Caroline Freissinet. Photo: Supplied / Caroline Freissinet
Headlines last month captured global excitement after astronomers claimed they detected the "strongest evidence to date" of life on another planet, but a world-leading astrobiologist says the science behind the claim is "fatally flawed".
However, one of the scientists behind the claims is standing behind their work.
In April, a team from Cambridge claimed to have recorded a possible biosignature, or signs of past or present life linked to biological activity, on an exoplanet named K2-18b.
Using the James Webb Space Telescope, the team detected chemical fingerprints that suggest the presence of dimethyl sulphide (DMS) and dimethyl disulphide (DMDS), molecules that on Earth are only produced by microbial life.
Carolyn Freissinet is a leading astrobiologist at the French National Center for Scientific Research. She told RNZ's Saturday Morning she believed the claims made by the Cambridge researchers were premature - and potentially misleading.
Freissinet, who collaborates with NASA and leads studies of Martian samples collected by the Curiosity rover, said the science behind the announcement simply did not stack up.
"This finding is super controversial," she said. "It's not based on a very serious scientific study. It has fatal flaws in the method that has been used."
According to Freissinet, just a week after the initial announcement, another team reviewed the same spectral data and found no trace of DMS at all.
"So first, there's a problem with the measurement itself."
Even if DMS was detected, she said, we shouldn't jump to the conclusion that it's a biosignature.
"We understand very poorly the sulphur chemistry of exoplanet atmospheres. There is this famous quote by Carl Sagan. He said that extraordinary claims, such as finding life, require extraordinary evidence."
One of the study's authors however said Freissinet's understanding of the findings was "clearly incorrect".
Nikku Madhusudhan, professor of astrophysics and exoplanetary science at the Institute of Astronomy, University of Cambridge, told RNZ their study was "the most advanced analysis conducted for an exoplanet with this [James Webb Space Telescope] instrument, and reported the first mid-infrared atmospheric spectrum of a potentially habitable planet outside the solar system ever".
"This is a major advancement in the field."
He said claims no other teams had detected molecules suggesting evidence of biological processes were also incorrect.
"None of them claim that the gases we reported cannot be found. The first study they mentioned, that came a week after ours, was a preliminary analysis which didn't even look for the gases so no conclusive statement can be drawn from it regarding our findings.
"The second and third papers which used more realistic models confirmed our calculations and suggested additional gases that could provide alternate explanations to at least some of the data. Our original suggestion of the gas DMS is still the most favoured at this point, considering all available data."
Piecing together a puzzle
Freissinet did not dismiss the search for extraterrestrial life - it was the core of her work.
But she argued that the search must be methodical, rigorous, and grounded in evidence.
"Right now, we're accumulating hints... pieces of a puzzle."
Some of those puzzle pieces are found on Mars. In 2013, Freissinet and her team also made headlines with the discovery of long-chain hydrocarbons in 3.7-billion-year-old Martian rocks at a site called Cumberland. These molecules, made up of ten or more carbon atoms, are incredibly fragile, especially on Mars, where conditions are harsh and preservation is rare.
"What we can say now is that if life ever existed on Mars... we could find those traces of life."
But she's careful not to overstate the findings.
"It's definitely not hints of life," she clarified. "We cannot tell if the origin of the molecules if they are biological or if they are pure chemical reactions."
So what would "life" look like? Freissinet emphasised just how difficult it was to identify alien life, especially when we only have one known example - Earth.
"We try to identify life as we don't know it," she said. "It's really hard."
One method scientists use is to look for chemical imbalances that suggest biological processes. For example, amino acids, the building blocks of proteins, exist in two mirror-image forms. On Earth, life uses only one of these forms. A similar imbalance found elsewhere could be a clue.
Still, even these signs require cautious interpretation. Natural processes can sometimes mimic the patterns life creates.
At the Cumberland site, Freissinet's team also found traces of nitrates and lighter isotopes of carbon and sulphur elements, which, on Earth, were often associated with biological activity. But even these, she said, had potential non-biological explanations.
Long road ahead
Madhusudhan said there was "usually a lot of debate" around the subject, but urged people "not to confuse debate with misinformation".
Freissinet's perspective was not one of scepticism, but of scientific integrity. She said discovering evidence of life beyond Earth would likely be a long, slow process and should not take the form of rushed conclusions or overhyped discoveries.
"For now, we're we are very far from biosignature detection. We are accumulating hints everywhere in the solar system and elsewhere in the universe, looking at exoplanets to make a story.
"We're accumulating pieces of a puzzle. And one day, hopefully, this puzzle will assemble."
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