system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do scientists determine the composition of planets many light years away?
gpt
So one of the properties inherent to every material is what wavelengths of light they tend to absorb and emit.

If we look at light bouncing off a distant object and it seems to be absorbing and emitting wavelengths of light characteristic of Hydrogen, we know that the object has hydrogen in it.