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Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b

Nature Astronomy volume 6, pages 537–540 (2022)Cite this article

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arising from A. Tsiaras et al. Nature Astronomy https://doi.org/10.1038/s41550-019-0878-9 (2019)

We modelled the composition of K2-18 b using the Exo-REM radiative-equilibrium model7,8,9 adapted for irradiated giant exoplanets10 and calculated the expected transit spectra. Beyond H2/He, Exo-REM incorporates 12 gaseous absorbers, including all potential ones in the HST transit data: that is, H2O, CO, CO2, CH4, NH3 and H2S, but no cloud opacity. Mixing ratio profiles are calculated either from thermochemical equilibrium or allowing for some non-equilibrium chemistry between C-, O- and N-bearing compounds11. We investigated a range of metallicity of 1–1,000, defined here as the ratio of the abundance of elements heavier than hydrogen or helium (relative to hydrogen) to that in the Sun. For the eddy-mixing coefficient (Kzz), a parameter that controls the transport-induced quenching of chemical equilibria, we investigated a range of 106–1010 cm2 s-1.

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Fig. 1: Transmission spectra of K2-18 b calculated for different atmospheric compositions.
Fig. 2: Relative contributions of H2O and CH4 to the transit depth.

Data availability

The observational data that support the findings of this study are included in refs. 4,6. The model data that support the plots within this paper are available from the corresponding author upon request.

Code availability

The Exo-REM source code and the input files used in this study are available from the corresponding author upon reasonable request. The current version of Exo-REM is available through the Observatoire de Paris GitHub website: https://gitlab.obspm.fr/Exoplanet-Atmospheres-LESIA/exorem.

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Acknowledgements

D.B. acknowledges financial support from the ANR (Agence Nationale de la Recherche) project e-PYTHEAS (ANR-16-CE31-0005-01). This work received funding from the Programme National de Planétologie (PNP) of the Institut National des Sciences de l’Univers (INSU) of the Centre National de la Recherche Scientifique (CNRS), co-funded by the Centre National d’Études Spatiales (CNES).

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Author notes

  1. Doriann Blain

    Present address: Max Planck Institute for Astronomy, Heidelberg, Germany

Authors and Affiliations

  1. LESIA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CNRS, Meudon, France

    Bruno Bézard, Benjamin Charnay & Doriann Blain

Authors
  1. Bruno Bézard
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  2. Benjamin Charnay
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  3. Doriann Blain
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Contributions

B.B. initiated the development of the Exo-REM model, performed the analysis and wrote the paper with the help of B.C. and D.B. B.C. contributed to the development of the Exo-REM model, compared Exo-REM and TauREx outputs and wrote the relevant section. D.B. calculated the tables of c-k absorption coefficients and adapted the Exo-REM model to irradiated planets, with contributions from B.B and B.C. All authors contributed to the interpretation of the results and commented on the manuscript at all stages.

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Correspondence to Bruno Bézard.

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Extended data

Extended Data Fig. 1 Comparison of model transmission spectra with the observational data set.

The transmission spectrum of K2-18b calculated from our best-fitting self-consistent model (red) is compared with the HST/WFC3 data6, the Kepler/K2 data point4 and the Spitzer/IRAC measurements4 at 3.5 and 4.5 μm. A model including only H2O absorption is shown in blue. For each model, the planet’s radius has been adjusted to minimize the χ2 residuals. The location of the main absorption bands of H2O (blue squares) and CH4 (green squares) are indicated. While both models are similar in the spectral range covered by the HST data, they vastly differ at longer wavelengths where the CH4 bands no longer coincide with the strong H2O bands. The rightmost vertical axis represents the pressure levels for our best-fitting model having a metallicity of 180 (red).

Extended Data Fig. 2 Best-fit atmospheric model.

Temperature profile (thick black line) and abundance profiles of selected molecular absorbers (coloured lines) for our best-fit Exo-REM model. The metallicity is 180 and the eddy mixing coefficient is 108 cm2 s−1. In this model, water vapour does not condense and water clouds are not expected to form. The mean molecular mass is 6.2 amu.

Extended Data Fig. 3 Transmission spectra of K2-18 b calculated for different atmospheric compositions.

Spectra calculated from our self-consistent model Exo-REM for a metallicity of 180 and an eddy mixing coefficient of 108 cm2 s-1, are compared with HST data as reduced by Benneke et al.4. All absorbers are included in the spectral calculation shown in red. Spectra including absorption from H2O only (blue) and from CH4 only (green) are also shown. The planet’s radius has been adjusted in each case to minimize the residuals with the whole dataset4.

Extended Data Fig. 4 Contributions of different absorbers to the transit depth.

Transmission spectra of K2-18 b calculated for the atmospheric composition shown in Extended Data Fig. 2 using the forward model of TauREx 3 (ref.23). The orange line corresponds to the spectrum with molecular absorption from H2O, CH4, CO, NH3 and collision-induced absorption (CIA) from H2-H2 and H2-He. Other coloured lines show the contribution of each molecule.

Extended Data Fig. 5 Posterior distributions obtained from free retrievals.

The retrieval tool TauREx 3 (https://taurex3-public.readthedocs.io/en/latest/) was applied to the HST data reduced by Tsiaras et al.6 with planetary radius (expressed in Jovian radius), temperature, log of H2O, CH4 and N2 volume mixing ratios, and cloud-top pressure (Pa) as free parameters. The posterior distributions of the log(H2O) and log(CH4) are very broad with a denser region for the ranges [−3.5:−1] and [−4:−0.3] respectively. A large fraction of the parameter space corresponds to a CH4-rich atmosphere whose absorption at 1.4 micron is dominated by CH4. The parameters of our best fit Exo-REM model having a metallicity of 180 (Extended Data Fig. 2), shown as an orange dot in the log(CH4)-log(H2O) space, are globally consistent with the TauREx retrievals.

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Bézard, B., Charnay, B. & Blain, D. Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b. Nat Astron 6, 537–540 (2022). https://doi.org/10.1038/s41550-022-01678-z

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