The transit-method of exoplanet-detection is pretty straightforward, conceptually: First you find a distant star. If the light from it dims, it could be due to a planet in its orbit that has passed between it and your telescope. But how do scientists determine how big these distant planets are, what they're made of, or what their atmospheres are like?
By observing periodic variations in the parent star's brightness and color, astronomers can indirectly determine an exoplanet's distance from its star, its size, and its mass. But to truly understand an exoplanet astronomers must study its atmosphere, and they do so by splitting apart the parent star's light during a planetary transit.
[Spotted on The Kid Should See This]