How to find an extrasolar planet

An extrasolar planet is a planet that revolves around a star other than our sun.  Because scientists have not found life on any planet yet in our solar system they began to look for other planets in other solar systems in our galaxy.  The problem with finding planets outside of our galaxy is (a) that they are very small compared to stars and (b) planets do not emit their own light, they only block and reflect light from other stars so general when looking through a telescope they are out-shined by the star they revolve around.  Astronomers are using a few different techniques to find extrasolar planets.  In order to understand these techniques you first must understand a few basic concepts.

Some matter in the universe emits or reflect light.  It may be visible and it may not, but all matter that we currently know about in the universe today react with light, all other matter we call dark matter.  For now, we will only be concerned with matter other than dark matter.  When an object is moving away from us or towards us, the light that is coming off of it is redshifted meaning that if the object is moving away the wavelengths will be longer and shifted more towards red  and if it is moving towards us then the wavelengths will be shorter and shifted more away from red.

star "wobble"

Another concept that astronomers use is a variation of a concept that most people learn as very young children.  Most people learn that the planets in a solar system revolve around their star, however this is not completely true.  In fact the planets in a solar system actually revolve around the center of mass of the entire solar system.  In addition, the central star of a solar system also revolves around the central mass of the solar system.  This was difficult to discover because the sun is very close to the center of mass of the solar system (about 1 of its own diameters away).  The sun will rotate around the central point at the same rate that the next largest mass, which in our solar system is Jupiter.  Jupiter is about 1/1000 the mass of the sun which means that the sun's orbital radius will be about 1/1000 that of Jupiter's.  A star will only revolve around the center of mass of its solar system if it has other masses orbiting it, when a star revolves it is called a "wobble".

Because the central star of a solar system is rotating about the center of mass of the system and all systems are moving away from us, this means that at some points in time the star will be moving away from us at a faster rate than other times.  Thus the light coming from the star will be redshifted more during half of its orbit and less during the other half.  The speed that a typical star wobbles is only about 50mph, however the typical star is also moving away from us at a speed of a few hundred thousand mph.  Because such a small difference in speed needs to be detected, scientists had to develop new methods of detecting differences in redshift.  This is the key to finding extrasolar planets and is one method used by astronomers.