In a first-of-its-kind research, James Webb Space Telescope (JWST) scientists have identified the strongest evidence yet of potential biological life on an exoplanet, detecting sulfur-based gases in K2‑18 b's atmosphere that on Earth are only known to originate from living organisms. Using near- and mid-infrared transmission spectroscopy, JWST captured indications of dimethyl sulfide (DMS), dimethyl disulfide (DMDS), methane (CH₄), and carbon dioxide (CO₂)—marking the first potential detection of sulfur-based biosignatures beyond our Solar System.
K2‑18 b: A Promising Hycean World
K2-18 b is a sub-Neptune-sized planet orbiting a red dwarf star 124 light-years away in the Leo constellation. It resides within its star’s habitable zone, where temperatures may support liquid water. Unlike Earth, K2-18 b is believed to host a hydrogen-rich atmosphere over a global ocean, qualifying it as a “Hycean” world—a newly theorized class of potentially life-supporting exoplanets.
Using the Near-Infrared Imager and Slitless Spectrograph (NIRISS) and Near-Infrared Spectrograph (NIRSpec), JWST detected CH₄ and CO₂ at a high confidence level (>10σ), indicating a water-rich, reducing atmosphere. Mid-infrared observations using the Mid-Infrared Instrument (MIRI) between 6 to 12 μm revealed absorption features consistent with sulfur-bearing compounds—features unaccounted for by known abiotic chemistry.
Biosignature Candidates: DMS and DMDS
On Earth, DMS and DMDS are emitted primarily by marine phytoplankton and anaerobic bacteria in ocean sediments. Though they exist in trace amounts—usually under 1 part per billion by volume (ppbv)—they play crucial roles in the sulfur cycle and cloud formation. In contrast, K2‑18 b shows estimated concentrations of >10 parts per million by volume (ppmv), indicating active replenishment processes potentially biological in nature.
Although scientists warn that abiotic processes like volcanic outgassing or hydrothermal chemistry might also produce these compounds in tiny quantities, simulations of Hycean conditions have so far failed to reproduce the observed levels, leaving biology as a leading—though unconfirmed—hypothesis.
Statistical Significance and Scientific Caution
Current DMS/DMDS detection stands at about a 3σ level, withover 99.7% confidence when these molecules are included in spectral models. However, the scientific community generally requires a 5σ threshold for discovery claims. Researchers urge caution, noting that instrumental errors, stellar activity, and atmospheric hazes could mimic biosignature signals.
The Cambridge-led team is advocating for follow-up studies using both JWST and upcoming ground-based observatories like the Extremely Large Telescope (ELT), Thirty Meter Telescope (TMT), and Giant Magellan Telescope (GMT). These instruments could resolve finer molecular features and isotopic ratios needed to differentiate between biogenic and abiotic sources.
What’s Next?
Future JWST observations aim to double the number of transit measurements of K2‑18 b, expanding the spectral coverage to 12–20 μm. This will allow more precise characterization of DMS and potential detection of other biosignature gases like methanethiol (CH₃SH) and methyl mercaptan—both associated with life on Earth.
Scientists will also refine atmospheric models using updated molecular cross-section data from laboratory studies. Parallel efforts to study Earth analog environments, such as deep-sea microbial mats, will inform the interpretation of remote exoplanet spectra.
Implications for Astrobiology and Society
If these sulfur-based biosignatures are confirmed, it would mark the first concrete evidence of life beyond Earth—an epochal discovery for science and society. Philosophers, ethicists, and policymakers are beginning to consider the implications for our understanding of life’s universality, interplanetary ethics, and cosmic responsibility.
Public excitement has fueled increased funding for astrobiology missions, including the Habitable Worlds Observatory and Mars Sample Return. Nonetheless, scientists remain cautious, reminding us that extraordinary claims require extraordinary evidence.
Conclusion
The detection of DMS and DMDS in K2‑18 b’s atmosphere represents a breakthrough in the search for extraterrestrial life. While not yet conclusive, these findings offer the strongest hints so far that life might exist elsewhere in the universe. With JWST leading the charge, and a new generation of telescopes on the horizon, the scientific community moves closer to answering one of humanity’s oldest questions: Are we alone?