ElI5: how do they know what other planets are made off, how can the be so sure? Jupiter could be made of a completely unknown gas?
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Have you ever looked at the sky an wondered why it is blue? It is blue because the amount of oxygen in the atmosphere ([Picture of liquid oxygen](https://en.wikipedia.org/wiki/Oxygen#/media/File:Liquid_oxygen_in_a_beaker_4.jpg), familiar shade isn’t it?). This is also why oceans are blue, because of oxygen.
When we look at other planets, we can look at the spectrum of the light they reflect to us. Each element has unique fingerprint on what they reflect. When we know these, we can break the composition of the planet’s atmosphere in to it’s parts and based on how strong the spectrum’s were, we can deduce the amounts of that element in the planet’s atmosphere.
You can do this simply with a telescope and a prism. You aim your telescope at the planet and then shine the light through a prism.
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