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Planetary gear vibration is a major source of noise and may lead to fatigue-induced failures in bearings or other drivetrain components. Gear designers use mathematical models to analyze potential designs, but these models remain unverified by experimental data. This paper presents experiments that completely characterize the dynamic behavior of a planetary gear by modal testing and spinning tests under operating conditions, focusing on the independent motion of planetary components. Accelerometers are mounted directly to individual gear bodies. Rotational and translational accelerations obtained from the experiments are compared to the predictions of a lumped parameter model. Natural frequencies and modes agree well. Forced response also agrees well between experiments and the model. Rotational, translational, and planet mode types presented in published analytical research are observed experimentally.