Astronomers using the NASA/ESA/CSA James Webb Space Telescope have detected the chemical fingerprints of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) in the atmosphere of the super-Earth exoplanet K2-18b. On Earth, DMS and DMDS are only produced by life, primarily microbial life such as marine phytoplankton. While an unknown chemical process may be the source of these molecules in K2-18b’s atmosphere, the results are the strongest evidence yet that life may exist on an extrasolar planet.
An artist’s impression of the super-Earth exoplanet K2-18b. Image credit: A. Smith / N. Mandhusudhan.
K2-18 is a red dwarf located approximately 111 light-years away in the constellation of Leo.
Also known as EPIC 201912552, the star hosts two massive exoplanets: K2-18b and K2-18c.
First discovered in 2015, K2-18b has a radius of 2.6 times that of Earth and is about 8.6 times as massive.
The planet orbits its star every 33 days at a distance of approximately 0.15 AU and has an Earth Similarity Index of 0.73.
It receives 1.28 times the light intensity of Earth, and its equilibrium temperature is minus 2 degrees Celsius (28 degrees Fahrenheit).
Earlier observations of K2-18b identified methane and carbon dioxide in its atmosphere. This was the first time that carbon-based molecules were discovered in the atmosphere of an exoplanet in the habitable zone.
Those results were consistent with predictions for a Hycean world: a habitable ocean-covered exoplanet underneath a hydrogen-rich atmosphere.
However, another, weaker signal hinted at the possibility of something else happening on K2-18b.
Transmission spectrum of K2-18b using Webb’s MIRI spectrograph. Image credit: A. Smith / N. Mandhusudhan.
“We didn’t know for sure whether the signal we saw last time was due to DMS, but just the hint of it was exciting enough for us to have another look with Webb using a different instrument,” said Professor Nikku Madhusudhan, an astronomer with the Institute of Astronomy at the University of Cambridge.
The earlier, tentative, inference of DMS was made using Webb’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph) instruments, which together cover the near-infrared (0.8-5 micron) range of wavelengths.
The new, independent observation used Webb’s MIRI (Mid-Infrared Instrument) in the mid-infrared (6-12 micron) range.
“This is an independent line of evidence, using a different instrument than we did before and a different wavelength range of light, where there is no overlap with the previous observations. The signal came through strong and clear,” Professor Madhusudhan said.
“It was an incredible realization seeing the results emerge and remain consistent throughout the extensive independent analyses and robustness tests,” added Dr. Måns Holmberg, an astronomer at the Space Telescope Science Institute.
DMS and DMDS are molecules from the same chemical family, and both are predicted to be biosignatures.
Both molecules have overlapping spectral features in the observed wavelength range, although further observations will help differentiate between the two molecules.
However, the concentrations of DMS and DMDS in K2-18b’s atmosphere are very different than on Earth, where they are generally below one part per billion by volume.
On K2-18b, they are estimated to be thousands of times stronger — over ten parts per million.
“While the results are exciting, it’s vital to obtain more data before claiming that life has been found on another world,” Professor Madhusudhan said.
“The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them” said Dr. Subhajit Sarkar, an astronomer at Cardiff University.
“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 Dr. Savvas Constantinou, an astronomer with the Institute of Astronomy at the University of Cambridge.
“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. That’s how science has to work,” Professor Madhusudhan said.
The study was published today in the Astrophysical Journal Letters.
_____
Nikku Madhusudhan et al. 2025. New Constraints on DMS and DMDS in the Atmosphere of K2-18b from JWST MIRI. ApJL 983, L40; doi: 10.3847/2041-8213/adc1c8
