Methane As A Dominant Absorber In The Habitable-zone Sub-Neptune K2-18 b

In their Letter, Tsiaras et al.1 reported the detection of water vapour in the atmosphere of K2-18 b, an exoplanet of 7 to 10 Earth masses located in the habitable zone of an M-dwarf star.

The detection is based on an absorption feature seen at 1.4 μm in observations of the transiting exoplanet with the Hubble Space Telescope/Wide Field Camera 3. We have simulated the mean temperature structure and composition of K2-18b using a radiative-convective equilibrium model2−4 and we present here the corresponding transit spectroscopy calculations. We argue that the reported absorption is most likely due to methane, a gas expected to be abundant in the hydrogen-helium atmosphere of cold sub-Neptunes. More generally, we show that the 1.4-μm absorption seen in transit spectra is not diagnostic of the presence of water vapour for sub-Neptunes having an effective temperature less than 600 K and that water vapour dominates over methane at this wavelength only at larger temperatures.

Bruno Bézard, Benjamin Charnay, Doriann Blain

Comments: Submitted to Nature Astronomy ("Matters Arising")
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2011.10424 [astro-ph.EP] (or arXiv:2011.10424v1 [astro-ph.EP] for this version)
Submission history
From: Bruno Bézard
[v1] Fri, 20 Nov 2020 14:42:13 UTC (1,139 KB)

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