Hycean worlds are a hypothetical class of habitable exoplanets with a thick hydrogen atmosphere overlying a liquid water ocean. The term was introduced by Cambridge astronomer nikku-madhusudhan in 2021 to describe a family of sub-Neptune planets that — unlike rocky Earth-analogs — could retain conditions suitable for life beneath a high-pressure hydrogen envelope.
Hycean worlds would be relatively easy to characterize atmospherically via jwst transmission spectroscopy, because their hydrogen atmospheres puff up to large scale heights.
Archetype: K2-18b
k2-18b is the leading hycean candidate. JWST transmission spectroscopy reported methane, CO₂, and controversial DMS/DMDS (src-jwst-k2-18b-biosignature-2025). A 2023 VLA + MeerKAT search for radio technosignatures on the same target returned a null result (src-k2-18b-technosignature-null-2026).
The methodology developed for these hycean searches is portable to any future candidate as the SKA and ngVLA come online.
Competing interpretations
sara-seager et al. (2025) treat the hycean reading of K2-18b’s spectrum as one of several valid interpretations — alongside mini-Neptune and hot-magma-ocean retrievals — under their “parallel interpretations” framework (src-jwst-biosignature-prospects-2025). JWST alone cannot arbitrate between these; next-gen telescopes are needed.
Methodological Comparison: TOI-1130b
The 2026 jwst characterization of warm mini-neptune toi-1130b (Barat et al.) is methodologically the “non-hycean” comparison case: a high-mean-molecular-weight-atmosphere (μ = 5.5 amu) and high metallicity argue for a volatile-rich, ex-situ formation beyond the water-ice-line — distinct from the hydrogen-dominated hycean retrieval applied to K2-18b. JWST sub-Neptune atmospheres span both regimes (src-jwst-toi-1130b-atmosphere-2026-05).
Photochemical support (Fujisawa et al., 2026)
Fujisawa et al. (Astrobiology, May 2026) modelled 1D photochemical + radiative-convective equilibrium atmospheres for K2-18b. Abiotic H₂-CH₄-H₂O chemistry reproduces the JWST spectrum without DMS; CO/CO₂ constraints remain ambiguous. The result does not rule out the hycean interpretation but demonstrates it is not required by the spectral data alone (src-k2-18b-hycean-photochemistry-2026-05).
Lunine & Bahcall survey (PNAS, 2025)
A PNAS overview by Lunine (JPL/Caltech) and Bahcall (Princeton) (src-pnas-jwst-exoplanet-characterization-era-2025) accepts k2-18b as a viable hycean world candidate while rejecting the DMS biosignature claim using their four-criterion evidentiary framework. The survey identifies sub-Neptunes as a class with no solar-system analogue, making interpretation of their interiors and habitability inherently uncertain. This reinforces the position that the hycean hypothesis remains scientifically live even as specific biosignature detections on K2-18b fail evidentiary standards.