Analyzing the vibration signals of planetary gearboxes has become critical for preventing failures. However, due to the complex mechanical kinematics and varying operating conditions, such as speed and torque, identifying faults in planetary gears through vibration analysis remains challenging. The time-varying nature of the fault characteristics of planetary gearboxes further compounds the difficulty of diagnosis. Nonetheless, time-frequency analysis methods are capable of handling non-stationary signals, providing a potential solution. In this study, we propose a novel indicator that exploits time-frequency representation to detect anisotropy in planet gears. After applying some pre-processing techniques, the signal is filtered around specific fault frequencies and segmented into one revolution of the planet carrier. The smoothed pseudo Wigner-Ville distribution is applied to each signal section, and their mean is calculated. The proposed indicator is based on the averaged time-frequency maps obtained for each signal and is correlated to the similarity between two adjacent maps. The effectiveness of the proposed indicator in detecting anisotropy in planet gears is demonstrated through experiments on two datasets containing planetary gearbox vibration signals, with and without anisotropic planet gears.