Three outstanding physical questions for K2-18 b and other temperate sub-Neptunes

Abstract

Recent transmission spectra of the temperate sub-Neptune K2-18 b obtained with JWST have attracted significant attention. Debates have quickly arisen over the interpretation of the spectral data, particularly the recent MIRI observation where dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) are claimed. Here we revisit K2-18 b as a case study to examine several key questions that are also broadly relevant to the temperate sub-Neptune population: i) Can the low water abundance be reconciled with water clouds driven by orbital eccentricity? ii) Are the observed and non-observed atmospheric compositions mutually consistent? iii) Is it kinetically possible to produce DMS under sub-Neptune conditions? To address these questions, we couple climate and photochemical models to obtain self-consistent climate-photochemistry states for K2-18 b with a moderate orbital eccentricity of 0.2, as suggested by radial-velocity measurements. In addition, we present new laboratory measurements of DMS and DMDS infrared opacities by HFML-FELIX and compile updated C$_2$H$_6$ (ethane) opacities that include weak overtone bands. Our results support the interpretation of a sub-Neptune scenario without invoking DMS, and we do not find strong evidence for a water-rich interior.

Figures (15)

• Figure 1: Temperature profiles of K2-18 b for different physical assumptions (see Section \ref{['sec:apo']} for details): (a) Thick atmosphere with instellation received at the semi-major axis (S $\approx$ 1.005; Benneke2019). (b) Thick atmosphere with instellation received at apoastron, assuming e = 0.2 and no clouds. (c) Same as (b), but including H2O clouds. (d) Thin atmosphere with a 1-bar surface, adopting the same instellation and composition from (b). (e) Same as (d), but including H2O clouds.
• Figure 2: The temperature and cloud profiles (left) and gas volume mixing ratios (right) for K2-18 b at apoastron in a sub-Neptune scenario. In the left panel, the calculation including H2O clouds is shown with a solid line, with the number density of clouds shown in blue. The temperature and H2O abundance profiles for the cloud-free case are shown in dashed lines in both panels for comparison. In the right panel, the error bars and arrows represent the joined 1-$\sigma$ ranges and 2-$\sigma$ upper limits adopted from multiple retrieval analysis (Table \ref{['tab:retrievals']}). For H2O, the thin line corresponds to Schmidt2025 and the thick line corresponds to Madhusudhan2023, respectively.
• Figure 3: The temperatures around water's cold trap as a function of stellar irradiation normalized by Earth's insolation. Simulations initialized from a cold state (from left to right) are shown in blue, while those initialized from a hot state (from right to left) are shown in orange. Circles indicate temperatures at 50 mbar, and the error bars represent the temperature range between 25 and 75 mbar, chosen to be near the cloud top region. The dashed line marks the onset of spontaneous water condensation and cloud formation, whereas the dotted line indicates the evaporation and dissipation of water clouds. The slash-line shaded region enclosed by the dashed and dotted lines corresponds to the bifurcation regime where two equilibrium solutions can exist.
• Figure 4: Top: Geometric albedo of the K2-18 b atmospheric model at apoastron, shown for the cloud-free case (blue) and with H2O clouds included (orange). Bottom: Corresponding transmission spectra for the cloud-free model (blue) and for a model with a gray cloud deck at a cloud-top pressure of 0.02 bar (orange).
• Figure 5: The atmospheric compositions of our cloudy Hycean model based on the temperature profile (e) in Figure \ref{['fig:TPs']}. The left panel shows the species whose abundances are fixed at the surface (see Section \ref{['sec:hycean']}). The right panel highlights the abundant carbon-bearing molecules under the assumed fixed abundances and Hycean conditions.