Hycean Worlds: Ocean Exoplanets & Habitability

• Hycean worlds are a class of exoplanets characterized by deep global water oceans and hydrogen-rich atmospheres that bridge rocky super-Earths and mini-Neptunes.
• Their layered interior, modeled using hydrostatic equilibrium and state equations, features a silicate core, extensive water layer, and a thin H₂/He envelope with nontrivial mass–radius relationships.
• Observational strategies using JWST focus on detecting atmospheric biomarkers like CH₄, CO₂, and DMS, highlighting the potential for habitability and biosignature discrimination.

Hycean worlds are a proposed class of exoplanets characterized by deep, planet-wide oceans of liquid water underlying substantial hydrogen-rich (H₂-dominated) atmospheres. These planets exhibit intermediate bulk properties between those of rocky super-Earths and more extended mini-Neptunes. Their internal structure typically consists of a compact silicate-rich core enveloped by an extensive water layer and capped with a low-density H₂/He atmosphere. With radii as large as 2.6 R⊕ for a 10 M⊕ planet and up to 2.3 R⊕ for a 5 M⊕ planet, Hycean planets extend the range of candidate habitable exoplanets beyond that defined by terrestrial analogues. Their unique physical and chemical properties, broad habitability zone, and accessible atmospheric signatures distinguish them as promising targets for the detection of habitability and biosignature gases in exoplanetary research (Madhusudhan et al., 2021).

1. Defining Properties and Internal Structure

Hycean planets are modeled with water mass fractions from 10% up to 90%, implying a global ocean hundreds of kilometers deep. The canonical interior structure comprises:

• A rocky (silicate) core, typically ~10% of total mass
• An overlying, extensive H₂O layer (ocean; forming the surface of the planet)
• An H₂/He-rich envelope, generally ≤ 0.1% of the total planet mass to permit habitable surface conditions without excessive pressure/temperature

The detailed internal structure is described using standard equations of hydrostatic equilibrium,

$\frac{dP}{dr} = -G\frac{M(r)\rho(r)}{r^2}$

and mass conservation,

$\frac{dM}{dr} = 4\pi r^2 \rho(r)$

along with equations of state (EOS) for H₂O (including temperature and pressure dependence), silicates, and H₂/He gas. The layer stratification modifies the canonical mass–radius scaling $R_p \propto M_p^{1/3}$, making the relationship nontrivial and sensitive to composition and boundary conditions at the ocean–atmosphere interface.

Ocean depths in Hycean worlds can span from a few tens to nearly 1000 km before reaching pressures ($\sim 6 \times 10^4$ bar) sufficient to form high-pressure ice phases, set by local gravity and surface temperature via the adiabatic temperature profile,

$$\nabla_\mathrm{ad} = \frac{\alpha T}{\rho c_p}; \quad \frac{dP}{dz} = \rho g$$

(Rigby et al., 19 Feb 2024).

2. Habitability Potential and the Hycean Habitable Zone

Habitability is assessed by the capacity to sustain a liquid water ocean at the base of the atmosphere, constrained by:

• Temperature $T$ at the liquid interface: 273 K $\leq T \leq$ 395–400 K
• Pressure $P$: from 1 to 1000 bar

Hycean habitable zones (HZ) are markedly broader than traditional terrestrial HZs. The inner edge accommodates equilibrium temperatures up to 430–510 K, significantly beyond the runaway greenhouse threshold for rocky planets, while the outer boundary is effectively unbounded: internal heating and large water inventories can maintain liquid oceans even at negligible stellar irradiation (Madhusudhan et al., 2021, Innes et al., 2023).

Atmospheric and radiative-convective models account for stellar flux, internal heat, molecular opacity (notably H₂O, CH₄, CO₂), and haze scattering (parametrized Bond albedo). Notably, the atmospheric profile is typically quasi-isothermal above the convective region, stabilizing the surface liquid phase. For M binary systems, the “dark Hycean” scenario posits habitable conditions confined to the permanent night side, even for planetary averages $T_\mathrm{eq}$ up to 510 K, and “cold Hycean” worlds may maintain oceans with surface temperatures as low as 10 K if internal heating is adequate (Madhusudhan et al., 2021, Rigby et al., 19 Feb 2024).

3. Interior and Atmospheric Chemical Regimes

Atmospheres are initially reducing (H₂-rich), with primary species including H₂O, CH₄, and NH₃. Photochemical and kinetic models predict atmospheric evolution toward more oxidized states (CO₂, CO, N₂, H₂O) under sustained stellar UV irradiation (Madhusudhan et al., 2023). The time-dependent abundance of these species is governed by the continuity equation: $$\frac{\partial n_i}{\partial t} = P_i - L_i - \frac{\partial \Phi_i}{\partial z}$$ where $P_i$ and $L_i$ are the production and loss rates, and $\Phi_i$ represents vertical diffusive flux, parameterized, for example, as: $K_\mathrm{zz} = 5.6 \times 10^4 P^{-0.5}$ with pressure $P$ (bar) (Madhusudhan et al., 2023).

Critical for habitability, Hycean ocean floors may be insulated from the rocky core by high-pressure ice. Nevertheless, delivery of bioessential metals (Fe, Mo, Ni, P, etc.) can occur via external impactors and atmospheric sedimentation. Estimates, adopting Archean Earth ocean concentrations, indicate that plausible impact histories would suffice to match nutrient requirements (Madhusudhan et al., 2023).

4. Observational Signatures and Biosignatures

Hycean atmospheres present extended scale heights and strong molecular signatures due to their low mean molecular weight. Simulated and observed transmission spectra exhibit pronounced bands of H₂O, CH₄, CO₂, and CO. The pattern of elevated CH₄ ($\sim 1\%$), CO₂ ($\sim 1\%$), reduced or undetectable NH₃ and H₂O, quantitatively retrieved in JWST spectra for K2-18 b and TOI-270 d, matches Hycean atmospheric predictions (Madhusudhan et al., 2023, Holmberg et al., 5 Mar 2024, Madhusudhan, 18 Jun 2024).

Secondary biomarkers such as dimethyl sulfide (DMS), methyl chloride (CH₃Cl), carbonyl sulfide (OCS), and nitrous oxide (N₂O) are anticipated at mixing ratios ranging from $\sim$1 to 10 ppmv for CH₃Cl and similarly for DMS. These can be detected at 3–6σ significance with modest JWST exposure times (few transits) in favorable cases, due to strong line contrasts supported by the planetary scale height (Madhusudhan et al., 2021, Leung et al., 19 Feb 2025, Madhusudhan et al., 16 Apr 2025).

Detection thresholds for CH₃Cl, a key biomarker, are established at $\sim$10 ppmv; DMS and DMDS may be detected at abundances $\gtrsim$10 ppmv as validated in recent MIRI LRS spectra for K2-18 b (Madhusudhan et al., 16 Apr 2025). The presence of these gases, particularly in conjunction with a Hycean-like equilibrium chemistry (CO₂/CO ratio $>$ 1; high CH₄-to-NH₃ ratio; depleted NH₃), constitute compelling biosignature scenarios. Photochemical modeling further demonstrates that significant CH₄ is only sustainable in the absence of rapid oxidation—either requiring an abiotically reducing atmosphere or continuous biological production, highlighting the potential for biosignature discrimination (Cooke et al., 9 Oct 2024).

5. Classification: Subtypes and Hycean Candidates

The Hycean class can be subdivided based on irradiation, interior dynamics, and atmospheric energy transport:

• Regular Hycean worlds: Efficient day-night heat redistribution; ocean surface temp. $<$ 395–400 K planet-wide
• Dark Hyceans: Tidally locked, with habitable conditions restricted to the nightside under inefficient heat transport; global mean $T_\mathrm{eq}$ up to 510 K is compatible
• Cold Hyceans: Internally heated, low-irradiation or free-floating; habitable oceans at temperatures as low as $\sim$10 K at high pressures

Case studies include K2-18 b, TOI-270 d, TOI-1468 c, TOI-732 c, and LHS 1140 b. For plausible interior scenarios, their modeled ocean depths span tens to hundreds of kilometers, while allowable H₂/He envelope mass fractions under Hycean conditions are consistently $\lesssim 10^{-3}$ (Rigby et al., 19 Feb 2024).

6. Research Implications and Observational Programs

Hycean worlds significantly broaden the target space for exoplanetary habitability. They are abundant in the exoplanet census, with radii placing them between rocky super-Earths and gas-rich mini-Neptunes. The wide HZ permits liquid water under elevated irradiation or internal heating; the observational properties—large scale heights and prominent spectral features—make them prime candidates for atmospheric retrieval and biosignature searches (Madhusudhan et al., 2021, Madhusudhan, 18 Jun 2024).

JWST’s multi-instrument strategy (NIRISS, NIRSpec, MIRI) enables robust detection of molecular features and constraints on atmospheric chemistry. Revised formation models, however, challenge the assumption that all sub-Neptunes beyond the snow line become water-rich; interior–atmosphere chemical equilibration may limit the accessible water mass fraction, so the observable “Hycean” signature might require alternative histories (e.g., formation inside the snow line) (Werlen et al., 1 Jul 2025).

Future research prioritizes refining photochemical models (e.g., cross-section databases, higher-dimension/dynamical modeling), targeted JWST and next-generation observations (e.g., the Habitable Worlds Observatory), and interdisciplinary studies linking atmospheric, interior, and dynamical evolution (Cooke et al., 9 Oct 2024, Hu et al., 20 Sep 2025). The presence or absence of key volatiles (e.g., CH₄, NH₃, CO₂, DMS, CH₃Cl), the measurement of atmospheric scale heights, and the identification of nutrient pathways and evolutionary rates will continue to constrain the prevalence and habitability of Hycean worlds across the exoplanet population.

7. Open Controversies and Future Challenges

The Hycean paradigm is subject to ongoing debate, especially regarding:

• The efficacy of planet formation and migration pathways in producing water-rich sub-Neptunes amenable to Hycean characterization; recent population models suggest chemical equilibration may suppress bulk water fractions below traditional Hycean thresholds (Werlen et al., 1 Jul 2025).
• The interpretation of atmospheric spectra; for example, percent-level CH₄ can be thermochemically explained in deep, gas-rich mini-Neptunes without invoking habitability, making biosignature inferences model-dependent (Wogan et al., 20 Jan 2024).
• The impact of dynamical (e.g., tidal heating) and evolutionary effects (e.g., atmospheric loss) in constraining the width and boundaries of the Hycean HZ, where even modest eccentricities due to companion bodies can internally heat and narrow the zone (Livesey et al., 14 Jun 2025).
• The ultimate detectability and unambiguous identification of biosignatures; degeneracy in mid-infrared features (DMS vs. DMDS), vertical transport in highly stratified atmospheres, and surface–interior exchange limitations (due to high-pressure ices) all complicate the assessment.

These challenges underscore the necessity of integrating structure, chemistry, dynamics, and evolutionary context in the paper of Hycean worlds and demand a multifaceted approach in the ongoing search for life outside the Solar System.