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We consider the time-frequency analysis of a scattered light curve of adirectly imaged exoplanet. We show that the geometric effect due to planetaryobliquity and orbital inclination induce the frequency modulation of theapparent diurnal periodicity. We construct a model of the frequency modulationand compare it with the instantaneous frequency extracted from thepseudo-Wigner distribution of simulated light curves of a cloudless Earth. Themodel provides good agreement with the simulated modulation factor, even forthe light curve with Gaussian noise comparable to the signal. Notably, theshape of the instantaneous frequency is sensitive to the difference between theprograde, retrograde, and pole-on spin rotations. While our technique requiresthe albedo map to be static, it does not need to solve the albedo map of theplanet. The time-frequency analysis is complementary to other methods whichutilize the amplitude modulation. This paper demonstrates the importance of thefrequency domain of the photometric variability for the characterization ofdirectly imaged exoplanets in future research.

Frequency Modulation of Directly Imaged Exoplanets: Geometric Effect as a Probe of Planetary Obliquity