1D atmospheric study of the temperate sub-Neptune K2-18b

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 H 2 O and an H 2-He atmosphere, but have not detected other gases, such as CH 4. 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 to 5 µm), assuming an H 2-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 H 2 O-ice clouds, but not liquid H 2 O clouds. We also confirm the findings of our previous study, which showed that CH 4 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 H 2 O-dominated spectrum interpretation is either due to the omission of CH 4 absorptions or a strong overfitting of the data. Finally, we investigated different scenarios that would allow for a CH 4-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 H 2 O to dominate the transmission spectrum and can fit the data but so far, this setup lacks a physical explanation. Simulations with a C/O ratio < 0.01 are not able to fit the data satisfactorily.

ARES.* V. No Evidence For Molecular Absorption in the HST WFC3 Spectrum of GJ 1132 b

“…It is important to note that the mass-radius models shown above are merely illustrative, as more complex setups are possible (e.g., Jespersen & Stevenson 2020;Modirrousta-Galian et al 2020a;Mousis et al 2020). The planets listed are Trappist-1 b, g (de Wit et al 2016;Grimm et al 2018) (Van Grootel et al 2014;Guo et al 2020), HD 3167 c (Christiansen et al 2017;Guilluy et al 2021;Mikal-Evans et al 2021), LHS 1140 b (Ment et al 2019), and K2-18b (Benneke et al 2019;Tsiaras et al 2019;Bézard et al 2020;Blain et al 2021). Each transit observation required four HST orbits and utilized the spatial scanning technique.…”

“…A number of studies using the Wide Field Camera 3 (WFC3) on board Hubble Space Telescope (HST) have found evidence for molecular absorption in sub-Neptunes (e.g., Guo et al 2020;Guilluy et al 2021). Of particular note are the studies of the habitable-zone planet K2-18 b, which likely has a hydrogen-helium envelope with a high concentration of water vapor (Benneke et al 2019;Tsiaras et al 2019) and possibly CH 4 (Bézard et al 2020;Blain et al 2021). Meanwhile, GJ 1214 b most probably hosts a thick cloud layer, with molecular features belonging to a cloud-free primary atmosphere, or one composed of 100% H 2 O and 100% CO 2 , having been ruled out (e.g., Kreidberg et al 2014).…”

“…It is important to note that the mass-radius models shown above are merely illustrative, as more complex setups are possible (e.g., Jespersen & Stevenson 2020; Modirrousta-Galian et al 2020a; Mousis et al 2020). The planets listed are Trappist-1 b, g (de Wit et al 2016; Grimm et al 2018), GJ 357 b (Luque et al 2019), Kepler-406 c (Marcy et al 2014), Kepler-414 b (Hadden & Lithwick 2014), Kepler-102 d (Marcy et al 2014), GJ 9827 d (Rice et al 2019), GJ 1214 b (Harpsøe et al 2013), CoRoT-7 b (Dai et al 2019), HD 97658 b (Van Grootel et al 2014; Guo et al 2020), HD 3167 c (Christiansen et al 2017; Guilluy et al 2021; Mikal-Evans et al 2021), LHS 1140 b (Ment et al 2019), and K2-18b(Benneke et al 2019;Tsiaras et al 2019;Bézard et al 2020;Blain et al 2021).…”

Possibilities for an Aerial Biosphere in Temperate Sub Neptune-Sized Exoplanet Atmospheres

“…Atmosphere models for the sub Neptune K2-18b, currently the only temperate sub Neptune with atmosphere observations, show that only water ice clouds and not liquid water clouds can form, given the exterior heating from the host star and the internal heating from interior energy [50,51]. This important point is made by [50,51], who show that K2-18 b has to have high water content, with a metallicity of greater than 100× solar, in order for water to condense, and that water condenses to the ice phase and not the liquid phase (as seen in Figure 4). Other than high water content, sub Neptunes must have lower interior energies than assumed for current K2-18 b models in order for liquid water clouds to exist, according to Figure 4.…”

“…One possibility is high water content in the atmosphere, which for a sub Neptune corresponds to high metallicity (see the liquid phase region of the water phase diagram in Figures 3 and 4). Atmosphere models for the sub Neptune K2-18b, currently the only temperate sub Neptune with atmosphere observations, show that only water ice clouds and not liquid water clouds can form, given the exterior heating from the host star and the internal heating from interior energy [50,51]. This important point is made by [50,51], who show that K2-18 b has to have high water content, with a metallicity of greater than 100x solar, in order for water to condense, and that water condenses to the ice phase and not the liquid phase (as seen in Figure 4).…”