This animated video demonstrates the transit method for detecting extrasolar planets.
A graph shows a temporary dip in the brightness of an observed star. This is explained by showing an orbiting planet passing across the face of its star.
Astronomers measure the brightness of many stars.
They are looking for planets that are orbiting stars other than our sun, which we call extrasolar planets.
As the data from each star is plotted on a graph, the astronomers look for a small dip in the brightness of a star.
If they find a dip in brightness, it could be caused by a planet orbiting the star.
The planet cannot be seen directly, but its effect on the light from the star can be measured.
During most of a planets orbit around a star, from earth we can not notice any difference. A graph of the stars brightness stays level at 100%.
But when the planet crosses in front of its star, in an event called a transit, it blocks a small amount of the stars light, this shows as a dip in the graph.
If the dip in brightness happens repeatedly astronomers can be fairly sure that they have found a planet orbiting the star.
This transit method is an important tool for detecting extrasolar planets.