If there are two planets, you may be able to detect that the periodic changes are not a pure sinusoid with a single frequency, but that there are two different beats. From the dynamics of the system, you can then estimate what the masses of the two planets have to be at their derived distances ( from their periods and Kepler's Third Law) to give the size of the perturbation seen. You proceed this way until, to your measurement accuracy, you can no longer detect any higher-order forcing terms. The above figure shows the spectroscopic data taken by Marcy and Butler at San Francisco State University and is from Planet Search . It shows the beating effect in the data for Upsilon Andromeda. The resulting planetary system solution looks like this:
Copyright 1997 Dr. Sten Odenwald
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