Recently, astronomers claimed to have discovered the strongest evidence yet of life on another planet. But some astronomers urged caution, calling for other research teams to confirm the findings and rule out other non-biological explanations. In 2015, astronomers first discovered the exoplanet K2-18b and quickly determined it was a promising place to find life. The planet is about eight times more massive than Earth and orbits a star 124 light-years away. It is located in the habitable zone of its host star, a region where liquid water can exist.
Researchers found evidence of water vapor during further observations in 2019, suggesting the planet may be covered in an ocean with a hydrogen-rich atmosphere above it. However, not all astronomers agree.
Artistic rendering of the planet K2-18b and its host star. Image credit: ESA/Hubble, M. Kornmesser
In 2023, Nikku Madhusudhan of the University of Cambridge in the United Kingdom and colleagues used instruments on the National Aeronautics and Space Administration (NASA) James Webb Space Telescope (JWST) to observe the atmosphere of K2-18b in near-infrared light and once again found evidence of the presence of water vapor, carbon dioxide, and methane.
At the same time, they discovered an exciting clue-dimethyl sulfide (DMS). On Earth, this molecule is produced only by biological organisms, primarily marine phytoplankton. However, the signs of DMS are very weak. Many astronomers believe that stronger evidence is needed to confirm the existence of this molecule.
Now, Madhusudhan and colleagues observed K2-18b using the mid-infrared camera, another instrument at JWST. They found a stronger DMS signal, as well as a potentially related molecule called dimethyl disulfide (DMDS), which is also produced only by living things on Earth.
"What we found is an independent piece of evidence, using different instruments and at different wavelengths, that there may be biological activity on this planet," Madhusudhan said.
The research team claims that the detection of DMS and DMDS reached a statistical significance level of 3 sigma, which means that there is a 3‰ probability that data patterns like this are just chance. In physics, the standard threshold for accepting a discovery as a true discovery is 5 sigma.
Nicholas Wogan of NASA Ames Research Center in the United States said that this evidence is more convincing than the 2023 results, but it still needs to be verified by other teams. Once the data are published, other researchers can begin to confirm the findings. But because of the difficulty of interpreting JWST data, this could take weeks or months. "It's not as simple as you download the data and see if there's a DMS. It's a super complex process."
Other scientists are skeptical of the findings. "The new JWST observations do not provide convincing evidence that DMS or DMDS are present in K2-18b's atmosphere," said Ryan MacDonald of the University of Michigan.
"With K2-18b, we are faced with a 'crying wolf' situation, where multiple previous detection results reaching the 3 sigma level are completely untenable after closer scrutiny. Any claim that there is life beyond Earth needs to be rigorously tested by other scientists." MacDonald added.
Madhusudhan's team estimates that 16 to 24 hours of further observations with JWST could help reach 5 sigma, but the difficulty of observing planetary atmospheres means they cannot guarantee this.
"The relative thickness of the planet's atmosphere is about the thickness of the apple peel on the surface of an apple." Thomas Beatty of the University of Wisconsin-Madison said that if further observations can prove that this is a real discovery, it will be "a huge improvement."
Wogan added that achieving 5 sigma may be fundamentally impossible due to the amount of noise in the data.
Madhusudhan and colleagues calculated that the likely concentration of DMS and DMDS on K2-18b appears to be more than 10 parts per million, thousands of times higher than concentrations in Earth's atmosphere. If this proves to be true, it may indicate that biological activity is much greater there than on Earth, but more work will be needed to confirm that these chemicals are biologically derived.
"We must be extremely cautious." Madhusudhan said, "At this stage, even if we detect DMS and DMDS, we cannot conclude that it is produced by biological activities. But referring to the existing literature, there is currently no non-living mechanism that can explain the phenomena we observed."
Sara Seager of the Massachusetts Institute of Technology in the United States said that because it is difficult to prove that the observed phenomena cannot be explained by non-biological explanations, K2-18b may be listed as a "candidate" for a long time. "This problem may never be fully resolved."