Introduction
K2-18 b, a sub-Neptune exoplanet located in the habitable zone of an M-dwarf star, is a candidate hycean world—a class of planets hypothesized to have global oceans and H₂-rich atmospheres. These characteristics make it a prime target for biosignature searches. Previous observations with the Hubble Space Telescope and JWST (NIRISS/NIRSpec) detected methane (CH₄) and carbon dioxide (CO₂), along with a tentative hint of dimethyl sulfide (DMS), a molecule linked to life on Earth. However, the DMS detection remained uncertain (≲2σ). This new study uses JWST’s MIRI instrument to observe K2-18 b in the mid-infrared (6–12 μm), aiming to confirm or refute the presence of DMS and explore other molecules like dimethyl disulfide (DMDS).
Methods: Observations and Data Analysis
JWST MIRI Data
Observations were conducted in April 2024 using the MIRI LRS spectrograph, covering a wavelength range of 5–12 μm. Two independent pipelines (JExoRES and JexoPipe) were employed for data reduction and light-curve analysis, accounting for instrumental effects (saturation, correlated noise, etc.). Transmission spectra were derived by comparing starlight during and outside the planet’s transit.
Atmospheric Modeling
Atmospheric retrievals were performed using the AURA code, incorporating up to 20 potential molecules (including CH₄, CO₂, DMS, DMDS, NH₃, HCN) and cloud/haze parameters. Bayesian evidence comparisons identified the most plausible scenarios.
Key Findings
- Significant Spectral Features
The MIRI spectrum reveals distinct features between 6.8–8 μm and 9–11 μm, inconsistent with a flat spectrum at 3.4σ. These features match the absorption signatures of DMS and DMDS, sulfur-bearing molecules biologically produced on Earth (Figure 2). - DMS-DMDS Degeneracy
Overlapping spectral bands of DMS (6.8–8 μm; 9.8 μm) and DMDS (6.8–8 μm; 10.5 μm) complicate their differentiation. Retrievals indicate a combined abundance of ≳10 ppmv for either molecule, with a significance of 3σ (Table 2). - Non-detection of Other Molecules
No other molecules (CH₄, CO₂, NH₃, etc.) were significantly detected in the MIRI range, though their abundances remain consistent with prior constraints. Observed features dominate over other species. - Robustness of Results
Analyses using both pipelines (JExoRES and JexoPipe) and varying assumptions (spectral binning, noise treatment) confirm the DMS/DMDS detection at ∼3σ. A “leave-one-out” analysis shows contributions from multiple data points, ruling out localized artifacts (Figure 6).
Implications and Discussion
Potential Biosignatures
- DMS and DMDS are robust biological markers on Earth, primarily emitted by phytoplankton. Their presence in K2-18 b’s atmosphere could indicate oceanic biogenic activity, akin to Earth’s processes. Photochemical models suggest biogenic fluxes ∼20 times Earth’s levels might explain the observed abundances (∼10⁻⁵–10⁻³ by volume).
- However, abiotic sources cannot be ruled out. Experiments show DMS/DMDS can form via UV irradiation of CH₄/H₂S mixtures, but such processes yield negligible quantities without continuous replenishment.
Challenges and Limitations
- Spectral Degeneracy: Overlapping DMS/DMDS features require follow-up observations to disentangle.
- Experimental Data Gaps: Absorption cross-sections for these molecules in H₂-rich atmospheres are poorly constrained, affecting abundance estimates.
- K2-18 b’s Environment: DMDS condensation in the cold stratosphere could limit gaseous accumulation, challenging high-abundance scenarios.
Future Directions
- Additional JWST Observations: 1–3 more transits with MIRI could boost significance to 4–5σ.
- Laboratory Work: Measure DMS/DMDS cross-sections under exoplanetary conditions (H₂-rich, low pressure).
- Photochemical Studies: Explore plausible abiotic pathways for these molecules in reducing atmospheres.
Conclusion
This study strengthens the hypothesis of biological activity on K2-18 b by independently detecting DMS and/or DMDS at ∼3σ in MIRI data. While inconclusive, this discovery highlights the potential of hycean worlds in the search for extraterrestrial life. Combining future JWST observations, refined models, and experimental data will be critical to confirm these results and assess their biological or abiotic origin. K2-18 b remains a priority target in the quest for biosignatures beyond our solar system.
Keywords: Exoplanets, Hycean, Biosignatures, JWST, DMS, DMDS, K2-18 b.
This summary is based on the peer-reviewed study:
Madhusudhan, N., Constantinou, S., Holmberg, M., Sarkar, S., Piette, A. A. A., & Moses, J. I. 2025, The Astrophysical Journal Letters, 983, L40. DOI: 10.3847/2041-8213/adc1c8.
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