Astronomers are still arguing about whether we have recently seen the “strongest evidence” for alien life yet, or simply nothing at all. Now, the researchers behind the original bold claim have reanalysed the data and say they have yet more evidence for molecules with no origin outside of biology – but critics say this new work undermines the original efforts.
Since at the University of Cambridge and his colleagues announced their remarkable finding that K2-18b, a super-Earth 124 light years away, showed “strong evidence” for an atmosphere containing the molecules dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) – both of which, on Earth, are produced solely by life – astronomers have closely scrutinised their findings.
at the University of Oxford showed that applying a different statistical test showed there was little evidence for DMS as the researchers had claimed. In a separate paper, at Arizona State University and his colleagues found that Madhusudhan and his team didn’t consider many alternative molecules that may fit the data, and that when the molecular pool under consideration is expanded from the original 20 to 92, then DMS is no longer the only explanation.
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Now, Madhusudhan and his team have gone far beyond that, comparing the chemical signatures of 650 different molecules to what they see in the spectrum of light from K2-18b’s atmosphere. They found that an additional two molecules, diethyl sulphide and methyl acrylonitrile, fit the data just as well as DMS. Both are complex molecules that also have no non-biological origin.
Madhusudhan says this new analysis is the most comprehensive chemical search of an exoplanet atmosphere ever performed and that it strengthens his team’s original claim, because the new molecules are even harder to explain than DMS both in their origin and chemical complexity – making it more likely that DMS is the best explanation for what they see. “After our most recent work, I am slightly more confident,” he says.
But Welbanks disagrees, and says it is notable that the DMDS detection has disappeared from the new results. “DMDS is no longer included among the highlighted species, despite being central to the original claim,” he says. “This retreat from the language of detection, and from DMDS altogether, strongly suggests the original interpretation was not robust.”
“This is a major change in interpretation within just one month, with no new data, no new retrieval framework, and no newly available [laboratory molecule] data,” says Welbanks.
Madhusudhan, however, says that their original claim didn’t rely upon DMDS, and the fact that DMS still remains in their data is consistent. “It had never been that we were saying it was only DMS that we were picking up. We are just saying now that it is DMS or even more complex molecules that we’re picking up,” says Madhusudhan.
Welbanks also argues that the statistical test that Madhusudhan and his team use, which compares whether the data better fits an atmosphere made from a certain molecule or a baseline model consisting of only methane and carbon dioxide, can lead to erroneous detections. “This approach inflates the apparent significance and deviates from standard practice in the field, where model comparisons are typically made relative to a more complete or physically motivated reference model.”
Madhusudhan disagrees with this criticism, saying that comparing the molecules to more complicated alternatives is difficult and could be computationally infeasible, and comparing to a simple baseline is standard practice.
Taylor says that Madhusudhan and his team’s new analysis is more statistically rigorous than their initial work, and is what he would have liked to see originally. However, he disagrees with Madhusudhan’s interpretation that DMS is the most likely interpretation of the data, and argues that the results show the data isn’t currently precise enough to determine exactly which molecules are present, which “supports the lack of detectable biosignatures in the current data” that his work showed.
arXiv
Mysteries of the universe: Cheshire, England
Spend a weekend with some of the brightest minds in science, as you explore the mysteries of the universe in an exciting programme that includes an excursion to see the iconic Lovell Telescope.
