that Madhusdan and his colleagues addressed the concerns raised, but they feel that the new paper is effectively returning to the central claims made in the original April study, Wellbanks said.
“The new paper implicitly acknowledges that DMS/DMD detection is not robust, but relies on the same flawed statistical framework and selective reading of unique results,” Wellbanks said in an email. “While the tone is more cautious (at times), methodology continues to obscure the true level of uncertainty. The statistical significance claimed in previous studies was the product of any unacceptable modeling decision.”
Luk said the new Cambridge team paper is a step in the right direction to explore other possible chemical biosignatures.
“But I think it lacks scope,” Ruk said. “I think it’s too limited to rebutting the (Wellbanks) paper.”
However, in another area, astronomers studying the K2-18B agree that advances in the study of deformation contribute to the scientific process.
“I think it’s a good, healthy scientific discourse to talk about what’s going on on this planet,” Wellbanks said. “Regardless of what the author group is saying now, we don’t have silver bullets, but that’s exactly why it’s exciting. And I think we might get it within our lifetimes, but we’re not there now. That’s not a failure. Testing bold ideas. ”
