image source: https://www.youtube.com/watch?v=bI-JIAmiL7A

JWST will look for the composition of exoplanets' atmosphere using the spectroscopy method. However as far as I know for elements of the periodic table to make the spectrograph you need to collect white light in the visible spectrum.

How will this be achieved in the JWST despite the fact that it is limited to operate only in the infrared band?

How we will be able to correctly identify using a spectrograph, an element of the periodic table if it contains lines in the visible spectrum with the JWST collecting light only in the infrared spectrum?

  • 5
    $\begingroup$ One can do spectroscopy in any region of the EM spectrum. $\endgroup$
    – Jon Custer
    Dec 23, 2021 at 18:24
  • $\begingroup$ "White light" in the image above is just any mixture of wavelengths, so that the human eye cannot distinguish them spatially. The light from a star or a planet comes in as the same dot/pixel, containing a mixture of wavelengths. This is then split and measured via an (infrared) spectrograph. $\endgroup$ Dec 23, 2021 at 18:43
  • 3
    $\begingroup$ Besides the fact that emission spectroscopy includes the infrared region and many atoms and molecules emit in the infrared, there is also the red-shifting of visible light into the infrared. The universe is expanding and the cosmic background radiation gets red-shifted so the new telescope needs infrared capability to observe it. $\endgroup$
    – Ed V
    Dec 23, 2021 at 19:10
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    $\begingroup$ @EdV Great comment that should be also put as an answer to my question. However, this would greatly reduce the span and number of the exoplanets investigation to those exoplanets only which are much further away from Earth and that have spectrographs totally red-shifted to infrared and I'm not so sure if the IR spectrograph charts we currently have can resolve all the different elements? Also for possible future colonization, these exoplanets will be too far away, $\endgroup$
    – Markoul11
    Dec 23, 2021 at 19:24
  • 2
    $\begingroup$ You seem to be assuming that elements & molecules only have emission lines in the visible spectrum. This is not the case. $\endgroup$ Dec 23, 2021 at 22:21

1 Answer 1


However as far as I know for elements of the periodic table to make the spectrograph you need to collect white light in the visible spectrum.

The infrared spectrum is in fact rich in absorption and emission features that are diagnostic of various chemical elements. As an example of the richness of the near IR spectrum and how it can be used to obtain the chemical abundances of stars you could look at Jonsson et al. (2020), describing the analysis of near-IR spectra from the APOGEE programme. This programme attempts to determine abundances for 24 chemical elements (including C, N, O, Mg, Na, K, Si, Ca Fe etc.) in hundreds of thousands of spectra.

Crucially though for exoplanet investigations, IR spectra also contain the signatures of many molecules (water, methane, ammonia and various oxides and silicates) that are very likely to be present in cooler exoplanetary atmospheres and are much more prominent in the IR than in the visible part of the spectrum. The picture below shows the cross-section of various species in the infrared spectrum for a hot Jupiter (from Burrows 2014 ).

A further point to consider is that, since they are cooler, in terms of contrast, exoplanets are likely to be relatively brighter with respect to their host stars in the infrared part of the spectrum.

As a final point, to address some comments below your question: as far as exoplanets are concerned, the use of the IR band has nothing to do with cosmological redshift. All the exoplanets that JWST will conduct a detailed study of will be in our own Galaxy and will have small radial velocities.

IR spectrum of a hot Jupiter


  • $\begingroup$ Great! Thank you so much! This explains a lot. I understand now, that they are seeking more on molecules rather than single atoms in the periodic table where IR spectroscopy is more prominent. $\endgroup$
    – Markoul11
    Dec 24, 2021 at 9:47

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