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View submission: How do we know what exoplanets look like?
This isn't just limited to transit spectroscopy, but works for emission and reflection spectroscopy as well.
Transmission spectra are formed as u/WarOnTime says, when a planet passes between us and its host star, the light of the star passes through the atmosphere around the planet, and the chemical composition of the atmosphere will imprint itself on the stellar light by absorbing characteristic wavelengths according to what chemical species are present in the atmosphere. This gives us information about about the planetary terminator, the line where day and night meet. From this we are able to gather information about atmospheric circulation such as what material is moving from day to night, and vice versa.
Emission spectra are formed by the heat of the planet and atmosphere causing it to glow of its own accord. Just like the temperature of the Earth and of human beings cause them to emit IR radiation, other planets do as well, and the hotter ones may also emit in visual wavelengths. It can be difficult sometimes to separate this from the light of the star, but doesn't require the system to be in the chance alignment that allows us to view a transit. Both the day and the night side of the planet can emit spectra, and the two are generally different according primarily to the temperature differences between the two hemispheres.
Reflection spectra are formed when the light of the star reflects off of the planet. This is modulated by something called the Albedo, which is a measure of the fraction of the light the planet reflects. In this scenario, the light passes down through the atmosphere, reflects off either the planetary surface or a cloud layer, and travels back out through the atmosphere again. Like transmission spectroscopy, passing through the atmosphere imprints the signature of the chemical composition, but it can be modulated again in different ways (for example, if there is a mismatch in the planet and star's rotation rates, https://arxiv.org/abs/2201.03600[1][2]). Reflection obviously only works for the dayside of the planet, but like emission, it doesn't require a transit geometry.
1: https://arxiv.org/abs/2201.03600
2: https://arxiv.org/abs/2201.03600
There's nothing here!