The atmosphere of a planet holds the keys to make it habitable, so we need to look at them to figure if exoplanets are habitable. They are too far to send probes to measure them directly like we do with Mars or Jupiter’s moons, but scientists can study them from right here, looking at how they block light.
When infrared light (the invisible electromagnetic waves that also carry heat) hits the atoms in a molecule, it makes them vibrate. Depending on the chemical bonds that keep a molecule together and the geometry of its atoms, each molecule vibrates differently. The molecules absorb the light with the same frequency as those vibrations (the resonant frequencies) and let the rest through.
It’s kind of the opposite of what a musical instrument does. Instruments only produce sounds of the frequencies their shape resonates with; these sounds combine to build the instrument’s characteristic voice.
Just as we can distinguish the sound of a piano from that of a trumpet, it is also possible to tell molecules apart by shining light through samples and looking at the blocked light.
To study the atmosphere of an exoplanet, scientists measure what light comes from the star it orbits. Then measure it again when the planet is passing in front of it, so the light appeared filtered by the planet’s atmosphere.
Like a trained ear can make out all the instruments playing in an orchestra, the scientists can figure what molecules make up the planet’s atmosphere looking at the missing light.
Whether or not jazz really is about the notes you don’t play, searching for life in the universe surely is about the light distant exoplanets don’t play.