A warm Neptune's methane reveals core mass and vigorous atmospheric mixing

The study addresses why many close-in gas giants exhibit strong methane (CH₄) depletion relative to chemical equilibrium expectations and seeks to quantify the role of non-equilibrium processes—vertical mixing and photochemistry—in shaping atmospheric compositions. WASP-107b, a low-density warm Neptune with temperatures conducive to methane, has previously shown only upper limits on CH₄ despite expectations of its presence. By targeting a spectral region where CH₄’s Q-branch at 3.32 µm is strongest, the work aims to directly measure the CH₄ abundance, constrain the strength of vertical mixing and interior temperature, and link atmospheric disequilibrium chemistry to planetary interior properties, including core mass. The findings are positioned to resolve uncertainties in atmospheric mixing rates, test predictions from core-accretion formation models, and shed light on mechanisms behind inflated radii of warm Neptunes and hot Jupiters.

Prior observations of exoplanet atmospheres often revealed methane depletion, with only recent robust detections in a few cases. Upper limits on CH₄ for WASP-107b and other planets left disequilibrium chemistry parameters (e.g., vertical mixing K_zz) poorly constrained. JWST has reliably detected H₂O, CO₂, CO, and photochemically produced SO₂ in several giant exoplanets, including WASP-39b, establishing CO₂ as a tracer of high metallicity and SO₂ as a photochemical by-product from H₂S under stellar UV irradiation. Theoretical estimates of vertical mixing cover wide ranges (∼10⁷–10¹⁰ cm² s⁻¹ for hot Jupiters), with large uncertainties and empirical constraints previously relying on unresolved Spitzer photometry. Models also link interior heat (T_int) and vertical mixing to cloud formation and atmospheric chemistry, suggesting that hotter interiors and lower gravities increase K_zz and can loft condensates like silicates to observable pressures.

• Observations: One transit of WASP-107b was observed with JWST/NIRSpec G395H (GTO program 1224; PI S. Birkmann), covering two detectors: NRS1 (2.70–3.71 µm) and NRS2 (3.83–5.16 µm). Time-series photometry yielded near photon-limited residuals. The transmission spectrum shows identifiable absorption from H₂O (shorter-wavelength slope), a strong CO₂ feature at 4.3 µm, CO between 4.5–5.0 µm, SO₂ near 4.0 µm, and a narrow CH₄ Q-branch at 3.32 µm.
• Retrievals: Atmospheric retrievals (ATMO framework implied) constrained molecular abundances and temperature. Robust detections of H₂O and CO₂ were obtained; CH₄ and SO₂ features were identified with high confidence. Retrieved volume mixing ratios: H₂O ≈ 10^(-1.85±0.22), CO₂ ≈ 10^(-3.33±0.27), CH₄ ≈ 10^(-6.03±0.21), SO₂ ≈ 10^(-5.06±0.13).
• Forward modeling: A grid of self-consistent non-equilibrium chemistry models including vertical mixing (eddy diffusion coefficient K_zz) and photochemistry was computed using radiative–convective equilibrium T–P profiles spanning intrinsic temperatures T_int. Vertical mixing quenches species at deeper, hotter levels and transports them upward. H₂O and CO largely trace metallicity, while CO₂ and CH₄ jointly constrain K_zz and T_int; SO₂ provides an upper bound on K_zz due to its photochemical origin. Best-fit parameters from the grid matched the retrieved abundances, yielding strong, joint constraints on metallicity, T_int, and K_zz. Interior structure and core mass were inferred by combining bulk properties (mass, radius, T_int) with atmospheric metallicity to estimate the distribution of heavy elements between atmosphere and interior (assuming a uniform-composition, isothermal 50:50 rock–water core for core-mass estimation).

• Spectroscopic detections: H₂O, CO₂, CO, SO₂, and CH₄ are identified in transmission with JWST/NIRSpec G395H.
• Methane detection and depletion: CH₄ detected at 4.2σ with abundance 1.0 ± 0.5 ppm (∼10^(-6.03±0.21)), depleted by ~3 orders of magnitude relative to equilibrium expectations at the probed pressures.
• Retrieved abundances: H₂O ≈ 10^(-1.85±0.22) (super-solar, ~40× solar); CO₂ ≈ 10^(-3.33±0.27) (lower than equilibrium by ~1 dex due to mixing); SO₂ ≈ 10^(-5.06±0.13) (consistent with photochemical production and prior JWST/MIRI results).
• Non-equilibrium chemistry and mixing: Best-fit vertical eddy diffusion K_zz = 10^(11.6±0.1) cm² s⁻¹, substantially higher (∼10³–10⁴×) than typical GCM-based estimates for hot Jupiters.
• Interior heat: Intrinsic temperature T_int = 460 ± 40 K (also reported as 458 ± 38 K in model caption), indicating a hot interior.
• Metallicity: Atmospheric metallicity Z = 43 ± 8 × solar from combined H₂O and CO features; bulk heavy-element fraction constrained to Z = 63.5 ± 3.8% from mass, radius, and T_int.
• Photochemistry: Minimal effect on CH₄ abundance; required to explain SO₂.
• Clouds: Elevated T_int and strong K_zz shift silicate cloud base to ∼bar pressures and loft particles to mbar levels, consistent with observed cloud signatures.
• Core mass: Interior modeling yields a statistically significant core (or interior heavy-element) mass M_core ≈ 11.5 ± 3.3–3.6 M_⊕, resolving prior tension with core-accretion models that require ≈10 M_⊕ to trigger runaway gas accretion.
• Context: Results provide some of the strongest evidence to date for vigorous vertical mixing driving atmospheric disequilibrium in exoplanets.

Quantifying CH₄ depletion directly links atmospheric composition to deep atmospheric and interior conditions, addressing long-standing uncertainties about non-equilibrium chemistry in warm Neptunes. The measured low CH₄ abundance, coupled with CO₂, tightly constrains vertical mixing (K_zz) and intrinsic heat (T_int), revealing vigorous mixing and a hot interior in WASP-107b. These parameters explain both the observed disequilibrium chemistry and cloud properties, providing a coherent picture in which strong interior heating and low gravity enhance vertical transport and loft silicate condensates to observable pressures. The high K_zz value, much larger than typical GCM-based estimates, suggests either unusually strong mixing in WASP-107b (possibly due to its thermal structure and gravity) or limitations in current GCM treatments of eddy dissipation. The elevated T_int offers insights into the inflated radius problem, consistent with mechanisms such as advection of potential temperature. By combining T_int and atmospheric metallicity with bulk properties, the study infers a significant interior heavy-element mass (~11.5 M_⊕), alleviating previous conflicts with core-accretion theory and placing WASP-107b along the solar-system-like mass–metallicity trend. Overall, the findings demonstrate that precise CH₄ measurements can serve as a thermometer for the deep atmosphere and a probe of planetary interiors, advancing our understanding of formation and evolution across warm Neptunes and hot Jupiters.

This work presents a definitive detection of methane in the warm Neptune WASP-107b and quantifies its strong depletion relative to equilibrium, enabling precise constraints on non-equilibrium chemistry, interior heat, and vertical mixing. The joint spectroscopic detections (H₂O, CO₂, CO, SO₂, CH₄) and modeling imply a super-solar atmospheric metallicity (~43× solar), a hot interior (T_int ≈ 460 K), and vigorous mixing (K_zz ≈ 10^11.6 cm² s⁻¹). Interior modeling yields the first statistically significant core (interior heavy-element) mass detection for a giant exoplanet (~11.5 M_⊕), resolving previous tensions with core-accretion models and aligning WASP-107b with solar-system mass–metallicity trends. Future work can extend this “CH₄ thermometer” approach to a broader exoplanet sample to constrain population-level distributions of K_zz, T_int, and core masses, refine GCM treatments of mixing and dissipation, and combine multi-wavelength JWST datasets (e.g., NIRSpec + MIRI) to further disentangle photochemical and mixing processes.

• Incomplete theoretical understanding of vertical mixing: Typical K_zz estimates from GCMs differ by orders of magnitude from the value inferred here, suggesting either planet-specific anomalies or limitations in GCM parameterizations of eddy dissipation.
• Interior structure assumptions: Core-mass estimates assume a uniform-composition, isothermal 50:50 rock–water core; actual interiors may be layered (e.g., icy mantles) or diffuse, so the reported core mass reflects total interior heavy elements irrespective of structure.
• Disequilibrium modeling sensitivities: Retrieved CO₂ is lower than equilibrium expectations due to mixing, highlighting sensitivities to non-equilibrium treatments; while photochemistry is found negligible for CH₄ here, it remains necessary for SO₂ and may vary with stellar UV environments.
• Affiliation and dataset context: The study reports on a single transit with one JWST/NIRSpec mode; broader phase coverage or additional instruments could further test vertical mixing and photochemistry constraints, though current results achieve near photon-limited precision.