Sensitive probing of exoplanetary oxygen via mid-infrared collisional absorption


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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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. (; LMD-G7 is available on request from M.T. (; PSG6 is available at


  1. 1.

    Timofeyev, Y. & Tonkov, M. Effect of the induced oxygen absorption bond on the transformation of radiation in the 6 μm region. lzv. Acad. Sci. USSR Atmos. Ocean. Phys. 14, 614–620 (1978).

  2. 2.

    Rinsland, C. P. et al. Stratospheric measurements of collision-induced absorption by molecular oxygen. J. Geophys. Res. Oceans 87, 3119–3122 (1982).

  3. 3.

    Rinsland, C. P., Zander, R., Namkung, J. S., Farmer, C. B. & Norton, R. H. Stratospheric infrared continuum absorptions observed by the ATMOS instrument. J. Geophys. Res. Atmos. 94, 16303–16322 (1989).

  4. 4.

    Baranov, Y. I., Lafferty, W. & Fraser, G. Infrared spectrum of the continuum and dimer absorption in the vicinity of the O2 vibrational fundamental in O2/CO2 mixtures. J. Mol. Spectrosc. 228, 432–440 (2004).

  5. 5.

    Luger, R. & Barnes, R. Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs. Astrobiology 15, 119–143 (2015).

  6. 6.

    Villanueva, G. L., Smith, M. D., Protopapa, S., Faggi, S. & Mandell, A. M. Planetary Spectrum Generator: an accurate online radiative transfer suite for atmospheres, comets, small bodies and exoplanets. J. Quant. Spectrosc. Radiat. Transf. 217, 86–104 (2018).

  7. 7.

    Wordsworth, R. D. et al. Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone. Astrophys. J. Lett. 733, L48 (2011).

  8. 8.

    Arney, G. et al. The pale orange dot: the spectrum and habitability of hazy Archean Earth. Astrobiology 16, 873–899 (2016).

  9. 9.

    Lincowski, A. P. et al. Evolved climates and observational discriminants for the TRAPPIST-1 planetary system. Astrophys. J. 867, 76 (2018).

  10. 10.

    Lustig-Yaeger, J., Meadows, V. S. & Lincowski, A. P. The detectability and characterization of the TRAPPIST-1 exoplanet atmospheres with JWST. Astron. J. 158, 27 (2019).

  11. 11.

    Gillon, M. et al. Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1. Nature 542, 456–460 (2017).

  12. 12.

    Misra, A., Meadows, V., Claire, M. & Crisp, D. Using dimers to measure biosignatures and atmospheric pressure for terrestrial exoplanets. Astrobiology 14, 67–86 (2014).

  13. 13.

    Wordsworth, R. & Pierrehumbert, R. Abiotic oxygen-dominated atmospheres on terrestrial habitable zone planets. Astrophys. J, 785, L20 (2014).

  14. 14.

    Schwieterman, E. W. et al. Identifying planetary biosignature impostors: spectral features of CO and O4 resulting from abiotic O2/O3 production. Astrophys. J. 819, L13 (2016).

  15. 15.

    Meadows, V. S. Reflections on O2 as a biosignature in exoplanetary atmospheres. Astrobiology 17, 1022–1052 (2017).

  16. 16.

    Meadows, V. S. et al. Exoplanet biosignatures: understanding oxygen as a biosignature in the context of its environment. Astrobiology 18, 630–662 (2018).

  17. 17.

    Des Marais, D. J. et al. Remote sensing of planetary properties and biosignatures on extrasolar terrestrial planets. Astrobiology 2, 153–181 (2002).

  18. 18.

    Kasting, J. F., Whitmire, D. P. & Reynolds, R. T. Habitable zones around main sequence stars. Icarus 101, 108–128 (1993).

  19. 19.

    Kopparapu, R. K. et al. Habitable zones around main-sequence stars: new estimates. Astrophys. J. 765, 131 (2013).

  20. 20.

    Kopparapu, R. K. et al. Habitable zones around main-sequence stars: dependence on planetary mass. Astrophys. J. 787, L29 (2014).

  21. 21.

    Grimm, S. L. et al. The nature of the TRAPPIST-1 exoplanets. Astron. Astrophys. 613, A68 (2018).

  22. 22.

    Zmuidzinas, J. Thermal noise and correlations in photon detection. Appl. Opt. 42, 4989–5008 (2003).

  23. 23.

    Thibault, F. et al. Infrared collision-induced absorption by O2 near 6.4 μm for atmospheric applications: measurements and empirical modeling. Appl. Opt. 36, 563–567 (1997).

  24. 24.

    Hopfner, M., Milz, M., Buehler, S., Orphal, J. & Stiller, G. The natural greenhouse effect of atmospheric oxygen (O2) and nitrogen (N2). Geophys. Res. Lett. 39, L10706 (2012).

  25. 25.

    Gordon, I. et al. The HITRAN2016 molecular spectroscopic database. J. Quant. Spectrosc. Radiat. Transf. 203, 3–69 (2017).

Download references


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.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

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

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

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

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Fauchez, T.J., Villanueva, G.L., Schwieterman, E.W. et al. Sensitive probing of exoplanetary oxygen via mid-infrared collisional absorption. Nat Astron (2020).

Download citation