The chemical composition of exoplanetary atmospheres can tell us much about the physical (and potentially life bearing) conditions on these worlds. The most successful method for measuring chemical composition of an exoplanetary atmosphere is the transit spectroscopy method. When an exoplanet passes in front of its host star from our point of view, a small fraction of the stellar light passes through the exoplanetary atmosphere, where molecules absorb light of some wavelengths while light of other wavelengths can pass through unhindered. By measuring the fraction of stellar light able to penetrate the atmosphere at different wavelengths, the chemical composition of the atmosphere can be inferred.
However, the fraction of stellar light that passes through a transiting exoplanet's atmosphere is very small, which constrains both the telescopes/instruments that can be used and the planetary system that can be observed. The on-going search for exoplanets helps us to find the best-suited systems for transit spectroscopy.