Sensitive probing of exoplanetary oxygen via mid-infrared collisional absorption

Abstract

The collision-induced fundamental vibration–rotation band at 6.4 μm is the strongest absorption feature from O₂ in the infrared¹̕²̓̕³ yet it has not been previously incorporated into exoplanet spectral analyses for several reasons. Either collision-induced absorptions (CIAs) were not included or incomplete/obsolete CIA databases were used. Also, the current version of HITRAN does not include CIAs at 6.4 μm with other collision partners (O₂–X). We include O₂–X CIA features in our transmission spectroscopy simulations by parameterizing the 6.4-μm O₂–N₂ CIA based on ref.³ and the O₂–CO₂ CIA based on ref.⁴. Here we report that the O₂–X CIA may be the most detectable O₂ feature for transit observations. For a potential TRAPPIST-1 e analogue system within 5 pc of the Sun, it could be the only O₂ signature detectable with the James Webb Space Telescope (JWST) (using MIRI LRS (Mid-Infrared Instrument low-resolution spectrometer)) for a modern Earth-like cloudy atmosphere with biological quantities of O₂. Also, we show that the 6.4-μm O₂–X CIA would be prominent for O₂-rich desiccated atmospheres⁵ and could be detectable with JWST in just a few transits. For systems beyond 5 pc, this feature could therefore be a powerful discriminator of uninhabited planets with non-biological ‘false-positive’ O₂ in their atmospheres, as they would only be detectable at these higher O₂ pressures.