A Planet's Telltale Signature
This diagram illustrates how a planet passing in front of a star blocks a tiny amount of starlight. In this case, it's a planet crossing the face of a nearby star called HD 209458, located 153 light-years from Earth in the constellation Pegasus. The dip in the center of the diagram represents a 1.7 percent drop in starlight over a three-hour period as the planet moves across the star's disk. By watching this eclipse, astronomers can confirm a planet's existence.
NASA, T.M. Brown, D. Charbonneau, R.L. Gilliland, R.W. Noyes, & A. Burrows.