New hope of extraterrestrial life? Scientists cautious about celebrating early, want recheck
Are we alone in this universe? A study led by the University of Cambridge provides a flicker of hope that it may not be so, and the answer may lie 120 light-years from Earth. The study, published in the Astrophysical Journal Letters last week, found hints of life in the distant planet named K2-18b. But astrophysicists are sceptical and say the study's results and the methodology need to be cross-checked by other researchers. According to the research, fingerprints of dimethyl sulphide and dimethyl disulphide molecules have been detected on the exoplanet's atmosphere. On Earth, these molecules are known to be produced by marine organisms.
Interestingly, the most common hypothesis is that life on Earth originated in the ocean.
The study provides evidence of 'three-sigma' significance -- a 99.7 per cent confidence that the results are not fortuitous -- about the strongest-yet of signs of life outside the Solar System, asserts the research team led by Nikku Madhusudhan, a professor of astrophysics and exoplanetary science. In an interview with PTI Videos last week, he said that given scope of the study's implications, his team was looking to increase the robustness of the results in future researches.
Jayesh Goyal, a reader at the School of Earth and Planetary Sciences at the National Institute of Science Education and Research (NISER), Bhubaneswar, feels that the findings of the study are a big step forward and "pushes the limits of our understanding of exoplanet atmospheres and their habitability". "The observations on K2-18b's atmosphere highlight the extent to which this class of sub-Neptune or super-earth exoplanets could be characterised as these targets are extremely challenging to study," he told PTI. The exoplanet is 8.6 times massive than Earth, but smaller and less massive than Neptune. Hence it has been classified as a 'sub-Neptune' exoplanet. Scientists who study exoplanets contest the interpretation of the observations, saying the results are not statistically sound, given the immensity of claims of detecting biological activity outside the Solar System.
"It is not a 'detection' according to the usual standards of exoplanet science," Ryan MacDonald, NASA Sagan Fellow at the University of Michigan in the US, told PTI.
Data recorded by the James Webb Space Telescope's MIRI instrument was analysed for the study. The infrared facility looked at starlight transmitted through the exoplanet's atmosphere. "The study assumes that half of the new data from the Webb telescope can only be explained by dimethyl sulphide and dimethyl disulphide, neglecting other possible gases, thereby attaching a much higher statistical significance to the claims than the data supports," the astronomer said. Asa Stahl, an astrophysicist whose PhD at the US' Rice University focused on exoplanets, said the study made use of a "hugely powerful tool" for peering into a faraway planet's atmosphere. "It's an immensely difficult task -- trying to piece together what a planet over a hundred light years away is like from how starlight filters through its atmosphere." However, it is also a relatively new method, and astronomers are still figuring out the best practices for this sort of thing, added Stahl, engaged in science communication projects. Madhusudhan, while asserting that the team of researchers would look into fortifying the robustness of the study's results, also said, "When you have big breakthroughs and big paradigm shifts, you want to be really sure because it changes the very fabric of science and society in fundamental ways." "So then, the measure of robustness there is that we want to be sure to a level that there is less than one part in a million chance of a fluke, which is a very, very, very small chance of a statistical fluke or a 'just by chance'. We want to be that robust," he added. However, the currently claimed 'three-sigma' significance -- or a 0.3 per cent of being wrong -- would need to be tested, astrophysicists said. "Using the statistics in the study, the actual probability of the molecules not being present (in K2-18b's atmosphere) is about 28 per cent. Therefore, the announcement projects (a) near-absolute confidence in a result that has a good chance of not being real," MacDonald said. And same is the case with Stahl. "We won't know for sure how robust the finding is until other researchers test it."
Astrophysicist Stephen Schmidt, a graduate research fellow at the Johns Hopkins University, US, re-analysed results from a 2023 study, in which Madhusudhan's team, using the James Webb space telescope, found abundant levels of carbon-containing gases -- methane and carbon dioxide -- and potential signs of dimethyl sulphide. Despite replicating the models used in the 2023 study, the results of the re-analysis (currently in a pre-print paper stage) differed from the original. Schmidt's team could not put limits on detectable amounts of carbon dioxide in K2-18b's atmosphere. The result indicated "a lot more methane compared to carbon dioxide" and therefore, "very difficult and unlikely for the exoplanet to have a habitable liquid water surface ocean, and also a biosphere or life", he explained. Schmidt said that while the University of Cambdrige-led study pushes the limits and tests the capabilities of the Webb telescope, which he stresses is important, "this can result in potentially exciting findings that aren't substantiated after further observations." Further, there are questions that need answers, to know for sure what is happening. One of these involves understanding the processes through which dimethyl sulphide and dimethyl sulphide molecules -- considered as predictors of life on exoplanets -- can form. Even as Madhusudhan's team is looking to address this aspect in future research, answers to the origins of the molecules could prove to be especially important as studies have found dimethyl sulphide on a comet and in the space between stars -- both 'lifeless' environments. Goyal said more observations of K2-18b using the Webb telescope, along with a detailed study of laboratory spectra of dimethyl sulphide and dimethyl disulphide, could help tighten or dispute the study's results. Further, a model's ability to detect chemicals accurately should be quoted in a paper, after "considering a wide variety of different molecules, rather than just those one assumes are there," MacDonald said. The astrophysicists said not one chemical or molecule, but a combination of chemicals or gases, produced in significant amounts, along with a deep understanding of the exoplanet's environment, would be needed before being confident of habitability.
However, the methods used in the Cambridge-led study could hold promise in these pursuits. Stahl said, "If we ever discover life in another world, this method could be how we find out."
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