Abstract
Context. The atmospheric composition of exoplanets with masses between 2 and 10 M⊕ is poorly understood. In that regard, the sub-Neptune K2-18b, which is subject to Earth-like stellar irradiation, offers a valuable opportunity for the characterisation of such atmospheres. Previous analyses of its transmission spectrum from the Kepler, Hubble (HST), and Spitzer space telescopes data using both retrieval algorithms and forward-modelling suggest the presence of H2O and an H2–He atmosphere, but have not detected other gases, such as CH4. Aims. We present simulations of the atmosphere of K2-18 b using Exo-REM, our self-consistent 1D radiative-equilibrium model, using a large grid of atmospheric parameters to infer constraints on its chemical composition. Methods. We compared the transmission spectra computed by our model with the above-mentioned data (0.4–5 μm), assuming an H2–He dominated atmosphere. We investigated the effects of irradiation, eddy diffusion coefficient, internal temperature, clouds, C/O ratio, and metallicity on the atmospheric structure and transit spectrum. Results. We show that our simulations favour atmospheric metallicities between 40 and 500 times solar and indicate, in some cases, the formation of H2O-ice clouds, but not liquid H2O clouds. We also confirm the findings of our previous study, which showed that CH4 absorption features nominally dominate the transmission spectrum in the HST spectral range. We compare our results with results from retrieval algorithms and find that the H2O-dominated spectrum interpretation is either due to the omission of CH4 absorptions or a strong overfitting of the data. Finally, we investigated different scenarios that would allow for a CH4-depleted atmosphere. We were able to fit the data to those scenarios, finding, however, that it is very unlikely for K2-18b to have a high internal temperature. A low C/O ratio (≈0.01–0.1) allows for H2O to dominate the transmission spectrum and can fit the data but so far, this set-up lacks a physical explanation. Simulations with a C/O ratio <0.01 are not able to fit the data satisfactorily.
Highlights
While fairly common among the thousands of exoplanets discovered to date1, the atmospheres of super-Earths and sub-Neptunes – with masses between 2 and 10 M⊕ – are poorly understood
Summary and conclusions We analysed the transmission spectrum of K2-18b between 0.43 and 5.02 μm using a combination of K2, HST, and Spitzer data retrieved from Benneke et al (2019) and Tsiaras et al (2019)
We studied the effect of irradiation, metallicity, clouds, internal temperature, eddy diffusion coefficient, and C/O ratio using our self-consistent model Exo-REM and assuming an atmosphere primarily composed of H2 and He
Summary
While fairly common among the thousands of exoplanets discovered to date (see Fig. 1), the atmospheres of super-Earths and sub-Neptunes – with masses between 2 and 10 M⊕ – are poorly understood. 1 exoplanet.eu using the Wide Field Camera 3 on the Hubble Space Telescope (HST/WFC3), as well as data from Kepler K2 and Spitzer IRAC channels 1 and 2 (Benneke et al 2019). These data have already been analysed by several teams (Benneke et al 2019; Tsiaras et al 2019; Madhusudhan et al 2020; Scheucher et al 2020; Bézard et al 2020). Bézard et al (2020) found that the HST/WFC3 spectrum is dominated by CH4 absorption and found abundances of CH4 and H2O, respectively, between 3% and 10% and 5% and 11% at a 1 − σ confidence level They assumed an H2-dominated atmosphere and varied the atmospheric metallicity, but their simulations did not include H2O clouds and they did not simulate non-solar C/O ratios.
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