Astronomers claim to have seen the strongest evidence so far for life on another planet. But other astronomers have urged caution until the findings can be verified by other groups and alternative, non-biological explanations can be ruled out.
“These are the first hints we are seeing of an alien world that is possibly inhabited,” Nikku Madhusudhan at the University of Cambridge told a press conference on 15 March.
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Astronomers first discovered the exoplanet K2-18b in 2015, and soon established that it was a promising place to look for life. About eight times as massive as Earth and orbiting a star 124 light years away from us, the planet sits in the habitable zone of its star, where liquid water can exist. Further observations, in 2019, found evidence of water vapour, which led to suggestions that the planet may be covered in oceans sitting under a hydrogen-rich atmosphere, though not all astronomers agreed.
In 2023, Madhusudhan and his colleagues used the instruments on the James Webb Space Telescope (JWST) to look at K2-18b’s atmosphere in near-infrared light, and again found evidence of water vapour, as well as carbon dioxide and methane. But they also found a tantalising hint of dimethyl sulphide (DMS), a molecule that, on Earth, is produced only by living organisms, mainly marine phytoplankton. The signs for DMS were extremely weak, however, and many astronomers argued that we would need much stronger evidence to be certain about the molecule’s presence.
Now, Madhusudhan and his colleagues have used a different instrument from JWST, the mid-infrared camera, to observe K2-18b. They found a much stronger signal for DMS, as well as a possible related molecule called dimethyl disulphide (DMDS), which is also produced on Earth only by life.
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“What we are finding is an independent line of evidence in a different wavelength range with a different instrument of possible biological activity on the planet,” Madhusudhan said.
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The team claims that the detection of DMS and DMDS is at the three-sigma level of statistical significance, which is equivalent to a 3-in-1000 chance that a pattern of data like this ends up being a fluke. In physics, the standard threshold for accepting something as a true discovery is five sigma, which equates to a 1-in-3.5 million chance that the data is a chance occurrence.
Nicholas Wogan at the NASA Ames Research Center in California says the evidence is more convincing than the 2023 results, but it still needs to be verified by other groups. Once the data is made public next week, other researchers can start to confirm the findings, but this could take weeks or months due to the difficulty of interpreting JWST data. “It’s not just like you download the data and you see if there’s DMS – it’s this super complicated process,” says Wogan.
Other scientists are more sceptical about the findings. “These new JWST observations do not offer convincing evidence that DMS or DMDS are present in K2-18b’s atmosphere,” says Ryan MacDonald at the University of Michigan. “We have a boy-who-cried-wolf situation for K2-18b, where multiple previous three-sigma detections have completely vanished when subject to closer scrutiny. Any claim of life beyond Earth needs to be rigorously checked by other scientists, and unfortunately many previous exciting claims for K2-18b haven’t withstood these independent checks.”
Madhusudhan and his team estimate that between 16 and 24 hours of further observations with JWST could help them reach the five-sigma level, but the difficulty of observing the planet’s atmosphere means they can’t guarantee this.
“The relative size of the atmosphere compared to the size of the planet is pretty close to the thickness of an apple skin on top of an apple. That’s what we’re trying to measure,” says Thomas Beatty at the University of Wisconsin-Madison, who was not part of the study team. Wogan adds that getting to five sigma might be fundamentally impossible due to the amount of noise in the data.
But if further observations can prove this is a real discovery, it would be a “tremendous advance”, says Beatty. “Ignoring whether or not it actually is produced by life for a second, it’s something that, a decade ago, people said would be evidence for life in the atmosphere of a planet that could feasibly host it.”
Madhusudhan and his colleagues calculate that the possible concentrations of DMS and DMDS on K2-18b appear to be over 10 parts per million, thousands of times greater than the concentrations in Earth’s atmosphere. This could indicate a far greater amount of biological activity than on Earth, if the signal proves to be correct, but establishing that the chemicals have a biological origin will take more work, he says.
“We have to be extremely careful,” said Madhusudhan. “We cannot, at this stage, make the claim that, even if we detect DMS and DMDS, that it is due to life. Let me be very clear about that. But if you take published studies so far, then there is no mechanism that can explain what we are seeing without life.”
Ruling out alternative mechanisms could take some time, says Wogan. “Something like this hasn’t really been studied. DMS in a hydrogen-rich atmosphere, we don’t know a tonne about it. There would have to be a lot of work.”
The difficulty in proving that it couldn’t have a non-biological explanation could put K2-18b in the category of a viable biosignature candidate for a long time, says Sara Seager at the Massachusetts Institute of Technology. “It may remain in that category for decades, since the question may never be fully resolved with the limited data exoplanets offer,” she says.
Madhusudhan, however, says the finding is important regardless of whether it came from life or not. “This is a revolutionary moment, fundamentally to me as an astronomer, but also to our species – that we have been able to come from single cellular life, billions of years ago, to an advanced technological civilisation which is able to peer through the atmosphere of another planet and actually find evidence for possible biological activity,” he said.
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.
