Exoplanets are light years away, hidden by their parent stars, and barely detectable.  Yet even though most have never been directly imaged, we can study the light from the parent star as the planet passes in front of it, and use this information to learn about the planet’s size and composition, especially if it has an atmosphere.  Once you know a little bit about how big and dense a planet is, and the major elements that form it’s crust and atmosphere, you can do a lot of Chemistry to figure out what it should be made of and how these...