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

TL;DR

This paper suggests that methane, not water vapour, is the main absorber at 1.4 μm in the atmosphere of the habitable-zone exoplanet K2-18 b, challenging previous interpretations based on transit spectroscopy.

Contribution

It demonstrates that methane can dominate absorption features at 1.4 μm in cold sub-Neptune atmospheres, revising the interpretation of spectral data for such planets.

Findings

Methane likely causes the 1.4 μm absorption in K2-18 b.

Water vapour is not the primary absorber at this wavelength below 600 K.

Transit spectra at 1.4 μm are not definitive for water detection in cold sub-Neptunes.

Abstract

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.