How do scientists measure what things like planets, stars, etc are made of? Things like “xyz planet is 60% hydrogen, 20% helium, ans 20% other.” Have always been mind boggling.
In: Chemistry
It is actually really fascinating.
So, to answer this, you have to know photos all have different wavelengths. We most commonly know this as color, but there is a massive spectrum that we can’t see. A photon we see as green has one wavelength, and one we see as red has another.
Different atoms absorb photons of different wavelengths. If you shine full spectrum light at a hydrogen atom, you will see that some of those photons at a very specific wavelength aren’t there anymore – that is because the hydrogen atom absorbed them. Each atom absorbs at a different wavelength.
So, we are able to take the light that is reflected off far away planets and see what wavelengths of light _aren’t_ there. If the wavelength is missing, it is because something in that atmosphere absorbed it. Since we assume a oxygen atom will behave the same (in terms of photon absorbtion) here as it will 1M lightyears away, we can determine what the atmosphere of the planet contains.
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