Have We Found Signs of Life on K2-18b? What JWST Detected and What Is Disputed
Short answer: not with any confidence. The James Webb Space Telescope (JWST) has clearly detected methane in the atmosphere of K2-18b, a planet about 2.4 times the width of Earth orbiting a small red star. One research team also reported signs of dimethyl sulfide (DMS), a gas proposed as a possible sign of life, first as a weak hint in 2023 and then at 3-sigma confidence in 2025. Independent teams that reanalysed the same data with many different methods found no statistically reliable evidence for DMS, and showed that other, non-biological molecules fit the data equally well. The planet is interesting; the life claim is unproven.
This page explains what K2-18b is, what each JWST study reported, why other scientists disagree, and what it would take to settle the question. All figures come from NASA and the papers listed at the end.
What K2-18b is
NASA's exoplanet catalogue lists K2-18b as a planet orbiting an M-type star, a small, cool red dwarf. Its discovery was announced in 2015. The catalogue gives these values:
| Property | Value (NASA exoplanet catalogue) |
|---|---|
| Mass | 8.92 times Earth |
| Radius | 2.37 times Earth |
| Orbital period | 32.9 days |
| Distance from its star | 0.1429 AU (Earth–Sun distance = 1 AU) |
| Eccentricity | 0.2 |
| Discovery | Announced 2015, by the transit method |
Source: NASA Science, Exoplanet Catalog, "K2-18 b", checked 24 October 2026.
NASA's catalogue classifies it as a "super Earth" by size, while the research papers below describe it as a temperate sub-Neptune: a planet between Earth and Neptune in size, with a thick, hydrogen-rich atmosphere. One reanalysis gives its temperature as roughly 250 K (about −23 °C). What lies beneath that atmosphere is the central question.
The "hycean world" idea
Researchers have proposed a class of planets called hycean worlds: temperate planets covered in oceans, under atmospheres rich in hydrogen. A 2023 paper by Nikku Madhusudhan and colleagues explains why these planets are attractive targets. Compared with rocky planets of the same mass, they are larger and have more extended atmospheres, so starlight passing through the atmosphere carries a stronger chemical fingerprint that JWST can read. K2-18b is the leading candidate.
The method is transmission spectroscopy. When the planet passes in front of its star, some starlight passes through its atmosphere, and molecules absorb particular wavelengths. Comparing light with and without the planet in front reveals which molecules are present.
2023: methane, carbon dioxide and a hint of DMS
Madhusudhan's team observed K2-18b with two JWST instruments, NIRISS and NIRSpec, covering wavelengths from 0.9 to 5.2 micrometres. They reported:
- methane (CH4) detected at 5-sigma confidence;
- carbon dioxide (CO2) at 3-sigma;
- each at roughly 1% of the atmosphere, in a hydrogen-rich mix;
- no ammonia detected, which they described as consistent with an ocean under a temperate hydrogen-rich atmosphere;
- "potential signs" of dimethyl sulfide, a molecule the authors had predicted to be an observable biomarker on hycean worlds.
The methane finding resolved what the authors called a long-standing "missing methane problem" for temperate exoplanets. The DMS signal was weak, and the team later described it as "a tentative hint" of "low statistical significance".
2025: a new instrument, a stronger claim
In a 2025 paper in Astrophysical Journal Letters, the same group reported a mid-infrared spectrum of K2-18b taken with JWST's MIRI instrument, covering about 6 to 12 micrometres. They found:
- the spectrum shows distinct features and is inconsistent with a featureless spectrum at 3.4-sigma, compared with their canonical model;
- most molecules predicted for K2-18b cannot explain the features, except DMS and dimethyl disulfide (DMDS), also proposed as a possible biosignature gas;
- "new independent evidence" for DMS and/or DMDS at 3-sigma, with a high abundance, of at least about 10 parts per million by volume, of one or both.
The authors themselves stated that more observations are needed to make the findings robust and to tell DMS and DMDS apart.
Why other scientists are not convinced
Two independent reanalyses took the same JWST data and tested how much the results depend on choices made in processing and modelling.
Schmidt and colleagues (2025) reanalysed the NIRISS and NIRSpec spectra, including the first analysis of NIRISS's second-order data. They noted that the original claims came from a single data-reduction and modelling framework, which did not allow standard robustness tests. Their reanalysis spanned 60 different data treatments and more than 250 atmospheric retrievals. They confirmed methane at about 4-sigma but found "no statistically significant or reliable evidence" for carbon dioxide or DMS. They also showed that the composition they found could be explained by an oxygen-poor mini-Neptune, a planet with no surface ocean.
Luque and colleagues (2025), in a paper published in Astronomy & Astrophysics, analysed the near-infrared and MIRI data together, from 0.6 to 12 micrometres, using three data pipelines and two retrieval codes. They found insufficient evidence for DMS or DMDS. Other molecules containing methyl groups, such as ethane, which absorb at similar wavelengths, fitted the data equally well. They concluded that marginal preferences for DMS came from limiting the number of molecules considered in the model and from oversensitivity to small differences between data reductions.
A further 2025 paper in Astronomy & Astrophysics examined whether aerosols, tiny particles or haze in the atmosphere, could reconcile the MIRI observations with the near-infrared data, another example of how many factors can shape the same spectrum.
Where things stand
| Claim | Original report | Independent reanalysis |
|---|---|---|
| Methane | 5σ (2023) | Confirmed, about 4σ (Schmidt et al.) |
| Carbon dioxide | 3σ (2023) | No reliable evidence (Schmidt et al.) |
| DMS | Tentative hint (2023) | No reliable evidence (Schmidt et al.) |
| DMS and/or DMDS | 3σ from MIRI (2025) | Insufficient evidence; ethane-like molecules fit equally well (Luque et al.) |
| Ocean world | Consistent with hycean conditions | Also explainable as an oxygen-poor mini-Neptune (Schmidt et al.) |
Sources: Madhusudhan et al. 2023 and 2025; Schmidt et al. 2025; Luque et al. 2025, as stated in their abstracts.
The firm result is methane. The life-related claims depend on signals near the limit of what the data can show, and on modelling choices that other teams find change the answer. Even the team making the claim has called for more observations.
Why 3-sigma is not enough here
A 3-sigma result means the signal would arise by chance only rarely if the model and its uncertainties are right. A common threshold in physics for calling something a discovery is 5-sigma. For a claim as extraordinary as a sign of life, two further tests matter: whether the result survives different ways of processing the data, and whether other, ordinary molecules could produce the same pattern. The reanalyses above are exactly those tests, and the DMS signal did not pass them.
Common questions
Is K2-18b in the habitable zone?
The papers describe it as a temperate planet, which is why it is studied as a possible ocean world. Whether it actually has liquid water at its surface is disputed: one reanalysis says its atmosphere can be explained by a mini-Neptune with no ocean.
What is DMS?
Dimethyl sulfide is a sulfur-containing gas. The 2023 paper describes it as predicted to be an observable biomarker on hycean worlds, which is why a detection would be significant.
Did JWST find life?
No. JWST detected methane. The reported DMS and DMDS signals have not held up in independent reanalyses, and the original team says more observations are needed.
Why do teams get different answers from the same data?
Turning raw telescope data into a spectrum, and then into a list of molecules, involves many choices. The reanalyses found that the DMS preference depended on which molecules were allowed in the model and on small differences between data reductions.
What would settle it?
More observations, as the 2025 claim paper itself states, analysed with multiple pipelines and models that include all plausible molecules. A signal that survives those tests at higher confidence would carry far more weight.
Sources and how to check
- NASA Science. Exoplanet Catalog: "K2-18 b." science.nasa.gov/exoplanet-catalog/k2-18-b (checked 24 October 2026).
- Madhusudhan N., Sarkar S. et al. "Carbon-bearing Molecules in a Possible Hycean Atmosphere." 2023. arXiv: 2309.05566
- Madhusudhan N., Constantinou S. et al. "New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI." Astrophysical Journal Letters 983, L40 (2025). DOI: 10.3847/2041-8213/adc1c8 · arXiv: 2504.12267
- Schmidt S. P., MacDonald R. J. et al. "A Comprehensive Reanalysis of K2-18 b's JWST NIRISS+NIRSpec Transmission Spectrum." 2025. arXiv: 2501.18477
- Luque R., Piaulet-Ghorayeb C. et al. "Insufficient evidence for DMS and DMDS in the atmosphere of K2-18 b." Astronomy & Astrophysics (2025). DOI: 10.1051/0004-6361/202555580 · arXiv: 2505.13407
- "Investigating aerosols as a way to reconcile K2-18 b JWST MIRI and NIRISS/NIRSpec observations." Astronomy & Astrophysics 702, L20 (2025). DOI: 10.1051/0004-6361/202556905
- Abstracts checked on arXiv and bibliographic details on Crossref on 24 October 2026. The temperature conversion (250 K ≈ −23 °C) is ours.