Strongest evidence of alien life on a distant planet has been found, claim astronomers

Astronomers believe they’ve found the strongest evidence yet for alien life—but they’re being very careful not to jump to conclusions.

Scientists studying a faraway planet called K2-18b have spotted chemical signs that could point to living organisms.

Still, experts are urging patience while other researchers work to confirm the discovery and rule out non-biological causes.

“These are the first hints we are seeing of an alien world that is possibly inhabited,” said Nikku Madhusudhan from the University of Cambridge during a press conference on March 15.

K2-18b was first discovered in 2015. It's about eight times heavier than Earth and sits in its star’s habitable zone—the sweet spot where temperatures might allow liquid water to exist.

In 2019, scientists detected water vapour in its atmosphere, sparking early hopes it could host oceans beneath a hydrogen-rich atmosphere. But not all astronomers agreed on how promising the planet really was.

In 2023, Madhusudhan’s team used the James Webb Space Telescope (JWST) to take a closer look at K2-18b’s atmosphere.

They found more signs of water vapour, plus carbon dioxide and methane—all ingredients that could hint at habitability.

But the most exciting clue was a faint signal of dimethyl sulphide (DMS), a molecule that on Earth is only made by living organisms, mostly ocean-dwelling plankton. However, the signal was weak, and many astronomers didn’t consider it enough proof.

Now, the team has used a different camera on JWST, one that sees in mid-infrared light, and found a stronger signal of DMS. They also picked up signs of a related compound called dimethyl disulphide (DMDS), another chemical that’s only produced by life here on Earth.

“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,” said Madhusudhan.

The team says their findings meet the three-sigma level of significance—meaning there’s only a 3-in-1000 chance the results are a random fluke. That’s promising, but not quite enough to declare it a real discovery. In physics, the gold standard is five-sigma, which equals a 1-in-3.5 million chance of a mistake.

Nicholas Wogan from NASA Ames Research Center says these results are “more convincing than the 2023 results,” but agrees it needs confirmation from other scientists. The data will be made public next week, but Wogan warns, “It’s not just like you download the data and you see if there’s DMS – it’s this super complicated process.”

Not everyone is convinced. Ryan MacDonald of the University of Michigan is skeptical: “These new JWST observations do not offer convincing evidence that DMS or DMDS are present in K2-18b’s atmosphere.”

He adds, “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.”

Even Madhusudhan agrees that the claim isn’t solid yet. “We have to be extremely careful,” he said.

“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.”

Wogan says more research is needed. “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.”

This uncertainty could keep K2-18b in the category of biosignature candidate for years to come.

Sara Seager from MIT says, “It may remain in that category for decades, since the question may never be fully resolved with the limited data exoplanets offer.”

Still, Madhusudhan says the discovery is exciting no matter what.

“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.