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

The study of exoplanet atmospheres provides crucial insights into planetary formation and evolution. A significant observation is the widespread depletion of methane (CH₄) in the atmospheres of many gas giants, contradicting equilibrium chemistry predictions. This depletion is hypothesized to result from disequilibrium processes like photochemistry or vigorous mixing from a hotter interior. However, the internal structure and mixing strength of these planets remain largely unconstrained, with only upper limits on CH₄ depletion previously available. WASP-107b, a warm Neptune with unusually low density and previously reported low core mass, presents a unique opportunity to investigate these processes. Its temperature profile is conducive to CH₄, yet previous observations failed to detect the molecule, creating a puzzle. This paper aims to address this issue by analyzing the detailed atmospheric composition of WASP-107b using data from the James Webb Space Telescope (JWST), specifically focusing on the detection and quantification of CH₄ depletion to better constrain the planet's interior structure and atmospheric dynamics. The importance of this study lies in the potential to refine our understanding of atmospheric processes and core formation in gas giants, potentially resolving inconsistencies between observations and theoretical models of planetary evolution.

Previous studies have noted the prevalence of methane depletion in exoplanet atmospheres (Stevenson et al., 2010; Kreidberg et al., 2018; Fu et al., 2022; Dyrek et al., 2024). Kreidberg et al. (2018) suggested water, high-altitude condensates, and possible methane depletion in WASP-107b's atmosphere. Recent detections of methane in some exoplanets (Bell et al., 2023) suggest that depletion is not universal, highlighting the variability of atmospheric processes. The detection of SO₂ in some exoplanets (Tsai et al., 2023) indicates the role of photochemistry. Studies on WASP-107b itself (Piaulet et al., 2021) have focused on its unusually low density, suggesting potential variations in formation mechanisms compared to Solar System planets. Existing models of giant planet formation (Pollack et al., 1996) are challenged by observations of low core masses, with WASP-107b's low core mass upper limits being particularly notable (Piaulet et al., 2021). These prior works establish a context for the present research and highlight the open questions surrounding methane depletion and core formation in warm Neptunes.

The research team observed one transit of WASP-107b using the G395H spectral grating of the JWST-NIRSpec. This mode has proven reliable for detecting H₂O, CO, CO₂, and SO₂ in giant exoplanets. The wavelength-integrated JWST-NIRSpec time-series photometry, shown in Fig. 1, was analyzed to create a transmission spectrum (Fig. 2). Model retrievals were used to determine atmospheric composition and temperature. The researchers used the ATMO model, a sophisticated atmospheric retrieval code that incorporates non-equilibrium chemistry from both vertical mixing and photochemistry. The vertical mixing was modeled using a turbulent flow with a vertical eddy diffusion coefficient (K_zz). A grid of forward atmospheric models was used to constrain photochemistry, vertical mixing, metallicity, and the temperature structure. This approach considered the abundances of several molecules (H₂O, CO, CO₂, CH₄, and SO₂) to determine the best-fit model. The core mass was estimated by considering the retrieved atmospheric metallicity and the interior temperature, assuming a uniform composition core of rock and water. The uncertainties in the measurements and model parameters were carefully propagated to obtain the error bars reported in the final results.

The JWST-NIRSpec transmission spectrum of WASP-107b revealed the presence of several molecules, including H₂O, CO, CO₂, SO₂, and importantly, CH₄. CH₄ was detected with a 4.2σ significance at an abundance of 1.0 ± 0.5 ppm, approximately three orders of magnitude lower than equilibrium expectations. The atmospheric analysis revealed a super-solar metallicity of 43 ± 8 times solar. The model retrievals indicated a hot interior temperature (*T*_int = 460 ± 40 K) and vigorous vertical mixing (*K*_zz = 10^11.6±0.1 cm² s⁻¹). Photochemistry plays a minor role in CH₄ abundance but is crucial for explaining the observed SO₂. Crucially, the study inferred a core mass of 11.5 ± 3.6 *M*_⊕, significantly higher than previous upper limits and consistent with core-accretion models predicting a massive enough core to accrete the observed H/He envelope. The high vertical mixing rate is approximately 1000-10000 times higher than typical estimates for hot Jupiters, potentially due to the planet's high equilibrium temperature or limitations in existing general circulation models. The hot interior temperature contributes to the planet's inflated radius, a long-standing problem in exoplanet research. The high metallicity of WASP-107b places it close to the mass-metallicity trend of the gas giants in the Solar System, suggesting similarities in formation processes despite the differences in interior temperature. The core mass estimate represents a statistically significant detection, challenging previous low-core-mass interpretations and aligning better with core accretion models.

The detection of methane in WASP-107b's atmosphere, despite its significant depletion, provides strong evidence for vigorous vertical mixing from a hot interior. The high core mass estimate, significantly higher than previously thought, resolves a tension with core-accretion models. The high vertical mixing rate, while unusual compared to typical estimates from general circulation models, could be due to the planet's high equilibrium temperature or imperfections in current models. The hot interior temperature is likely a significant contributor to the planet's inflated radius. The high metallicity of WASP-107b, despite its low density, is consistent with the mass-metallicity relation observed in Solar System gas giants. The findings suggest a core-accretion formation scenario, similar to the Solar System's gas giants, with the key difference being the planet's unexpectedly hot interior. Future research should focus on applying similar techniques to a larger sample of exoplanets to explore the prevalence of high vertical mixing and its implications for planetary evolution.

This study presents the first statistically significant detection of a core in a gas giant exoplanet, using CH₄ depletion as a proxy for deep atmospheric temperature in WASP-107b. The findings reveal a hot interior, vigorous vertical mixing, and a surprisingly large core mass, challenging previous understanding of this planet's formation and evolution. The methods employed here can be applied to other exoplanets to further constrain the diversity of planetary interiors and atmospheric dynamics. Future work could focus on investigating the physical mechanisms responsible for the anomalously high vertical mixing observed in WASP-107b and examining the relationship between core mass, atmospheric metallicity, and interior temperature in a broader sample of exoplanets.

The study relies on a specific atmospheric model (ATMO), and the results might be sensitive to the assumptions and parameters used in the model. The inferred core mass depends on the assumed composition of the core, which could introduce uncertainties. The observed high vertical mixing rate is significantly higher than typical estimates, warranting further investigation into the underlying mechanisms and potential limitations of existing general circulation models. The analysis focuses on a single planet, so extrapolating these results to the broader population of exoplanets requires caution.