To introduce about extrasolar planets one important word we must know is eclipses. A common misunderstanding is that eclipses are only solar or luna eclipse. However, many phenomena are classified as eclipses rather than just the aforementioned occurrences. The proper definition of eclipse is 'The passage of one celestial body in front of another, cutting off the light from the second body'.
Extrasolar planets are difficult to find due to their distance, and the fact that the star they orbit tend to conceal the planets due to their sheer brightness and size relative to the planet. Quoted from a documentary 'The universe' in (discover channel or was it history channel) its like finding a flee in front of a headlight. However, it is possible to find it when an eclipse occurs. When a planet's orbit is positioned exactly at the astronomer's vantage point, a slight drop in the brightness of the sun at a specific point can be detected which suggests that a crossing planet is blocking out part of the sun where it is currently at as it travels. Of course there are many false alarms, but this method can enable astronomers to determine the size of the planets. Sadly most of the planets that are detected are huge gas planets which are big enough to cause a drop in brightness of us to detect.
Definition of eclipse: http://space.about.com/od/glossaries/g/eclipse.htm
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