An infrared, rotating, rotationally shearing interferometer may be used for a detection of a potential planet orbiting around a nearby star. We derive an expression for the signal, generated by a star and its faint companion, and detected by a rotationally shearing interferometer. It shows that the planet signal may be detected, despite the presence of a much larger star signal, because the planet produces a faint modulation superimposed on a large star signal when the aperture rotates. In the particular case of a rotating, rotationally shearing interferometer, the argument of the cosine term is shown to depend only on the planet and observational parameters. However, the amplitude of the modulation term in the interferometric signal is shown to be proportional to the star intensity.