# When examining an exoplanet's atmosphere is the star's emission spectra or planet's light used?

My understanding of the main method we use to figure out an exoplanet atmosphere composition is that when a exoplanet transits their sun, visible light passes through the planet's atmosphere, and absorbed by the elements in the atmosphere. We then observe these absorption lines to figure out the chemical composition right? Or am I getting some details wrong here?

My main question is, does the light that passes through the planet's atmosphere is the sun's light or the sun's emission spectra?

Also will different stars starlight cause different absorption lines? As in say some planet X orbiting a M class star as opposed to the same planet X orbiting a G class star. Considering both the stars are different but the planet are same, would the absorption bands of the planet be different in each case?

### References

The three common techniques used for aquiring the spectra of exoplanets and their atmosphereseres are:

Considering both the stars are different but the planet are same, would the absorption bands of the planet be different in each case?

It will not make any difference as the atmosphere's absorption lines are solely planet dependent and star's polychromatic light ensures same lines will show no matter what sun it is.

• So does the first method mean we measure the absorption spectra at different wavelengths?
– Hash
Mar 3, 2021 at 14:39
• Yes indeed. Here is an image of the wavelengths corresponding to Na I D absorption lines beign measured: aanda.org/articles/aa/full_html/2015/05/aa25729-15/F2.html Mar 3, 2021 at 14:45
• so from my understanding an element absorbs different wavelengths, and might absorb a certain wavelength of uv rays just as it might absorb that of visible light. But why do we measure uv Ray absorption also instead of visible light? Is it due to issues of red shifting and turning into background noise?
– Hash
Mar 3, 2021 at 14:48
• The absorpion lines are discrete, which means that given element will allways absorb the same frequencys. You could think about it as a 'fingerprint' form the elements. The frequency showed in the image above mentioned is in visible light range at $589 nm$. More about absorpion: astronomy.swin.edu.au/cosmos/a/absorption+line Mar 3, 2021 at 14:54

If you look for a simple answer by easy principles and not technicalities this would be the picture...

When the exoplanet transit between our point of sight and its star, its disk block part of the light. This results in a dimming of the star light we receive. The latter can be analyse as we do for light of any source. It is well approximated by a black body spectrum with dimmer lines corresponding to the absorption lines of the elements/species present in the star atmosphere.

If the exoplanetary disk that block the light has an atmosphere as well, the constituents of the latter will absorb their typical lines and this results on the appearance of dimmer and/or new lines in the star spectrum as received.

In principle there could be a scattering effect of the exoplanetary atmosphere, too. For instance an observer on the moon during a sun eclipse would observe the sun fainting overall, the absorption lines due to terrestrial air, and a global different spectral distribution toward the red due to scattering, also due to air.

However, in case of far exoplanetary systems, it is already astonishing that we can do the line analysis as for all the above just happen to the little portion of the star flux that is intercepted by the planet and at same time by our telescope.

My main question is, does the light that passes through the planet's atmosphere is the sun's light or the sun's emission spectra?

Emission by the star atmosphere to us or to the exoplanet is mixed with the proper photosphere emission. Practically, it is the star light whatever you like to call it. It has not really an influence on the difficult above measurement. They are difficult right because we must see tiny changes relatively to a much brighter and bigger (angular) sized source.

Considering both the stars are different but the planet are same, would the absorption bands of the planet be different in each case?

No. In principle there could be a difference in the easy of detection, but the absorption lines are typical of the absorber not of the excitation source. Obviously we can't search for absorption happening in the UV using just visible light. But the very broad emission of stars provides quite the same spectral coverage in term of wavelengths.

• Can you explain what you meant by the absorption lines by the star's atmosphere? I'm not able to understand. How can star have absorption line when it is the thing that emits light? There is no other light for it to absorb right?
– Hash
Mar 3, 2021 at 14:34
• @HankRyan Read the paper I attached on Transmission on my answer below. Great example how the absorption lines are measured! Mar 3, 2021 at 14:36
• @Wilhelmroentgen I did try reading it but I'm not able to understand much of it as the language is far too complex for my understanding, sorry
– Hash
Mar 3, 2021 at 14:42
• @HankRyan yes the stars emits light but what it is around it does absorb and re-emits. Being re-emission randomly oriented, the light reaching us - at that particular wavelength which is absorbed - is dimmed. Is not much dissimilar (for what is relevant here) from placing a transparent coloured filter between a white lamp and us. If the filter abs red, the light reaching us is untouched in the green and dimmer in the red. Though, replace the colour with discrete wavelengths. Mar 3, 2021 at 15:39
• @Hash an isolated atom emits one photon after it has absorbed one. But not along the direction of the incoming one. As net result the photon won't reach your eyes/scope, it is absorbed anyway in term of net absorbance. There are many phenomena in molecules etc but I referred to re-emission just to prevent a classical question (why there are atomic or ionic absorption lines if each re-emit what it can absorb?) Mar 5, 2021 at 12:25