<p>It is only possible to detect gases in transiting Exoplanets. The spectrum of the star is taken when the Exoplanet is not in transit and again when in transit. The differences between the spectrums is due to a small amount of light from the parent star passing through the upper atmosphere of the Exoplanet and being absorbed by gases in the atmosphere.
The effect is VERY small and difficult to detect.</p>

CC BY-SA 4.0
2020-12-04T13:52:26.077Z
This isn't the only method. e.g. You  can simply subtract a spectrum of the star when the planet is *behind* it, from the spectrum of the star+planet. This is what is done at infrared wavelengths.
ProfRob
CC BY-SA 3.0
2016-05-30T16:00:25.173Z
2016-05-30T16:00:25.173Z
James Screech
<p>Exoplanets are planets that are located outside our solar system - whether that be orbiting a star or drifting past one. Now, the closest star to us is Alpha Centauri which is just over four light-years away. So, how can astronomers detect gases in the atmospheres of planets over this distance with any degree of accuracy (or at all)?</p>

CC BY-SA 3.0
2016-05-30T18:15:30.690Z
See called2voyage's answer [here].(http://astronomy.stackexchange.com/questions/2/how-are-the-compositional-components-of-exoplanet-atmospheres-differentiated)
Jerard Puckett
<exoplanet><planetary-atmosphere><gas>
How do astronomers detect gases that are in the atmosphere of exoplanets?
HotSaucey
# How do astronomers detect gases that are in the atmosphere of exoplanets?

(Asked by: username_0 on 2016-05-30)

Exoplanets are planets that are located outside our solar system - whether that be orbiting a star or drifting past one. Now, the closest star to us is Alpha Centauri which is just over four light-years away. So, how can astronomers detect gases in the atmospheres of planets over this distance with any degree of accuracy (or at all)?

**Tags:** exoplanet, planetary-atmosphere, gas
> **Comments:**
> **username_1**: See called2voyage's answer [here].(http://astronomy.stackexchange.com/questions/2/how-are-the-compositional-components-of-exoplanet-atmospheres-differentiated)  

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## 1 answers

### Answer by username_2 on 2016-05-30. Score: 6 | Accepted answer

It is only possible to detect gases in transiting Exoplanets. The spectrum of the star is taken when the Exoplanet is not in transit and again when in transit. The differences between the spectrums is due to a small amount of light from the parent star passing through the upper atmosphere of the Exoplanet and being absorbed by gases in the atmosphere.
The effect is VERY small and difficult to detect.

> **Comments:**
> **username_3**: This isn't the only method. e.g. You  can simply subtract a spectrum of the star when the planet is *behind* it, from the spectrum of the star+planet. This is what is done at infrared wavelengths.  

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