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Methane on a temperate mini-Neptune ?

13 mai 2022

The announcement in 2019 of the detection of water vapor in the atmosphere of K2-18 b, an exoplanet of about 8 Earth masses located in the so-called "habitable zone" of its star, made a big noise. A study led by researchers from the Observatoire de Paris-PSL at LESIA questions this detection and is published as Matters Arising in Nature Astronomy, on May 12, 2022.

Vue d’artiste de l’exoplanète K2-18 b, de son étoile-hôte, et d’une seconde planète du système planétaire
ESA/Hubble, M. Kornmesser

Because of its mass (between 7 and 10 Earth masses) and radius (about 2.5 Earth radii), the exoplanet K2-18 b, discovered in 2015 by the Kepler satellite, is considered a "super-Earth" or "mini-Neptune." Moreover, it receives about the same amount of energy from its star, a red dwarf, as the Earth does from the Sun.

By analyzing data obtained by the Hubble Space Telescope recorded during the passage of K2-18 b in front of its star, two teams had announced in 2019 the detection of water vapor in its atmosphere : a first for a planet orbiting in the "habitable zone" of its star. This detection was based on an absorption observed at a wavelength of 1.4 micrometers. These teams had also concluded to the presence of significant quantities of hydrogen and helium and probably of water clouds.

Numerical simulations in support

A LESIA team has produced numerical simulations of the atmosphere of K2-18 b using a coherent physical and chemical model (based on the computer program Exo-REM) and assuming that the exoplanet had a chemical composition close to that of "Neptune". They succeeded in reproducing the Hubble data for K2-18 b, with the best fit being obtained for a proportion of heavy elements (oxygen, carbon, nitrogen, etc.) to hydrogen of about 200 times the solar proportion.

Absorption du flux stellaire par la planète K2-18b lorsqu’elle passe devant son étoile
Le spectre en transit calculé avec le modèle d’atmosphère Exo-REM pour une composition chimique de type Neptune reproduit l’absorption observée par HST. Dans ce modèle, celle-ci est majoritairement due au méthane et non à la vapeur d’eau comme annoncé précédemment.
B. Bézard et al. 2022

The spectra of the passage of K2-18 b in front of its star, as calculated from this model, show that the absorption observed by Hubble is in fact mostly due to methane, even if it is about half as abundant as water vapor. Methane is a gas that is expected to be abundant in the atmosphere of temperate mini-Neptunes.

More generally, these simulations show that absorption at 1.4 micrometers wavelength is not diagnostic of the presence of water vapor for giant exoplanets with equilibrium temperatures below 600 K.

To differentiate water vapor from methane, it is therefore necessary to observe in parallel at other wavelengths, as the Webb Telescope launched in late 2021 or the European space mission Ariel planned for 2029.

References

B. Bézard, B. Charnay, D. Blain, Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b, 2022, Nature Astronomy.

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