As the planet transits across its star a small portion of the starlight will filter through the atmosphere before reaching our telescopes. Imprinted on this light are the fingerprints of different atoms and molecules in the planets atmosphere. We measure the transmission spectra of exoplanets to determine their composition and gain an understanding of their origin and evolution.

When the planet passes behind the star from our point of view we can measure the difference between the light before and after the planet is eclipsed. The difference between the eclipsed light (just the star) and the light just prior/after (star+planet) will be just the planet emission. Depending on the wavelength of light we will measure the reflection or emission which can tell us about the thermal structure and the albedo.

A phase curve is the measure of the light from the combined star + planet system over a full or half orbit of the planet around the star. As the planet orbits the star different phases of the planet will appear which contribute more or less light to the measured total flux. Measurements of a planets phase curve can inform us about the redistribution of energy in the planets atmosphere, and changing thermal structure from day-to-night side.