I have read that since roughly the mid 90's they have discovered nearly 350 exoplanets orbiting some 250 different stars. They do this using the doppler effect which looks at the spectrum of that star for a wobble in the spectral lines. As the planets orbits it pulls on the star causing it to move in a circular motion and as it moves towards us the light coming from it is compressed to the blue end of the spectrum. When ot moves away from us the light is shifted red. Much like as an ambulance approaches the the sound is shifted to a high frequency and as it passes it is stretched to a lower frequency. What a useful tool.
Another method requires the plane of the exoplanets orbit to be viewed from earth so that when the planet passes in front of the star its brightness dims and does so for the time it takes the planet to pass. The reduction in brightness is extremely small and the Keppler telescope launched in March this Yr will use this method to monitor 100000 stars with its 96 mega pixel camera to see how many dim or blink. from that they will roughly be able to tell how many stars in the Milky way have planets orbiting and what type of star has earth like planets. they say they will know this in about 3 Yr. Very exciting. I heard at a conference I attended for school that they can measure the speed of the wobble of a star to 1 meter per second.
