Hycean worlds — temperate sub-Neptune exoplanets with deep liquid-water oceans beneath H2_2-rich atmospheres — were proposed by Madhusudhan, Piette & Constantinou (2021) as a large, observationally accessible class of candidate habitable planets, with a habitable zone far wider than the terrestrial one and biomarkers such as dimethyl sulfide (DMS) potentially detectable with the James Webb Space Telescope (JWST). Open issues standing between the proposal and its confirmation were identified partly in that paper and partly by subsequent work: mass–radius compositional degeneracy, uncharacterized geochemical and climate regulation, atmospheric escape under M-dwarf irradiation, parameterized haze microphysics, untested day–night energy redistribution requiring 3D modeling, unquantified biogenic fluxes and abiotic false positives, incomplete molecular opacity data, and untested observational systematics. Follow-up work in 2023–2026 has engaged every one of these fronts — JWST detections of CH4_4 and CO2_2 on K2-18 b and TOI-270 d, contested ~3σ claims for DMS and dimethyl disulfide (DMDS) and a disputed independent reanalysis, runaway-greenhouse and tidal constraints narrowing the habitable zone, the first dedicated general circulation models, escape-based interior constraints, and first-generation biosphere models — yet the two governing questions remain unanswered: are any of the candidate Hycean worlds actually habitable ocean planets, and can a biosignature claim in a Hycean atmosphere be validated? The problem remains open.

Background

Sub-Neptunes, planets with radii between Earth's and Neptune's, are the most common known exoplanet class, yet have no solar-system analogue; their interiors could be rocky cores with thin H2_2 envelopes (gas dwarfs), water-rich worlds, or rocky or icy bodies under massive gaseous envelopes (mini-Neptunes), and observed mass and radius alone cannot distinguish these possibilities. Madhusudhan, Piette & Constantinou (2021) proposed that one subset, which they named Hycean worlds, could be habitable: planets with water-rich interiors and planet-wide liquid-water oceans tens to hundreds of kilometers deep, underlying H2_2-rich atmospheres of order bar-level surface pressure. Because H2_2 has low molecular weight, such atmospheres are vertically extended and produce large spectral signatures in transmission spectroscopy — the technique in which starlight filtered through a planet's atmosphere during transit is used to infer its composition — making Hycean candidates around nearby small stars (M dwarfs, the coolest main-sequence stars) far easier to characterize than Earth-like planets. Using one-dimensional radiative-convective atmosphere models with a parameterized haze scattering factor and assumed day–night energy redistribution, the paper derived a Hycean habitable zone (the orbital range allowing surface liquid water) much wider than the terrestrial one, including tidally locked 'Dark Hycean' worlds habitable only on their permanent nightsides and 'Cold Hycean' worlds with negligible irradiation, and showed via synthetic retrievals — statistical inferences of atmospheric properties from simulated spectra — that trace biomarkers (gases produced by life), such as the organosulfur compound dimethyl sulfide (DMS) alongside CH4_4, carbon disulfide (CS2_2) and methyl halides, could be detectable with modest James Webb Space Telescope (JWST) time on the archetype candidate K2-18 b. Confirming this picture, however, faced the following obstacles, some flagged by the paper itself and others inherent in its methodology or exposed by later work: (i) for a given measured mass and radius, multiple interior solutions (rocky mini-Neptune, 100% water world, Hycean ocean plus H2_2 envelope) remain admissible, so spectroscopy must break the degeneracy; (ii) the geochemical cycles that regulate climate and atmospheric composition on Earth over geological time, such as the carbonate–silicate weathering thermostat (the feedback in which atmospheric CO2_2 removal by rock weathering balances volcanic input, stabilizing surface temperature), are uncharacterized for oceans isolated from any exposed rock surface; (iii) high-energy irradiation, stellar winds and coronal mass ejections (eruptions of magnetized plasma from a star's outer atmosphere) from active M-dwarf hosts may photodissociate H2_2O and drive atmospheric escape, threatening long-term retention of both the envelope and the ocean; (iv) hazes were represented only by a Rayleigh enhancement factor rather than explicit microphysics; (v) the day–night redistribution profiles underlying the Dark Hycean limits were parameterized rather than computed with general circulation models (GCMs, three-dimensional atmospheric simulations); (vi) quantitative models of biogenic gas fluxes from aquatic biospheres, and of abiotic production pathways that could mimic them (false positives), were lacking; (vii) laboratory absorption cross sections for several candidate biomarkers had insufficient pressure–temperature coverage at Hycean conditions; and (viii) detectability had been demonstrated only on synthetic spectra, not against real instrument systematics and stellar contamination. Subsequent context sharpened the stakes: JWST observations of K2-18 b (Madhusudhan et al. 2023) detected CH4_4 and CO2_2 and hinted at DMS, prompting competing mini-Neptune and magma-ocean interpretations (Wogan et al. 2024; Shorttle et al. 2024), while radiative-convective modeling showed that moist convective inhibition — the suppression of convection when a radiatively heated stratosphere overlies a condensable steam layer — sharply narrows the accepted inner edge of the Hycean habitable zone (Innes et al. 2023). These developments define the current frontier: the habitability of the proposed class, and the credibility of its biosignatures, are actively contested rather than established.

Problem Statement

Determine whether Hycean worlds — temperate sub-Neptune planets with deep liquid-water oceans beneath H2_2-rich atmospheres — actually exist as a habitable class, and whether a biosignature detection in such an atmosphere can be validated. This is the completion target set by the founding paper (Madhusudhan, Piette & Constantinou 2021), which subsequent work has developed into jointly linked open questions, including: (1) breaking the mass–radius–spectroscopy degeneracy for the candidate population (Hycean ocean versus rocky or gas-rich mini-Neptune versus magma-ocean interiors) — including formation-side and atmospheric-escape constraints on whether planets with the required ~10^-3-level H/He envelope mass fractions overlying water oceans exist at all; (2) establishing the actual boundaries of the Hycean habitable zone under realistic climate physics — runaway-greenhouse limits (the instability in which a thickening steam atmosphere traps heat until the ocean evaporates) with moist convective inhibition, tidal heating, geochemical cycling without exposed rock surfaces, and the bistable/multistable climate states these admit; (3) the three-dimensional atmospheric structure and day–night energy redistribution that govern whether tidally locked or Dark Hycean configurations can sustain habitable nightside oceans; (4) the quantitative biomarker side — biogenic fluxes from plausible aquatic biospheres, abiotic production pathways and their discrimination (false positives), the photochemistry setting steady-state atmospheric abundances of dimethyl sulfide (DMS), dimethyl disulfide (DMDS) and related candidate biosignature gases, and the laboratory opacity data at Hycean pressure–temperature conditions needed to interpret their spectral features; and (5) the end-to-end observational validation — whether real JWST-class data, with instrument systematics, stellar heterogeneity and retrieval model degeneracies, can confirm or refute both the Hycean nature of specific candidates such as K2-18 b and TOI-270 d and any biosignature claim on them. An answer is complete when the habitability of the class (or of the identifiable candidate population) is established or refuted with quantified conditions, and when the criteria under which a Hycean biosignature detection would count as validated — required significance, excluded abiotic alternatives, demonstrated cross-instrument robustness — are met or shown to be unattainable. The five numbered items above are the linked fronts bearing on these two governing questions rather than five separately mandatory deliverables; the two-pronged bar stated here — habitability established or refuted with quantified conditions, and the biosignature-validation criteria met or shown unattainable — defines completion at the class level or for the identifiable candidate population.

References

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