Astronomers have found the clearest evidence yet that life might exist beyond the solar system, from the atmosphere of a planet 124 light years away from Earth, setting off rare excitement – tinged with caution – in the global scientific community.
Using the James Webb Space Telescope, researchers led by astronomers at the University of Cambridge in the United Kingdom have found chemical signatures of two compounds that on Earth are only produced by living creatures.
“These are the first hints we are seeing of an alien world that is possibly inhabited,” Nikku Madhusudhan, an astronomy professor at Cambridge and the lead researcher behind the discovery, told reporters at a media briefing on April 15. “This is a revolutionary moment.”
So where is the planet that might possibly host life, what evidence have scientists found, and is there reason for scepticism?
Where did the scientists find this evidence?
The researchers relied on data captured by NASA’s James Webb telescope, which was carried into outer space in 2022, and sits about 1.5 million kilometres (930,000 miles) away from Earth, as humanity’s watchtower peering into the universe.
They focused on one particular planet, K2-18b, because it had already shown promising signs as a candidate extraterrestrial body with conditions similar to those on Earth.
K2-18b lies in a constellation called Leo, and is so far away from Earth that a spaceship would need to travel for 124 years at the speed of light to get there. In reality, it would take much, much longer since the laws of physics don’t allow anything other than light to travel that fast.
The planet is 8.6 times heavier than Earth, and 2.6 times as large. Critically, it sits in what is known as the “Goldilocks Zone” of its sun: that’s the region around a star where a planet’s temperature could, in theory, support water in its liquid form on the surface.
In 2023, Cambridge astronomers found methane and carbon dioxide in the atmosphere of the planet.
That was the first time that carbon-based molecules had been discovered in the atmosphere of any planet in the habitable zone of its sun – the distance from a sun where it’s neither too hot, nor too cold, and so possible for life to survive. The scientists said that a surface covered first by an ocean, and then a hydrogen-rich atmosphere, would explain the presence of carbon-based molecules. Simply put, it was possible that the planet could have water.
What have the scientists found now?
Researchers have now found much harder evidence suggesting that the planet might not only have the conditions to host life – but could, at least in theory, be hosting life itself.
To explore planets light years away from Earth, scientists wait for them to pass in front of their suns. They study the light from the suns as it streams through the atmospheres of these planets, searching for clues.
That’s how the team found traces of either dimethyl sulfide (DMS) or dimethyl disulfide (DMDS) – or both – in the atmosphere of K2-18b.
On Earth, these compounds are only produced by living beings, particularly microbes such as marine phytoplankton. What’s more, what the scientists found suggests that the concentration of these chemicals in the K2-18b atmosphere was thousands of times higher than on Earth.
“It was an incredible realisation seeing the results emerge and remain consistent throughout the extensive independent analyses and robustness tests,” said co-author Mans Holmberg, a researcher at the Space Telescope Science Institute in Baltimore, United States, in a media statement on the Cambridge University website.
How reliable are the findings?
The scientists published their findings in the peer-reviewed Astrophysical Journal Letters publication, which means that other experts in the field who studied their paper found it convincing.
But that does not mean that the scientists have found irrefutable evidence of life. Far from it.
Madhusudhan acknowledged that it is possible that the traces of DMS and DMDS found in the atmosphere of K2-18b are the outcome of chemical phenomena that are as of now, unknown to humanity.
“It’s important that we’re deeply sceptical of our own results, because it’s only by testing and testing again that we will be able to reach the point where we’re confident in them,” Madhusudhan said. “That’s how science has to work.”
His colleagues in the research team agreed.
“Our work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings,” said co-author Savvas Constantinou, also from Cambridge’s Institute of Astronomy.
The findings of the Cambridge-led team follow a series of breakthroughs in recent years that have excited scientists about the possibilities of finding life beyond Earth.
In 2011, NASA scientists announced that they had found chemicals that are components of DNA on meteorites that had landed in Antarctica. The chemical traces they had discovered couldn’t have been the result of contamination after the meteorites landed on Earth. The only explanation – that asteroids and comets could contain the building blocks of life.
A year later, astronomers at Copenhagen University tracked down a sugar molecule in a distant star system. That molecule is an essential component of ribonucleic acid or RNA, a molecule that is critical for most biological functions.
In 2023, astronomers found traces of organic molecules in the gases around one of Saturn’s moons, Enceladus.
And in mid-2024, scientists identified five greenhouse gases that they said would be telltale signs of life on any other planet.
But the journey of science is also about setbacks. In 2005, two NASA scientists claimed they had found potential traces of extraterrestrial life on Mars after they discovered signs of methane there. Yet those findings eventually did not hold up to scientific scrutiny and NASA distanced itself from their conclusions.
What’s next?
The Cambridge-led team has found DMS and DMDS with 99.7 percent certainty. But while that might sound like a near-perfect score, it is far from what is accepted as the benchmark for a new discovery by the exacting standards of science.
For their conclusions to be considered bulletproof, they need to get to what is known as the five-sigma threshold – 99.99994 percent certainty.
The astronomers believe that more hours on the James Webb telescope could help them reach that level of confirmation.
“Decades from now, we may look back at this point in time and recognise it was when the living universe came within reach,” said Madhusudhan. “This could be the tipping point, where suddenly the fundamental question of whether we’re alone in the universe is one we’re capable of answering.”
