Astronomers have detected the most promising signs yet of a possible biosignature outside the solar system, although they remain cautious. Using data from the James Webb Space Telescope (JWST), the astronomers, led by the University of Cambridge, have detected the chemical fingerprints of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS), in the atmosphere of the exoplanet K2-18b, which orbits its star in the habitable zone.
In a nutshell, scientists detected dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in the atmosphere of exoplanet K2-18 b – chemicals that on Earth are primarily produced by living organisms. The detection has a 99.7% confidence level (three-sigma), with researchers estimating 16-24 more hours of observation needed to reach the definitive five-sigma threshold. K2-18 b is a “hycean world” located 124 light-years away that likely has a global ocean beneath a hydrogen-rich atmosphere – potentially ideal conditions for life.
CAMBRIDGE, England — Astronomers may have detected the strongest evidence thus far of alien life beyond our solar system. Using NASA’s James Webb Space Telescope (JWST), researchers have found chemicals in the atmosphere of exoplanet K2-18 b that on Earth are typically produced by living organisms. The team, led by researchers from the University of Cambridge, discovered compounds called dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in the planet’s atmosphere. These sulfur compounds are particularly exciting because on our planet, they’re mainly created by marine microbes.
“We report new independent evidence for DMS and/or DMDS in the atmosphere … with high abundance of at least one of the two molecules,” the researchers write in their paper, published in The Astrophysical Journal Letters. This builds on earlier hints of DMS found in the planet’s atmosphere, making the case stronger that this world might harbor life.
A Watery World That Could Support Life
K2-18 b sits in the sweet spot around its star where temperatures could allow liquid water oceans to exist. Located about 124 light-years away, this world is what scientists call a “hycean world”—a planet that might have global oceans covered by hydrogen-rich atmospheres. These planets have become prime targets in the search for life beyond Earth.
The discovery came from analyzing data captured by the Webb Telescope’s MIRI instrument, which observed the planet in mid-infrared wavelengths (6-12 micrometers). This complemented earlier observations that had already found carbon-bearing molecules like methane and carbon dioxide.
“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 JWST using a different instrument,” says lead researcher Nikku Madhusudhan, a professor at Cambridge’s Institute of Astronomy, in a statement.
When K2-18 b passes in front of its star from our viewpoint, starlight filters through the planet’s atmosphere. By studying which wavelengths go missing, scientists can determine what compounds exist in the atmosphere. The researchers found distinctive absorption patterns that closely matched the signatures of DMS and DMDS. The statistical confidence reached about 99.7% that the signal is genuine rather than a fluke or error.
“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,” says Madhusudhan. “The signal came through strong and clear.”
Why These Chemicals Matter
What makes this finding remarkable is that DMS and DMDS are considered “biosignature gases,” which are chemicals that, at least on Earth, typically come from living things. While non-biological processes could potentially create these compounds, maintaining them at the detected levels would be difficult without biological production. The concentrations of DMS and DMDS in K2-18 b’s atmosphere are very different than on Earth, where they are generally below one part per billion by volume. On K2-18 b, they are estimated to be thousands of times stronger – over ten parts per million.
“Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds,” says Madhusudhan. “And now we’ve observed it, in line with what was predicted. Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have.”
The research team carefully examined alternative explanations, but the biological explanation remains most compelling.
The Next Steps
Although these findings represent the best evidence yet for possible biological activity on an exoplanet, more observations are needed for certainty. Additional Webb Telescope observations could increase detection confidence from the current 99.7% level to an even more definitive 99.99% level. “Decades from now, we may look back at this point in time and recognize it was when the living universe came within reach,” says Madhusudhan. “This could be the tipping point, where suddenly the fundamental question of whether we’re alone in the universe is one we’re capable of answering.”
This discovery marks a major step in humanity’s search for life beyond Earth. If confirmed through further study, it could represent the first detection of a biosignature on a planet outside our solar system—bringing us one step closer to answering whether we share our universe with other forms of life.
Paper Summary Methodology
The research team observed exoplanet K2-18 b using the JWST MIRI LRS (Low Resolution Spectrograph) instrument in the slitless prism configuration, covering wavelengths from approximately 6-12 micrometers. The observations were conducted over 5.85 hours, with 2.68 hours of in-transit data. The team employed two independent data reduction pipelines (JExoRES and JexoPipe) to process the raw data, both yielding consistent results. They performed atmospheric retrievals using the AURA framework, testing a maximal model with 20 possible molecular species as well as more focused canonical models. The team utilized Bayesian statistics and nested sampling algorithms to determine which molecules best explained the observed spectral features, with particular attention to differentiating between closely related compounds DMS and DMDS whose spectral features overlap in this wavelength range.
Results
The research detected spectral features that strongly indicated the presence of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) in K2-18 b’s atmosphere, with a detection significance of approximately 3-sigma (99.7% confidence). The abundance of these compounds was estimated to be approximately 10 parts per million, which the researchers argue would be difficult to maintain without biological production. The team found that the spectral features could not be adequately explained by other expected atmospheric compounds. The study also found these results were robust across different data reduction techniques, trend removal approaches, and spectral binning methods.
Limitations
The study acknowledges several limitations. The 3-sigma detection confidence is at the lower end of what’s typically considered robust scientific evidence. Due to spectral degeneracy, the team couldn’t definitively determine whether DMS, DMDS, or both were present. The absorption cross-sections used for DMS and DMDS were obtained at Earth-like conditions (1 bar, 298K) rather than the specific conditions of K2-18 b’s atmosphere, which may affect the derived abundance estimates. The researchers also note that while abiotic production of these compounds is unlikely to explain the observed concentrations, it cannot be completely ruled out without further research into potential alternative chemical pathways.
Funding/Disclosures
The research was supported by the UK Research and Innovation (UKRI) Frontier Grant (EP/X025179/1) with Nikku Madhusudhan as Principal Investigator. Co-author J.M. received support from JWST-GO-02722 provided by NASA through a grant from the Space Telescope Science Institute. The researchers acknowledge computational resources provided by the Cambridge Service for Data Driven Discovery at the University of Cambridge.
Publication Information
The paper “New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI” was authored by Nikku Madhusudhan, Savvas Constantinou, Måns Holmberg, Subhajit Sarkar, Anjali A. A. Piette, and Julianne I. Moses. It is published in The Astrophysical Journal Letters.
