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Possibly—but no ocean has been observed. K2-18 b’s atmosphere has been studied as it passes in front of its star, and scientists have used those spectra to test models of the planet’s interior. Some interpretations allow a liquid-water ocean beneath a thin atmosphere; others explain the available evidence with a gas-rich mini-Neptune that has no liquid surface. The disagreement turns partly on which atmospheric gases are reliably detected and what those gases imply about the planet’s interior.
What has actually been observed?
During a transit, a planet passes in front of its star. Some starlight filters through the planet’s atmosphere before reaching a telescope, and molecules in that atmosphere can leave signatures in the light. This is an indirect way to study an atmosphere; it does not show the planet’s surface or reveal its interior directly.
In 2019, Tsiaras and colleagues reported a water-vapor signature in K2-18 b’s atmosphere from Hubble Space Telescope observations. They gave the result an Atmospheric Detectability Index of 5.0, approximately 3.6 sigma, and inferred that the atmosphere contained some hydrogen. That finding concerned water vapor in the atmosphere—not liquid water on the surface.
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Later JWST spectra were interpreted as evidence of methane and carbon dioxide. But analyses of the JWST data do not agree on whether carbon dioxide is reliably detected, and atmospheric composition alone does not settle whether the planet has an ocean.
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Why does an ocean remain one possible interpretation?
K2-18 b is described as a temperate sub-Neptune. Its atmosphere and interior cannot be inspected directly, so scientists compare observations with models: proposed combinations of atmospheric gases, temperature, chemistry and interior structure that might produce the measured spectrum.
One possibility is a water-rich planet whose hydrogen-rich atmosphere sits above a liquid-water ocean. A 2025 preprint by Hu and colleagues, analyzing four new JWST/NIRSpec transit observations, reports methane and carbon dioxide and argues that its results support a water-rich interior. In that interpretation, the planet could have either a thick envelope containing more than 10% water by volume or a thin atmosphere above an ocean. These are modeled possibilities, not measurements of an ocean or of the planet’s bulk water content.
The preprint also says alternative models can reproduce the spectrum within its uncertainties and calls for deeper observations. Its ocean interpretation should therefore be treated as a proposed explanation, not a settled finding.
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A mini-Neptune can fit the gases without a biosphere
A 2024 study by Wogan and colleagues compared a lifeless Hycean scenario—an ocean-world interpretation with a hydrogen-rich atmosphere—with a gas-rich mini-Neptune. Under that study’s assumptions, the modeled lifeless Hycean atmosphere produced less than 1 part per million methane, while the data were taken to suggest about 1% methane. A modeled mini-Neptune with 100 times solar metallicity produced 4% methane and nearly 0.1% carbon dioxide. These are values from that study’s model comparison, not definitive measurements of K2-18 b or universal limits on what an ocean world can contain.
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The comparison illustrates a key limitation: even if an atmospheric molecule is detected, different kinds of planet can potentially explain it. Matching a spectrum with a mini-Neptune model does not prove that there is no ocean, just as a water-rich model that permits an ocean does not prove that one exists.
A December 2025 reanalysis questions the carbon-dioxide evidence
Schmidt and colleagues’ December 2025 reanalysis tested earlier JWST NIRISS and NIRSpec spectra using 60 data treatments and more than 250 atmospheric retrievals. It reported methane at approximately 4 sigma, but found no statistically significant or reliable evidence for carbon dioxide or dimethyl sulfide (DMS) in its analysis. The authors concluded that their revised composition could be explained by an oxygen-poor mini-Neptune without requiring a liquid-water surface or life.
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This is a conclusion about that reanalysis, not a direct determination of the interior or a universal consensus. It differs from Hu and colleagues’ interpretation of four new NIRSpec transits, which reports methane and carbon dioxide and allows an ocean-bearing model. The analyses use different observations and approaches, so their results should not be collapsed into a single claim that JWST has either confirmed or ruled out an ocean.
How to read the competing results
| Question | Hu and colleagues, 2025 preprint | Schmidt and colleagues, December 2025 reanalysis |
|---|---|---|
| Data analyzed | Four new JWST/NIRSpec transit observations | Earlier JWST NIRISS and NIRSpec spectra, assessed across 60 data treatments and more than 250 atmospheric retrievals |
| Molecular interpretation | Reports robust methane and carbon dioxide detections | Reports methane at approximately 4 sigma; finds no statistically significant or reliable evidence for carbon dioxide or DMS in its analysis |
| Interior implication | Allows a water-rich interior, including an ocean below a thin atmosphere | Finds that an oxygen-poor mini-Neptune without a liquid surface can explain the revised composition |
| Evidence status | Preprint interpretation; the authors note alternative models and call for deeper observations | Published reanalysis and model interpretation; not a direct observation of the interior |
The disagreement is not simply a contest between “ocean detected” and “ocean disproved.” It involves which spectra and data treatments are used, how securely particular molecules are identified, and whether the resulting atmospheric composition requires a water-covered surface. Neither atmospheric analysis directly observes the planet’s interior.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Do the candidate gases suggest life?
No. A gas discussed as a possible biosignature—a substance that might be associated with life in some settings—is not proof of life. In Hu and colleagues’ preprint, the proposed signals of DMS, methyl mercaptan and nitrous oxide are marginal: none exceeds 3 sigma in model preference, and they fall below about 2 sigma without a strong super-Rayleigh haze assumption. The same preprint notes that DMS and methyl mercaptan may form through non-biological chemistry in some massive, hydrogen-rich atmospheres. Schmidt and colleagues’ reanalysis found no reliable DMS evidence.
These claims are also separate from the ocean question. Even a robust atmospheric gas detection would not, by itself, establish that the planet has a liquid surface or that the gas was produced by life.
What remains unknown?
The evidence described here does not resolve whether K2-18 b has a liquid-water surface. The new-transit preprint presents an ocean as one interpretation but also acknowledges alternative models; the December 2025 reanalysis shows how a no-ocean mini-Neptune can account for its revised composition. Further observations and independent analyses may change how the competing explanations are assessed.
For now, the careful description is that K2-18 b is a temperate sub-Neptune whose atmospheric observations have prompted models that include a possible ocean. “Water-rich,” “potentially habitable” and “has a liquid-water ocean” are not interchangeable: the last is an interior claim that has not been directly established.
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