Modern searches for extrasolar planets attempt to find them by looking for the characteristic dip in brightness that happens when a planet passes in front of its parent star. But if you want to know what the planet’s atmosphere is made of, you have to watch for the second, smaller dip that happens when a planet passes behind its star. This eclipse blocks the star’s reflected light; by studying the reflection, astronomers can piece together the molecular composition of the atmosphere.

For more on how scientists are looking at exoplanet atmospheres to seek signs of extraterrestrial life read "Astronomers Search for Signs of Life in the Skies of Distant Expolanets" in the July issue of Scientific American.