Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b. (arXiv:2011.10424v2 [astro-ph.EP] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Bezard_B/0/1/0/all/0/1">Bruno B&#xe9;zard</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Charnay_B/0/1/0/all/0/1">Benjamin Charnay</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Blain_D/0/1/0/all/0/1">Doriann Blain</a>

In their Letter, Tsiaras et al. 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 $mu$m 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 model 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-$mu$m 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.

In their Letter, Tsiaras et al. 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 $mu$m 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 model 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-$mu$m 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.

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