Since the discovery of the first planet outside the solar system in 1995, the detection of exoplanets has been an attractive and engaging scientific field. This article intends to present briefly the radial velocity method for detecting the presence of an unseen planet orbiting a star. Based on an experimental setup, the presentation resorts to the analogy between sound waves and light waves. In particular, they can undergo the Doppler effect from which the radial velocity can be determined. Because the Doppler effect is easier to observe for sound waves than for light waves, a Bluetooth speaker simulates a star. It rotates in a horizontal circle with a constant angular speed, while emitting at two user-selected frequencies, simulating two spectral lines of a star. From the analysis of spectrograms, the radial velocities are deduced. Very good agreement is found between the experiment and theory.

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As illustrated in Fig. 3, the radial component vr is the projection of the velocity v on the r-axis. The magnitude of vr is maximum when the angle between v and the r-axis is zero or π. In this case, the sides R and r are perpendicular to each other; they form the legs of a right triangle, with L as hypotenuse. This allows to write cosωt=R/L=m. The radial velocity is vr=+v when cosωt=m and the source moves away from the microphone. Its value is vr=v when cosωt=m and the source moves toward the microphone.
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An asymmetry in the frequency shift should appear in the experimental results of Fig. 4. However, the bandwidth of 43Hz (also the frequency uncertainties) is quite large for detailed investigation. For instance, using f=f0(1/1+(vr/vs)), with vr/vs±0.03, for f0=15000Hz, the minimum frequency is fmin14563Hz corresponding to a shift of 437Hz with respect to f0 whereas the maximum frequency is fmin15464Hz corresponding to a shift of 464Hz. The approximation of Eq. (1) eliminates this asymmetry. Experimentally, a double frequency resolution would make that asymmetry detectable.
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