I just can’t compute how this is possible. How can a telescope detect a gas, which isn’t even visible to the naked eye, on a planet that is an incomprehensible distance away.
[Source](https://www.bbc.co.uk/news/science-environment-66786611)
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When atoms or molecules emit or absorb energy, this energy comes from/goes into vibrations that they perform. A given atom or molecule has a series of very specific frequencies corresponding to different kinds of vibrations (e.g. in a water molecule, you can get a vibration in which the molecule bends, one in which it stretches, etc.). It will be especially good at emitting/absorbing radiation at those specific frequencies (this is similar to how bridges have resonant frequencies at which they are very good at absorbing energy from external vibrations).
So when light from a star passes through a planet’s atmosphere, some frequencies get absorbed much more than others. By splitting the light coming from a planet into different frequencies with a spectrometer, and comparing the results with what you see in a lab when you pass light through a known sample of gas, you can tell which atoms and molecules are there. However, there are two complications:
* the light coming from a distant object will be red-/blueshifted by the Doppler effect, so the frequencies will not be exactly the same as you see in the lab, but will all be shifted by the same amount
* the light that has passed through the planet’s atmosphere will be swamped by the light coming directly from the star (which has its own pattern of absorption from the gas in the outer layers of the star) – there are some different ways of dealing with this. One thing you can do is compare how the absorption spectrum changes over time as the planet passes in front of the star. It’s also possible to use coronagraphy to block out some of the light from the star, which simply involves placing a carefully positioned object in front of the telescope.
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