<p>The three common techniques used for aquiring the spectra of exoplanets and their atmosphereseres are:</p>
<ul>
<li><strong>Transmission:</strong> The brightness of a star decreases as the subject planet moves in front of it. If the planet has an atmosphere, it absorbs the suns emitted light. By measuring the brightness decrease at different wavelengths, the wavelength dependent transmittance of the atmosphere can be obtained. Here is a great example of how the chemical composition of the atmosphere is obtained from this measurments: <a href="https://www.aanda.org/articles/aa/full_html/2015/05/aa25729-15/aa25729-15.html" rel="nofollow noreferrer">Spectrally resolved detection of sodium in the atmosphere of HD 189733b with the HARPS spectrograph</a></li>
<li><strong>Reflectance:</strong> The star's light bounces off the planets atmosphere and gets reflected towards Earth. This can happen at allmost all points during the planets orbit, but ocurs specially when close to a secondaty eclipse (stars ocultation). Of course the measured light reflected from the planet is almost in its entirety the stars light, however, the host and the sun's emissions are separated by a fraction of an arcsecond. A good example of this method beign used: <a href="https://www.aanda.org/articles/aa/full_html/2018/02/aa31192-17/aa31192-17.html" rel="nofollow noreferrer">Searching for reflected light from τ Bootis b with high-resolution ground-based spectroscopy: Approaching the <span class="math-container">$10^{-5}$</span> contrast barrier</a></li>
<li><strong>Thermal emission</strong>: If the planet is hot enough, one can measure the radiation emission as if it were a blackbody. This can only be used (as of today) on big hot planets. Planets such as Earth are not measurable. A good example  of this method: <a href="https://iopscience.iop.org/article/10.1088/0004-637X/795/2/150" rel="nofollow noreferrer">Atmospheric Circulation of Eccentric Hot Jupiter HAT-P-2b</a></li>
</ul>
<blockquote>
<p>Considering both the stars are different but the planet are same, would the absorption bands of the planet be different in each case?</p>
</blockquote>
<p>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.</p>

CC BY-SA 4.0
2021-03-03T14:39:33.797Z
So does the first method mean we measure the absorption spectra at different wavelengths?
Hash
CC BY-SA 4.0
2021-03-03T14:45:17.523Z
Yes indeed. Here is an image of the wavelengths corresponding to *Na I D* absorption lines beign measured: https://www.aanda.org/articles/aa/full_html/2015/05/aa25729-15/F2.html
Wilhelmroentgen
CC BY-SA 4.0
2021-03-03T14:48:13.983Z
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
CC BY-SA 4.0
2021-03-03T14:54:16.780Z
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: https://astronomy.swin.edu.au/cosmos/a/absorption+line
Wilhelmroentgen
CC BY-SA 4.0
2021-03-03T14:29:59.650Z
2021-03-03T14:29:59.650Z
Wilhelmroentgen
<p>If you look for a simple answer by easy principles and not technicalities this would be the picture...</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<blockquote>
<p>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?</p>
</blockquote>
<p>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 <em>relatively</em> to a much brighter and bigger (angular) sized source.</p>
<blockquote>
<p>Considering both the stars are different but the planet are same, would the absorption bands of the planet be different in each case?</p>
</blockquote>
<p>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.</p>

CC BY-SA 4.0
2021-03-03T14:34:34.263Z
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
CC BY-SA 4.0
2021-03-03T14:36:36.450Z
@HankRyan Read the paper I attached on *Transmission* on my answer below. Great example how the absorption lines are measured!
Wilhelmroentgen
CC BY-SA 4.0
2021-03-03T14:42:59.190Z
@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
CC BY-SA 4.0
2021-03-03T15:39:45.397Z
@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.
Alchimista
CC BY-SA 4.0
2021-03-03T20:23:15.967Z
@Alchimista What does remission randomly oriented mean? Also as the star will have its own emission spectra, will this cause larger stars with different absorption spectra to cause planets give off different absorption spectra? Or does the absorption spectra of stars not affect that of a planet's absorption spectra? As for the transparent filter part, is it a phenomenon of the 'absorption spectra' of the film? Also if it reflects color red so we see it as red, how does it get dimmer in red?
Hash
CC BY-SA 4.0
2021-03-03T21:08:25.617Z
@HankRyan it means that an atom that absorb a photon can re-emit it in all directions, not necessarily and even not likely along the direction of arrival. This result on net absorption even if atoms can absorb and re-emit the same wavelength. The rest is exactly what is already in my answer. The absorption of a system is a property of itself. The two stars can at most make the observations difficult, but it unrelated to simple principles we discuss.
Alchimista
CC BY-SA 4.0
2021-03-03T21:08:42.297Z
Finally, if the filter that you see is red instead than blue or green as in my example, of course the dimmed part would be the blu green. Forget even the term reflection here. There is none in my example.
Alchimista
CC BY-SA 4.0
2021-03-05T12:04:22.427Z
@Alchimista So when an atom emits a photon it emits it in all direction means it doesnt just shoot one photon out? It shoots many out?
Hash
CC BY-SA 4.0
2021-03-05T12:25:35.463Z
@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?)
Alchimista
CC BY-SA 4.0
2021-03-03T13:19:57.607Z
2021-03-05T12:27:59.970Z
2021-03-05T12:27:59.970Z
Alchimista
<p>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?</p>
<p>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?</p>
<p>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?</p>
<h3>References</h3>
<ul>
<li>ESA: <a href="https://esahubble.org/images/heic1802f/" rel="nofollow noreferrer">Starlight yields clues to exoplanets’ atmospheres</a></li>
<li>Lecture by  Michael Richmond: <a href="http://spiff.rit.edu/classes/resceu/lectures" rel="nofollow noreferrer">Spectroscopy of exoplanets</a>/spectra/spectra.html</li>
</ul>

<exoplanet><planetary-atmosphere><spectroscopy><spectra><esa>
When examining an exoplanet's atmosphere is the star's emission spectra or planet's light used?
Hash
# When examining an exoplanet's atmosphere is the star's emission spectra or planet's light used?

(Asked by: username_0 on 2021-03-03)

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

* ESA: [Starlight yields clues to exoplanets’ atmospheres](https://esahubble.org/images/heic1802f/)
* Lecture by Michael Richmond: [Spectroscopy of exoplanets](http://spiff.rit.edu/classes/resceu/lectures)/spectra/spectra.html

**Tags:** exoplanet, planetary-atmosphere, spectroscopy, spectra, esa

---

## 2 answers

### Answer by username_1 on 2021-03-03. Score: 2

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

* **Transmission:** The brightness of a star decreases as the subject planet moves in front of it. If the planet has an atmosphere, it absorbs the suns emitted light. By measuring the brightness decrease at different wavelengths, the wavelength dependent transmittance of the atmosphere can be obtained. Here is a great example of how the chemical composition of the atmosphere is obtained from this measurments: [Spectrally resolved detection of sodium in the atmosphere of HD 189733b with the HARPS spectrograph](https://www.aanda.org/articles/aa/full_html/2015/05/aa25729-15/aa25729-15.html)
* **Reflectance:** The star's light bounces off the planets atmosphere and gets reflected towards Earth. This can happen at allmost all points during the planets orbit, but ocurs specially when close to a secondaty eclipse (stars ocultation). Of course the measured light reflected from the planet is almost in its entirety the stars light, however, the host and the sun's emissions are separated by a fraction of an arcsecond. A good example of this method beign used: [Searching for reflected light from τ Bootis b with high-resolution ground-based spectroscopy: Approaching the $10^{-5}$ contrast barrier](https://www.aanda.org/articles/aa/full_html/2018/02/aa31192-17/aa31192-17.html)
* **Thermal emission**: If the planet is hot enough, one can measure the radiation emission as if it were a blackbody. This can only be used (as of today) on big hot planets. Planets such as Earth are not measurable. A good example of this method: [Atmospheric Circulation of Eccentric Hot Jupiter HAT-P-2b](https://iopscience.iop.org/article/10.1088/0004-637X/795/2/150)

> 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.

> **Comments:**
> **username_0**: So does the first method mean we measure the absorption spectra at different wavelengths?  
> **username_1**: Yes indeed. Here is an image of the wavelengths corresponding to *Na I D* absorption lines beign measured: https://www.aanda.org/articles/aa/full_html/2015/05/aa25729-15/F2.html  
> **username_0**: 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?  
> **username_1**: 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: https://astronomy.swin.edu.au/cosmos/a/absorption+line  

---

### Answer by username_2 on 2021-03-03. Score: 2

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.

> **Comments:**
> **username_0**: 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?  
> **username_1**: @HankRyan Read the paper I attached on *Transmission* on my answer below. Great example how the absorption lines are measured!  
> **username_0**: @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  
> **username_2**: @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.  
> **username_0**: @Alchimista What does remission randomly oriented mean? Also as the star will have its own emission spectra, will this cause larger stars with different absorption spectra to cause planets give off different absorption spectra? Or does the absorption spectra of stars not affect that of a planet's absorption spectra? As for the transparent filter part, is it a phenomenon of the 'absorption spectra' of the film? Also if it reflects color red so we see it as red, how does it get dimmer in red?  
> **username_2**: @HankRyan it means that an atom that absorb a photon can re-emit it in all directions, not necessarily and even not likely along the direction of arrival. This result on net absorption even if atoms can absorb and re-emit the same wavelength. The rest is exactly what is already in my answer. The absorption of a system is a property of itself. The two stars can at most make the observations difficult, but it unrelated to simple principles we discuss.  
> **username_2**: Finally, if the filter that you see is red instead than blue or green as in my example, of course the dimmed part would be the blu green. Forget even the term reflection here. There is none in my example.  
> **username_0**: @Alchimista So when an atom emits a photon it emits it in all direction means it doesnt just shoot one photon out? It shoots many out?  
> **username_2**: @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?)  

---

