This episode of "Hubble at 25" uncovers Hubble's key role in the study of planets beyond our solar system. Thousands of "exoplanet" candidates have been discovered. While Hubble is not responsible for most exoplanet detections, it is able to examine the chemical compositions of their atmospheres. Since these planets are too far away to ever visit in the forseeable future, analyzing their atmospheres provide critical clues about the existence of life elsewhere in the universe.