Identifying atmospheres around exoplanets

How Well Can Astronomers Study Exoplanet Atmospheres? (Universe Today) 

At this point, how well can astronomers study the atmosphere around an exoplanet? 

Currently, there are only a handful of methods researchers can use to make estimates of exoplanet atmospheres. Interestingly enough, one method makes use of the light coming from the host star. The basic principle is that the light from a star can be analyzed both before and after an exoplanet crosses in front of the star. By comparing the spectrum from the host star, and the spectrum of an exoplanet, the tell-tale signs of atmospheric contents can be detected.