Doubts about the discovery of possible "evidence" of extraterrestrial life

 

New studies are revisiting the discovery of potential traces of biological activity outside the solar system announced by a US-UK team in April, suggesting that this evidence of extrasolar life is not particularly promising.

Last month, astronomers caused a stir when they published in The Astrophysical Journal Letters the results of observations of K2-18b, an exoplanet located 124 light-years from Earth in the constellation Leo.

Using the James Webb Space Telescope, scientists confirmed that they had detected signs of chemical compounds in the planet's atmosphere that have long been considered "biomarkers" for extraterrestrial life. These compounds are dimethyl sulfide (DMS) and dimethyl disulfide (DMDS), which on Earth are produced only by living organisms, particularly phytoplankton.

"I think this discovery is the closest we can get to a feature that we can attribute to life," said Nikku Madhusudhan, an astrophysicist at the University of Cambridge and lead author of the study, emphasizing the need for further observations.

In 2023, the James Webb Space Telescope detected methane and carbon dioxide in the atmosphere of the exoplanet K2-18b. This was the first discovery of an exoplanet located in the "habitable zone," meaning it's neither too close nor too far from its star for a key ingredient for life: liquid water.

Although the signs were more pronounced this time, they still fall far short of the statistical significance threshold that scientists consider crucial for validating the discovery.

Since the study was published, several astronomers have expressed skepticism. These conclusions have been reinforced by recent research that has not yet undergone peer review.

Among the researchers who reexamined the data are two of Madhusudhan's former students: Louis Wilbanks of Arizona State University and Matthew Nixon of the University of Maryland.

By using other statistical models, "the assurances about the possibility of detecting a biosignature evaporate," the scientists wrote in a preliminary study published last month.

They expanded the number of potential chemicals that could explain the signatures observed by the James Webb Space Telescope to 90, up from 20 in the original study.

More than 50 provided a "result." "When you detect everything, have you actually detected anything?" Nixon told AFP.

The researchers emphasized the need for further observations, without ruling out the possibility that the atmosphere of K2-18b contains dimethyl sulfide.

"This kind of discussion is valid," Madhusudhan told AFP.

Last week, his team published its own preliminary study, expanding the number of potential chemicals to 650.

Among the three most promising results is dimethyl sulfide, but not dimethyl disulfide, a key aspect of the initial announcement.

"Within a month, without any observational data, laboratory data, or new models that would completely change their analysis," Wilbanks said.

Telescopes can observe exoplanets as they pass in front of their star, allowing astronomers to analyze how molecules filter the wavelengths of light passing through their atmospheres and infer their composition.

A team of astrophysicists from the University of Chicago combined James Webb Space Telescope observations of the star K2-18b in the near and mid-infrared. They found "no statistical significance for dimethyl sulfide and dimethyl disulfide," according to a study published last week.

Using a basic statistical test, Oxford University astrophysicist Jake Taylor found no evidence of biosignatures.

Madhusudhan rejected this latest study, asserting that the test did not take into account the physical phenomena observed. He also defended his research, saying he was "quite confident" about it now than he was a month ago, ahead of the release of new data on K2-18b expected next year.

Many scientists believe that space telescopes may one day be able to gather sufficient evidence to determine the presence of extraterrestrial life.

"But we have to use existing frameworks and gather (the evidence) reliably rather than rushing into things, as is particularly the case here," Nixon said.