How do astronomers determine the chemical composition of the atmospheres of other planets

My review sheet is asking for a certain type of "study" that determines this.

Planets and asteroids shine reflect the light of their parent star. The reflected light contains absorption bands due to minerals in the rocks present for rocky bodies, or due to the elements and molecules present in the atmospheres of the planets. Asteroids can be classified into three main types, according to their spectra: the C-types are made of carbonaceous material, S-types consist mainly of silicates, and M-types are 'metallic'. C- and S-type asteroids are the most common.

Put simply, the light reflected from a planet has certain wavelengths missing. They're missing because those wavelengths were absorbed by certain chemicals in the atmosphere. Scientists can see what's missing & calculate what's in the atmosphere/what it's made of.