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 infrared1,2,3, 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.

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Fig. 1: Earth-like transmission spectra of TRAPPIST-1 e.
Fig. 2: Number of TRAPPIST-1 e transits needed for a 5σ detection of the O2 A band (R = 100), the O2–O2 CIA at 1.27 μm (R = 100) and the O2–X CIA at 6.4 μm (R = 30) with JWST for the TRAPPIST-1 system moved from its distance from the Sun (12.1 pc) to 2 pc.
Fig. 3: Transmission spectra for a 1-bar O2 desiccated atmosphere on TRAPPIST-1 e assuming various isothermal profiles.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request.

Code availability

Atmos8 is available on request from G.A. (giada.n.arney@nasa.gov); LMD-G7 is available on request from M.T. (martin.turbet@lmd.jussieu.fr); PSG6 is available at https://psg.gsfc.nasa.gov/

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Acknowledgements

T.J.F., G.L.V., G.A., R.K.K., A.M. and S.D.D.-G. acknowledge support from the GSFC Sellers Exoplanet Environments Collaboration (SEEC), which is funded in part by the NASA Planetary Science Divisions Internal Scientist Funding Model. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement 832738/ESCAPE. This work was also supported by the NASA Astrobiology Institute Alternative Earths team under Cooperative Agreement NNA15BB03A and the NExSS Virtual Planetary Laboratory under NASA grant 80NSSC18K0829. E.W.S. is additionally grateful for support from the NASA Postdoctoral Program, administered by the Universities Space Research Association. We thank H. Tran for useful discussions related to O2–X CIAs.

Author information

T.J.F. led the photochemistry and transmission spectroscopy simulations. G.L.V., E.W.S. and M.T. derived parameterizations of the O2–N2 and O2–CO2 CIA bands. T.J.F. and G.A. wrote most of the manuscript. All the authors contributed to the discussions and to the writing of the manuscript.

Correspondence to Thomas J. Fauchez.

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Peer review information Nature Astronomy thanks Sergei Yurchenko and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Information

Supplementary Figs. 1-3, Table 1, text and references.

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Fauchez, T.J., Villanueva, G.L., Schwieterman, E.W. et al. Sensitive probing of exoplanetary oxygen via mid-infrared collisional absorption. Nat Astron (2020). https://doi.org/10.1038/s41550-019-0977-7

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