A water-rich interior in the temperate sub-Neptune K2-18 b revealed by JWST (2507.12622v1)

Abstract: Temperate sub-Neptunes are compelling targets for detecting liquid-water oceans beyond the Solar System. If water-rich and lacking massive hydrogen-helium envelopes, these planets could sustain liquid layers beneath their atmospheres despite sizes larger than Earth. Previous observations of the temperate sub-Neptune K2-18 b revealed an H2-dominated atmosphere rich in CH4, with moderate evidence for CO2 and tentative signs of dimethyl sulfide (DMS). Here we present four new JWST/NIRSpec transit observations of K2-18 b. The resulting high-precision transmission spectrum robustly detects both CH4 and CO2, precisely measuring their abundances and firmly establishing the planet's water-rich nature: either a thick envelope with >10% H2O by volume or a thin atmosphere above a liquid-water ocean. The spectrum reveals no detectable H2O, NH3, or CO. The absence of atmospheric water vapor suggests an efficient cold trap, while the nondetections of NH3 and CO support the scenario of a small H2-rich atmosphere overlying a liquid reservoir. However, alternative models that include these gases can also reproduce the spectrum within uncertainties, highlighting the need for deeper observations. The spectrum only contains marginal signals of DMS, methyl mercaptan (CH3SH), and nitrous oxide (N2O), with none exceeding 3 sigma in model preference and all falling below ~2 sigma without imposing a strong super-Rayleigh haze. Meanwhile, our self-consistent photochemical models show that DMS and CH3SH may form abiotically in massive H2-rich atmospheres of high metallicity, making it important to consider additional indicators for their potential use as biosignatures. K2-18 b, a cool, water-rich world, stands out as one of the most promising temperate sub-Neptunes for exploring the emergence of liquid-water environments in non-Earth-like planets, motivating further characterization of its atmosphere and interior.

• The paper presents detailed JWST NIRSpec observations integrated with archival data to characterize K2-18 b's atmosphere.
• The detected CH4 and CO2 provide precise mixing ratios and imply a water-rich interior with a potential cold trap for H2O.
• The study proposes a refined roadmap emphasizing diagnostic gas ratios and trace compounds to assess temperate sub-Neptune habitability.

Okay, here is the essay:

JWST Reveals Water-Rich Interior of K2-18 b

The paper "A water-rich interior in the temperate sub-Neptune K2-18 b revealed by JWST" (2507.12622) presents new JWST observations and analysis of exoplanet K2-18 b, focusing on its atmospheric composition and potential for hosting a liquid-water ocean. By integrating new NIRSpec transit observations with existing data, the paper offers refined constraints on atmospheric constituents and proposes a revised roadmap for characterizing temperate sub-Neptunes.

Observational Data and Reduction

The investigation incorporates five NIRSpec transit observations, including new data from the JWST GO Program 2372, supplemented by archival data from Program 2722. These observations span a wavelength range of 1.67-5.16 $\mu$m. The data reduction process employs Eureka! for NIRSpec data and NAMELESS for NIRISS/SOSS data, with independent reductions using ExoTEDRF for validation. Special attention is given to mitigating systematic noise and visit-to-visit discrepancies using a "shifted average" technique.

Figure 1: Spectroscopic light curves extracted from five NIRSpec visits, demonstrating the data's precision and coverage.

The consistency between the Eureka! and ExoTEDRF reduction pipelines is confirmed, ensuring the robustness of the derived transmission spectra. The reduction of the NIRISS/SOSS data using NAMELESS incorporates the Order 2 spectrum and employs a more conservative treatment of uncertainties.

Atmospheric Modeling and Retrieval

The paper employs three independent Bayesian spectral retrieval frameworks: , AURA, and SCARLET, to explore a broad range of atmospheric scenarios. Self-consistent atmospheric models are developed using the EPACRIS framework. These models incorporate planetary physics and chemistry, linking retrieved gas abundances to the planet's internal structure. Key parameters explored include the \ce{H2O}-to-\ce{H2} ratio and the eddy diffusion coefficient ($K_{\rm zz}$).

Figure 2: JWST transmission spectra from NIRSpec and NIRISS data reductions, alongside a comparison of RMS values and independent data reductions.

The retrieval analysis includes a baseline case, incorporating \ce{H2O}, \ce{CH4}, \ce{CO2}, \ce{NH3}, \ce{CO}, and \ce{HCN}, and explores the impact of adding DMS, CH$_3$SH, and N$_2$O. The paper assesses detection significances using Bayesian evidence and Bayes factors.

Key Findings and Implications

A primary result is the robust detection of both \ce{CH4} and \ce{CO2} in K2-18 b's atmosphere.

Figure 3: Transmission spectrum of K2-18~b compared with models, highlighting contributions from various gases.

The retrieved \ce{CH4} volume mixing ratio lies between $10^{-1.4}$ and $10^{-0.8}$, while \ce{CO2} ranges from $10^{-4.2}$ to $10^{-2.7}$. The carbon-to-hydrogen ratio (C/H) is estimated to be approximately 100 times solar. The paper infers that the bulk \ce{H2O} mixing ratio lies between 10% and 25% by volume. A notable non-detection is that of \ce{H2O}, implying the operation of a water cold trap.

Self-consistent models indicate that a massive atmosphere with $100\times$ solar metallicity overpredicts \ce{NH3} abundance, suggesting nitrogen sequestration in the interior. Additionally, the paper identifies abiotic pathways for organosulfur compounds, challenging their reliability as biosignatures.

Figure 4: Self-consistent atmospheric chemistry models illustrate the impact of varying \ce{H2O}-to-\ce{H2} ratios on atmospheric composition.

The analysis of the CO$_2$-to-CO ratio suggests a value greater than 3, supporting the presence of a relatively small atmosphere. However, constrained retrievals reveal that ratios less than 1 cannot be entirely ruled out.

Figure 5: A roadmap illustrating the range of possible internal compositions and characterization methods for temperate sub-Neptunes.

A refined roadmap for characterizing temperate sub-Neptunes is presented, emphasizing the importance of detecting key trace gases and considering the CO$_2$-to-CO ratio as a diagnostic for distinguishing between small atmospheres and massive envelopes.

Updated Roadmap for Characterizing Temperate Sub-Neptunes

The updated roadmap incorporates new JWST insights, highlighting key chemical diagnostics such as the \ce{CO2}-to-CO ratio and the presence of \ce{SO2}. The refined approach emphasizes the systematic detection of \ce{CH4}, \ce{CO2}, and \ce{NH3}, followed by a detailed characterization of the \ce{CO2}-to-CO ratio and sulfur chemistry.

Conclusion

This paper refines our understanding of K2-18 b, presenting evidence for a water-rich interior and potential liquid-water ocean. The detection of \ce{CH4} and \ce{CO2}, combined with stringent upper limits on other species, constrains the planet's atmospheric composition and internal structure. The work underscores the need for high-precision JWST transmission spectroscopy and advanced atmospheric models to characterize temperate sub-Neptunes and assess their potential habitability.