This is an artist's impression of a Jupiter-sized planet passing in front of its parent star. Such events are called transits. When the planet transits the star, the star's apparent brightness drops by a few percent for a short period.
Through this technique, astronomers can use the Hubble Space Telescope to search for planets across the galaxy by measuring periodic changes in a star's luminosity. The first class of exoplanets found by this technique are the so-called "hot Jupiters," which are so close to their stars they complete an orbit within days, or even hours.
Artwork : NASA ESA and G. Bacon (STScI)