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James Webb detects possible gas linked to life on ocean world 124 light-years away, but scientists disagree on data interpretation.
According to the University of Cambridge, a team of astronomers led by Professor Nikku Madhusudhan detected, in April 2025, using the James Webb Space Telescope, chemical signatures of dimethyl sulfide in the atmosphere of the exoplanet K2-18 b. Dimethyl sulfide is a compound that, on Earth, is strongly associated with marine biological activity, primarily phytoplankton. The detection reached a statistical significance level of three sigma, indicating about a 0.3% probability that the signal is a result of noise.
Weeks later, a team from the University of Chicago analyzed the same data and concluded that the signal could be explained by instrumental noise, opening a direct scientific debate about the interpretation of the results.
Exoplanet K2-18 b is a sub-Neptune world located in the habitable zone 124 light-years from Earth
The planet K2-18 b was discovered in 2015 by the Kepler telescope during its extended K2 mission, which monitored variations in stellar brightness caused by planetary transits.
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Located 124 light-years from Earth in the constellation Leo, the planet exhibits unusual characteristics: it has about 2.6 times the radius of Earth and approximately 8.6 times its mass.
This combination places it in the category of so-called sub-Neptunes, a class of planets common in the galaxy but nonexistent in the Solar System, which limits direct comparisons with known worlds.
Physical characteristics indicate presence of water on a large scale and hydrogen-rich atmosphere
The density of K2-18 b suggests that it is not composed solely of rock. Models indicate a significant presence of less dense materials, with a strong possibility of large amounts of water.
The planet orbits within the habitable zone of its star, receiving an amount of energy comparable to Earth, which allows for the potential existence of liquid water.
This combination led to the formulation of the concept of a Hicean world, characterized by a global ocean covered by a hydrogen-rich atmosphere.
Observations in 2023 confirmed methane and carbon dioxide in the planet’s atmosphere
In September 2023, initial observations from the James Webb detected methane and carbon dioxide in the atmosphere of K2-18 b with a high level of confidence.
The absence of ammonia, expected in gas giants without oceans, reinforced the hypothesis that the planet has characteristics compatible with an oceanic environment.
A preliminary signal of dimethyl sulfide was also identified, but with low statistical significance at the time.
New analysis with MIRI instrument in 2025 strengthened hypothesis of presence of sulfur compounds
In April 2025, the team used the MIRI instrument, operating at a different wavelength, allowing for an independent analysis of the previous data.
The results indicated spectral characteristics compatible with dimethyl sulfide and dimethyl disulfide, with estimated concentrations exceeding 10 parts per million.
This level is significantly higher than that observed in the Earth’s atmosphere, where the compound appears in much lower concentrations.
Dimethyl sulfide is associated with marine biological activity and generates interest in the search for biosignatures
On Earth, dimethyl sulfide is linked to marine biological processes and contributes to the characteristic odor of the ocean environment.
Its possible detection on another planet raises the hypothesis of biological activity, although it does not constitute conclusive evidence of life. The interpretation depends on the exclusion of non-biological mechanisms capable of producing the same compound.
The scientific community’s reaction was immediate, with experts highlighting the need for caution. NASA indicated that a single possible biosignal is not sufficient to confirm the existence of life, as multiple independent pieces of evidence are necessary.
Researchers also pointed out inconsistencies in atmospheric models, such as the absence of ethane, which would be expected under certain chemical conditions.
Reanalysis of data in August 2025 points to possibility of instrumental noise
The team led by Rafael Luque from the University of Chicago conducted a complete reevaluation of the data. The results indicated that much of the analysis did not find statistically significant evidence of the presence of dimethyl sulfide.
According to this interpretation, the signal may be the result of instrumental variations or calibration artifacts.
The current detection is at the three sigma level, considered insufficient for definitive conclusions in science. The more robust five sigma standard drastically reduces the probability of statistical error and is used in high-impact discoveries.
Previous cases, such as the alleged detection of phosphine on Venus, have shown that preliminary results may not hold up after reanalyses.
Additional studies indicate the presence of water, but do not confirm signs of biological activity
Collaborative research involving NASA and other institutions has confirmed that K2-18 b has a high concentration of water.
However, the evidence related to dimethyl sulfide remains inconclusive. New observations with the James Webb are underway and may raise the confidence level of the detection.
K2-18 b marks an advancement in the capacity to study exoplanet atmospheres in the habitable zone
Regardless of the confirmation of biosignatures, K2-18 b represents a significant advancement in astrobiology.
It is one of the first exoplanets in the habitable zone with an atmosphere analyzed in sufficient detail to detect complex compounds. The use of the James Webb in this context demonstrates the technological advancement in the search for signs of life beyond Earth.
Now we want to know: do you believe we are close to confirming the first evidence of life beyond Earth?
The case of K2-18 b shows that science can already detect complex chemical signals on distant planets, but still faces challenges in interpreting this data.
In your view, are we facing a possible historic discovery or still at an early stage of scientific interpretation?
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