In a potential breakthrough in the search for extraterrestrial life, astronomers using the James Webb Space Telescope (JWST) have detected strong chemical signatures in the atmosphere of a distant exoplanet that, on Earth, are produced only by living organisms. The planet, K2-18 b, is located 124 light-years away in the constellation Leo and is now being closely studied as one of the most promising candidates for life beyond the solar system.
Unprecedented Detection of Biosignature Gases
The chemicals detected – dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) – are closely linked to biological activity on Earth, primarily generated by marine phytoplankton. According to Professor Nikku Madhusudhan, an astrophysicist at the University of Cambridge and lead author of the new study, this is the strongest evidence yet of possible biological activity beyond Earth. “If we can detect these molecules on habitable planets, this is the first time we’ve been able to do that as a species,” Madhusudhan said. “It’s mind-boggling that this is possible.”
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These findings, published in The Astrophysical Journal Letters, are reported with a 99.7% confidence level, or “three-sigma” statistical significance — meaning there’s a 0.3% chance the observation is a fluke. However, this still falls short of the gold standard in physics (five-sigma), and scientists are urging caution.
A ‘Hycean World’ in the Habitable Zone
K2-18 b is a sub-Neptune-class planet — 8.6 times the mass and 2.6 times the diameter of Earth. It orbits a cool red dwarf star in the star’s habitable zone, where temperatures may allow liquid water to exist.
Earlier data from Hubble suggested water vapor might be present. That was later revised to methane, but Madhusudhan’s team argued the overall profile of K2-18 b could indicate a “hycean world” — a theoretical type of planet with a hydrogen-rich atmosphere and a vast ocean beneath, potentially warm enough to support microbial life.
“The only scenario that currently explains all the data obtained so far… is one where K2-18 b is a hycean world teeming with life,” said Madhusudhan. However, he was quick to add, “We need to remain open and pursue other possibilities.”
Not a Discovery of Life — Yet
Crucially, scientists are not claiming they have discovered alien life. Instead, they stress that the detection of DMS and DMDS could represent a biosignature, a possible indicator of life, rather than definitive proof. “There may be processes that we don’t know about that are producing these molecules,” Madhusudhan said. “But I don’t think there is any known process that can explain this without biology.”
Sceptics in the scientific community have pointed out alternative scenarios. Some propose that geological or chemical processes, such as volcanic activity or lightning storms, could produce similar gases under exotic conditions. Others argue that DMS has been found on comets, suggesting abiotic origins are possible. “Life is one of the options, but it’s one among many,” said Dr Nora Hänni, a chemist from the University of Berne. “We would have to strictly rule out all the other options before claiming life.”
The Transit Method
Because planets like K2-18 b are too distant to observe directly, scientists rely on the transit method. As the planet passes in front of its host star, a portion of the starlight is filtered through its atmosphere. The JWST can analyse this light to identify chemical fingerprints, revealing the planet’s atmospheric composition.
The latest observations found that certain wavelengths of light dropped sharply, consistent with the presence of DMS and DMDS. These gases were detected at concentrations thousands of times higher than on Earth, potentially hinting at robust biological processes.
New Era for Astrobiology?
While Madhusudhan and his team call the findings “transformational,” many scientists remain cautious. Dr Jo Barstow of the Open University said her “scepticism dial” for such claims is always “turned up to 11.” Dr Caroline Morley, an astrophysicist from the University of Texas, also noted that while biosignatures are important, technosignatures — such as intercepted communications from intelligent life — may ultimately provide clearer evidence.
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Nevertheless, the detection of potential biosignatures on a planet so far from Earth marks an exciting step in what researchers are calling the dawn of “observational astrobiology.” As new instruments and missions come online, the chances of confirming life elsewhere in the universe may no longer be the stuff of science fiction. “We’re trying to establish if the laws of biology are universal,” said Madhusudhan. “This could be the tipping point where we finally start to answer the question: are we alone?”