Detectability of biosignatures on LHS 1140 b

Rocky extrasolar planets orbiting M dwarfs are prime targets in the search for habitable surface conditions and biosignatures with near-future telescopes like the James Webb Space Telescope (JWST) and the Extremely Large Telescope (ELT). Even for the closest known targets the capabilities to characterize Earth-like or CO₂-dominated atmospheres with JWST or ELT might still be limited to a few molecules such as CO₂ or CH₄. Hence it would be difficult to draw conclusions on the surface conditions and potential habitability of these planets. In clear H₂-He atmospheres the molecular features in transmission spectra could be much larger and hence potential biosignatures might be detectable.

In this study, we investigate the detectability of the potential biosignatures NH₃, PH₃, CH₃Cl, and N₂O, assuming different H₂-He atmospheres for the habitable zone super-Earth LHS 1140 b. Recent observations of the atmosphere of LHS 1140 b suggest that the planet might hold a clear H₂-dominated atmosphere and might show an absorption feature around 1.4 µm due to H₂O or CH₄ absorption. Here we use the coupled convective-climate-photochemistry model 1D-TERRA to simulate H₂ atmospheres of LHS 1140 b with different amounts of CH₄ and assuming that the planet has an ocean and a biosphere.

The destruction of the potential biosignatures NH₃, PH₃, CH₃Cl, and N₂O shows a weak dependence on the concentrations of CH₄. For weak abundances of CH₄ only 5 to 10 transits are required to detect these molecules with JWST or ELT. However, for CH₄ surface mixing ratios of a few percent only NH₃ and N₂O might be detectable with less than 10 transits. A scenario with large abundances of CH₄ is consistent with the spectral feature at 1.4 µm and such an atmosphere might allow habitable surface temperatures. If this spectral feature at 1.4 µm originates from H₂O absorption, the planet is likely not habitable at the surface.