Hycean Planets: H2-Rich Ocean Worlds

• Hycean planets are sub-Neptune exoplanets featuring global liquid water oceans beneath hydrogen-dominated envelopes, supporting habitable surface conditions.
• Their extensive, low-density atmospheres yield large scale heights, enabling strong transmission signatures and detection of key molecules like CH4 and CO2.
• Research emphasizes formation dynamics, climate stability, and biosignature detection strategies to assess the potential habitability of these distinctive worlds.

Hycean planets constitute a recently defined class of sub-Neptune exoplanets characterized by extensive, deep liquid water oceans overlain by hydrogen-rich, low mean molecular weight ($\mu \lesssim 4$) atmospheres. Their unique physical, chemical, and observational characteristics position them as leading candidates in the search for habitable exoplanetary environments and atmospheric biosignatures, particularly in the era of JWST and next-generation facilities.

1. Definition and Bulk Properties

Hycean planets are sub-Neptune to mini-Neptune mass/radius objects ($\sim1.5$–$3\;R_\oplus$, $2$–$10\;M_\oplus$) that maintain a global liquid water ocean beneath a hydrogen-dominated envelope. Canonical structure models adopt interior layering of an iron/silicate core, high-pressure ice, a liquid $H_2O$ ocean (tens to $\sim$1000 km thick), and an $H_2$-He atmosphere with surface pressures spanning $\sim$1–100 bars. The envelope mass fraction required to inflate radii to $\sim2\;R_\oplus$ is only a few percent by mass, yet this is sufficient to support habitable surface pressures and temperatures (273–350 K) via collision-induced absorption and moderate greenhouse warming, so long as the envelope is not so thick as to drive supercritical conditions (Leung et al., 19 Feb 2025, Rigby et al., 19 Feb 2024, Madhusudhan et al., 2021).

Table: Representative Parameters for Candidate Hycean Worlds

Name	$M_\text{p}$ ($M_\oplus$)	$R_\text{p}$ ($R_\oplus$)	$T_\text{eq}$ (K)	H/He Mass Fraction	Ocean Depth (km)
K2-18 b	$8.63\pm1.35$	$2.61\pm0.09$	$250$–$297$	$0.005$–$0.022\%$	$50$–$350$
TOI-270 d	$4.20\pm0.16$	$2.19\pm0.07$	$326$–$387$	$1.7\times10^{-6}$–$0.0195\%$	$220$–$500$
LHS 1140 b	$5.60\pm0.19$	$1.73\pm0.03$	$198$–$235$	$8\times10^{-5}$–$0.036\%$	$100$–$300$

Surface conditions supporting habitability—i.e., liquid water at the ocean–atmosphere interface—require $273\;\text{K} \leq T_\text{surface} \leq 400$ K, and $1$–$1000$ bar at the interface. Deeper ocean layers reach pressures up to $6\times10^4$ bar before transitioning into high-pressure ice phases (Rigby et al., 19 Feb 2024).

2. Atmospheric Structure, Dynamics, and Convective Inhibition

Hycean atmospheres are $H_2$-dominated ($>90\%$ by volume), yielding atmospheric scale heights $H = kT/(\mu g)$ of $100$–$200$ km—an order of magnitude greater than Earth's and a primary source of strong transmission signatures (Barrier et al., 10 Nov 2025, Leung et al., 19 Feb 2025). Typical atmospheric compositions include:

• $H_2$: Major constituent
• $H_2O$: $1$–$10\%$ near tropopause, up to $50\%$ at condensation base
• $CH_4$: $10^{-4}$–$10^{-2}$
• $CO_2$: $10^{-6}$–$10^{-3}$
• $CO$: $\lesssim10^{-6}$
• $NH_3$: $\lesssim10^{-7}$–$10^{-6}$ (rapidly depleted by ocean uptake)

Global atmospheric circulation models (adapted ExoCAM) demonstrate regimes resembling slow rotators, with weak Coriolis forces, broad equatorial or mid-latitude jets, and a weak temperature gradient in the free troposphere ($\Delta T_\text{horiz} \lesssim 5$ K) above $\sim$1 bar. Dayside–nightside surface temperature contrasts reach $20$–$80$ K, and surface heat transport is dominated by divergent, thermally direct overturning (Barrier et al., 10 Nov 2025).

A defining dynamical feature is "moist convective inhibition": due to increasing mean molecular weight with $H_2O$ mixing, a compositionally stratified inhibition layer develops, suppressing convection near the surface (Gao et al., 28 Oct 2025, Barrier et al., 10 Nov 2025). Climate states exhibit multiple regimes—fully convective, bistable, oscillatory, or multistable—depending on instellation and atmospheric structure. Bistability allows for two stable surface states at identical external conditions, with temperature differences across the habitable range. Multistability at high instellation or envelope mass relaxes the mapping between incident flux and surface temperature, complicating habitable zone definitions (Gao et al., 28 Oct 2025).

3. Formation, Internal Evolution, and Water Inventories

Conventional scenarios hypothesize that Hycean candidates accrete substantial water ($\gtrsim10\;\text{wt}\%$) beyond the snow line, migrate inward, and retain thin $H_2/He$ envelopes. However, recent population synthesis and equilibrium chemistry calculations show that most sub-Neptunes lose nearly all primordial $H_2O$ via interface redox reactions in early magma oceans (e.g., $\ce{H2O + H2 -> 2H2 + 1/2O2}$) and sequestration into silicates, yielding final $H_2O$ mass fractions $\lesssim1.5\;\text{wt}\%$—insufficient for classical Hycean status (Werlen et al., 1 Jul 2025). Endogenic $H_2O$-rich atmospheres are restricted to rare, hydrogen-poor, inside-snow-line objects with small envelopes, and are immiscible with $H_2$, precluding distinct ocean layers.

Thus, the formation and survival of canonical Hycean interiors are subject to severe chemical and dynamical limits. This casts doubt on the prevalence of true Hycean planets as predicted by bulk mass–radius/ice accretion arguments alone, and suggests observable $H_2O$ signatures may instead trace unique formation pathways or ongoing (photo)chemical cycling (Werlen et al., 1 Jul 2025).

4. Atmospheric Chemistry, Biosignatures, and Spectral Accessibility

Hycean worlds provide uniquely favorable conditions for atmospheric spectroscopy: large transit depths (due to extended scale heights), strong absorption bands, and rich suites of chemical species. JWST observations of K2-18 b and TOI-270 d have revealed robust $CH_4$ ($\sim1\%$), $CO_2$ ($\sim1\%$), and nondetection of $NH_3$, a pattern matched by photochemical models only in Hycean regimes (Madhusudhan et al., 2023, Holmberg et al., 5 Mar 2024, Madhusudhan, 18 Jun 2024, Cooke et al., 9 Oct 2024).

Key biosignature candidates include:

• Methyl chloride ($CH_3Cl$): Produced by marine microbes, detected at $\sim$10–30 ppm is feasible for $10$–$50\times$ Earth’s global oceanic flux, detectable via JWST in 5–14 transits at $4.0\,\mu$m (Leung et al., 19 Feb 2025).
• Dimethyl sulfide (DMS): Predominantly from eukaryotic phytoplankton, tentatively observed on K2–18 b with a posterior peak at $X_{DMS} \sim 10^{-4}$–$10^{-7}$ (Madhusudhan et al., 2023, Mitchell et al., 11 Feb 2025). Strong DMS signals are likely only in warm ($T_{ocean}>288$ K), evolutionarily mature biospheres.
• $CH_4/CO_2$ disequilibrium, $CS_2$, $CH_3Cl$, and water-soluble gases (NH$_3$ depletion) as indirect indicators (Leung et al., 19 Feb 2025, Madhusudhan et al., 2021).

Abiotic sources of methyl halides and DMS are several orders of magnitude weaker than plausible biospheric production, and high photolysis rates in $H_2$-rich atmospheres necessitate large continuous sources to maintain detectable abundances.

Modeling of N-based chemistry emphasizes that, for $N_2$-dominated atmospheres, photochemical daughter species (NH$_3$, HCN, HC$_3$N) are present only at sub-ppm levels. Detections of N-containing species at tens to hundreds of ppm are feasible only if ammonia is a major N-source, which is inconsistent with observed data for K2-18 b (Radecka et al., 3 Sep 2025).

5. Climate Stability, Habitability Limits, and the Hycean Habitable Zone

The Hycean habitable zone (HZ) is distinct in both breadth and structure compared to terrestrial HZs. The inner edge of the Hycean HZ is governed by several processes:

• Runaway greenhouse: For $1$–$10$ bar atmospheres, superadiabatic radiative layers (due to convective inhibition) lower the maximum outgoing longwave radiation and move the inner HZ outward relative to classical models. For G stars, inner HZ may shift from $\sim1$ AU (Earth) to $1.6$–$3.9$ AU ($1$–$10$ bar $H_2$ envelopes); for M stars, from $0.17$ AU to $0.28$–$0.54$ AU (Innes et al., 2023).
• Cloud and haze effects: Bond albedo ($A_b$) and enhanced Rayleigh scattering are required to avoid runaway greenhouse; e.g., $A_b \gtrsim0.27$–$0.48$ for stable climates depending on envelope pressure (Barrier et al., 10 Nov 2025).
• Tidal heating: Modest eccentricities ($e\sim0.01$–$0.1$), especially in multi-planet systems, can shift the inner HZ outward by a factor $\sim3$–$5$ for low-mass stars, restricting habitability for close-in Hycean candidates (Livesey et al., 14 Jun 2025).
• Convective inhibition: Bistability, oscillatory, or multistable atmospheric states further complicate the mapping between stellar flux and surface liquid water. This creates climate regimes where multiple surface temperatures, including both habitable and uninhabitable, are stable under identical external forcing (Gao et al., 28 Oct 2025).

The outer edge of the Hycean HZ is set by the stability of the ocean under high pressure ($\lesssim1000$ bar), and radiogenic/internal heat, allowing habitability to persist at extreme orbital distances or even for rogue (cold Hycean) planets.

6. Observational Strategies and Diagnostic Discriminants

The elevated scale heights and $H_2$-dominated atmospheres of Hycean planets enable detection of major and trace gases via transmission spectroscopy within realistic integration times. JWST observations of K2-18 b and TOI-270 d demonstrate the ability to retrieve $CH_4$, $CO_2$, and upper limits on $NH_3$ and $H_2O$ with only a few transits (Madhusudhan et al., 2023, Holmberg et al., 5 Mar 2024). Discriminants between Hycean, mini-Neptune, and super-Earth scenarios include:

• $CH_4$ at percent levels with suppressed $NH_3$ and $CO$ favors a Hycean ocean boundary over a deep, convective mini-Neptune envelope (Wogan et al., 20 Jan 2024, Cooke et al., 9 Oct 2024).
• Detection of methylated biosignatures (e.g., $CH_3Cl$) at $\gtrsim10$ ppm and/or DMS at $\sim$ppm–ppb can only be sustained by high biological surface fluxes in $H_2$-rich atmospheres due to rapid photolysis (Leung et al., 19 Feb 2025, Mitchell et al., 11 Feb 2025).
• Muted $H_2O$ features in transmission imply a tropospheric cold trap, consistent with a shallow $H_2$ envelope and liquid ocean (Madhusudhan et al., 2023).
• Future MIRI and Habitable Worlds Observatory-class facilities are expected to expand access to mid-IR bands and additional biosignatures (Leung et al., 19 Feb 2025).

Models stress the need to account for convective inhibition and vertical mixing, as atmospheric abundances at observable altitudes may be decoupled from oceanic or surface fluxes (Gao et al., 28 Oct 2025). Multi-wavelength and phase-curve observations, coupled with forward modeling including diverse atmospheric processes, are required to break degeneracies and assess the presence of liquid surfaces and biological activity.

7. Open Questions and Future Prospects

Fundamental uncertainties remain regarding Hycean planet formation, long-term preservation of surface oceans, and the coupling of interior, atmospheric, and biospheric evolution. Chemical modeling demonstrates that generic mini-Neptunes cannot reproduce the low $NH_3$ and $CO$ together with high $CH_4$ and $CO_2$ seen in leading Hycean candidates without resorting to fine-tuned or biogenic sources (Cooke et al., 9 Oct 2024, Wogan et al., 20 Jan 2024). Conversely, the survivability of shallow $H_2$ envelopes in the face of XUV-driven escape, and the delivery and cycling of bioessential elements within kilometers-thick oceans capped by high-pressure ices, remain areas of active paper.

The Hycean hypothesis predicts that systematic detection of $CH_4$, $CO_2$, $H_2O$ (when not cold-trapped), and depleted $NH_3$ across a sample of temperate sub-Neptunes would serve as an empirical fingerprint for these worlds (Holmberg et al., 5 Mar 2024). The rate of biological evolution and strength of detectable biosignatures such as DMS are tightly linked to ocean temperature and evolutionary history; warmer Hyceans could manifest biosignatures earlier and at higher amplitude (Mitchell et al., 11 Feb 2025).

Upcoming JWST, ARIEL, and ELT campaigns will clarify the prevalence and diversity of Hycean atmospheres, map the full chemical parameter space, and test for multiple, independent biosignature channels. Simultaneously, improvements in multi-dimensional climate and transport models, laboratory spectroscopy (e.g., for near-IR methyl-halide opacities), and refined population synthesis will address current model systematics and formation pathway constraints (Leung et al., 19 Feb 2025, Barrier et al., 10 Nov 2025, Werlen et al., 1 Jul 2025).