A team of scientists claimed in April that a planet orbiting a distant star bore a possible signature of life, but the latest observations suggest otherwise. At the University of Cambridge, Nikku Madhusudhan and his colleagues claimed in April that they found hints of the molecules dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) in the atmosphere on K2-18b using the James Webb Space Telescope (JWST) data.
On Earth, these sulfur-based compounds are primarily produced by marine microorganisms. Madhusudhan said those were the "first hints we are seeing of an alien world that is possibly inhabited".
K2-18b is a super-Earth located 124 light-years away in the constellation Leo. It is approximately 2.6 times the diameter and 8.6 times the mass of Earth, orbiting a cool red dwarf star within the habitable zone.
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What did the other researchers say?
Other researchers also analysed the same data using different statistical models and didn't find evidence for the presence of these molecules. Renyu Hu at the California Institute of Technology and his colleagues teamed up with Madhusudhan and his group to analyse the observations of K2-18b.
"The paper does not provide conclusive evidence for the existence of this molecule in the atmosphere," Hu said as quoted by the New Scientist. The second team found no statistical evidence..
However, Madhusudhan told New Scientist that his colleagues analysed the data again, which made him "more confident" that DMS was the best explanation.
James Webb's near-infrared camera was used by the second group of researchers to look at the light coming from K2-18b's star. It can reveal what molecules exist in the atmosphere after passing through the planet's atmosphere.
The camera looked at a different wavelength of light compared to the mid-infrared measurements that were used for the analysis done in April.
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"This model dependency just speaks to the fact that it is a very weak signal, if there is any signal at all," Hu said. "I would just exercise caution".
"This paper is very clear in saying that there is no evidence for dimethyl sulphide. There is no statistical evidence for any of these gases," Luis Welbanks at Arizona State University said as quoted.
"We seem to be coming to the end of the debate on whether DMS is present in detectable levels in the [K2-18b] atmosphere, as the increased precision has not helped to detect it at a higher significance," Jake Taylor at the University of Oxford said, as quoted.
Scientists previously proposed that K2-18b could be a "Hycean world," a planet with a hydrogen-rich atmosphere and vast ocean, making it a prime candidate for hosting life.
Hu and his team found that for certain hydrogen-rich atmospheres, chemicals can produce DMS without the presence of life.
"A key takeaway is that biosignatures are going to be hard, no matter what kind of planet we are talking about," Jacob Bean, an astronomer at the University of Chicago, who was not involved in the study, said as quoted by The New York Times.
All researchers, however, agreed that the planet is rich in water. Strong evidence for the presence of methane and carbon dioxide was also found by Hu and his team. It implies the existence of water, Hu said.
However, additional studies could provide insights into the planet's atmospheric composition, temperature profile and potential biosignatures.
