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 a planet passes in front of a star, even if the planet is absolutely tiny compared to the star, the overall brightness we see from the star drops a teeny tiny bit. In most cases, this isn’t even detectable on non-specialized equipment. Our telescopes looking for exoplanets are specifically designed to be sensitive enough to detect these dips in brightness.
Then, certain gasses absorb certain wavelengths of light. When white light (presence of all wavelengths) passes through a gas, some of the light doesn’t come out, but only certain wavelengths. When looking at the brightness of individual wavelengths, there will be seemingly random valleys compared to the rest of the light curve. We call this an absorbtion spectrum, and it can act as a kind of fingerprint for gasses (in this case. This phenomenon isn’t exclusive to gasses).
So not only is the body of the planet lowering the brightness across the spectrum, but the atmosphere will cause certain wavelengths to drop even more. We can see where these valleys of brightness are and match them to known absorbtion spectrums and get the atmospheric composition.
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