The Wobble Method
With over 60 extraterrestrial planets found to date, the wobble method has been the most successful in finding these distant planets. Through careful observation of a star, astronomers are able to detect if the star is slightly moving, or wobbling. This wobbling lets astronomers know that there is some force tugging on the star causing it to move. This tug is the gravitational force from an orbiting planet.
The force of gravity needed to ‘wobble’ the star back and forth must be very strong. For such a gravitational force the planet must be very large, about the size of Jupiter. Because the planets have to have such a strong force to move a star, we have yet to detect any Earth sized planets. Until our instruments are sensitive enough and technology advanced enough, we never will. Astronomers are able to tell that a planet is Wobbling through Doppler shifts in the stars spectrum. The Doppler Effect is the apparent change in wave length of radiation caused by the motion of the source. Basically if the star is moving towards the Earth then the wavelengths are going to be blue-shifted, towards the blue end of the spectrum. If the star is moving away from Earth the wavelengths will be red-shifted. Like in Dr. Alexander’s example of the train; when it is coming towards us it is higher pitched while when it is moving away from us it will be lowered pitched.