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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In the spirit of ELI5, here’s a relatively simple explanation:
Scientists on Earth have tested what happens to light when it passes through gases. We know that when a beam of white light passes through a gas, the beam coming out of the other end will be missing some of its colors.
Each unique gas will remove a unique set of colors in the exact same way every time, so you can think of these missing colors as the gas’s “signature”.
We’ve done this for pretty much all gases, so we know what each gas’s signature looks like.
Now, when observe the light from a distant planet, the light we observe will also be missing some colors. If you look at the missing colors, you can find out what gases the light passed through using the gas signatures we already know.
It doesn’t matter how far away the planet is, all we need is a single beam of light to be able to analyze the missing colors and find the gas signatures.
With this method we can then conclude what gases are present in a planet’s atmosphere.
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