A team from the University of Cambridge, led by Nikku Madhusudhan, reported the detection of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) in the atmosphere of exoplanet K2-18b, based on observations from the James Webb Space Telescope (JWST). On Earth, these molecules are associated with biological activity and are primarily produced by organisms such as ocean plankton.
The detection has a 3σ confidence level, meaning there’s a 99.7% chance the signal is real, where sigma (σ) denotes the standard deviation used to measure statistical certainty in scientific data. A 3σ result is promising but below the 5σ level (99.99994% certainty), astronomers prefer confirmation. The molecules were found at a level of at least 10 parts per million, far higher than Earth’s trace amounts of DMS.
The observations used JWST’s Mid-Infrared Instrument, which studied light from K2-18b’s star as the planet passed in front of it. Data was collected for six hours on April 25–-26, 2024. The instrument picked up specific patterns in the light that suggest DMS and/or DMDS are present.
K2-18b is a sub-Neptune planet, about 8.6 times Earth’s mass and 2.6 times its size, orbiting a small star in its habitable zone. It has a hydrogen-rich atmosphere and possibly a liquid water ocean, making it a “hycean” world—a type of planet that might support life. Earlier JWST data found methane and carbon dioxide there.
The team used a technique called transmission spectroscopy, which analyzes starlight passing through a planet’s atmosphere. The data showed unique patterns between 6.8 and 8 μm, with additional signs at 9–10 μm for DMS and 10–11 μm for DMDS. However, the patterns are similar, so it’s unclear if one or both molecules are present.
To study the atmosphere, researchers used a method called atmospheric retrieval, testing 20 possible molecules. Only DMS and DMDS matched the patterns seen in the data. Their model, including these molecules plus methane and carbon dioxide, showed a strong fit with a 3.2σ confidence level.
The team processed the data with two separate systems to ensure accuracy, and both gave similar results. Additional checks, like testing for noise or data adjustments, showed the detection held up, with confidence levels between 2.9σ and 3.4σ. The data clearly differed from what an atmosphere without these molecules would show.
The high levels of DMS and/or DMDS—possibly 10 to 1000 parts per million—raise questions about their source. On Earth, DMS comes from living organisms, but the team considered whether chemical reactions in the atmosphere could produce them. But found that such reactions seem unlikely to create such large amounts without life.
One challenge is that the data on how DMS and DMDS absorb light are based on Earth’s atmosphere, which is different from K2-18b’s hydrogen-rich atmosphere. This could affect estimates of the molecules’ amounts and the atmosphere’s temperature, measured at about 422 K.
The 3σ result is convincing but not final. The team suggests 1–3 more JWST observations, taking 8–24 hours, could confirm the signal at a higher confidence level, potentially reaching 5σ. Looking at other wavelengths, like near-infrared, might also help figure out if DMS, DMDS, or both are present.
Future work will include lab tests to better understand how DMS and DMDS behave in hydrogen-rich atmospheres. Researchers also plan to model whether these molecules could form without life. These steps will help determine if the signal is truly a sign of life.
This is the first time a mid-infrared spectrum has been captured for a habitable-zone sub-Neptune, showing JWST’s power to study distant planets. The findings add to earlier discoveries of methane and carbon dioxide, making K2-18b a top focus for life-hunting research. The results fit predictions for life-friendly hycean worlds.
References:
1 New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI – Nikku Madhusudhan, Savvas Constantinou et al. – The Astrophysical Journal Letters – April 17, 2025 –