Answer:
Center of mass
Explanation:
The Doppler technique is a good method for discovering exoplanets. It uses the Doppler effect to analyze the motion and properties of the star and planet. Both the planet and the star are orbiting a common center of mass. This means that the star and the planet gravitationally attract one another, causing them to orbit around a point of mass central to both bodies.
If we use the Doppler method to measure the period with which a star alternately moves toward and away from planet Earth (us) due to an orbiting planet, then we also know the: orbital period of the planet.
The Doppler effect is also referred to as Doppler shift and it can be defined as a change in wave frequency with respect to an observer who is moving relative to a wave source. Thus, it is an effect that is caused by a moving wave source with an apparent upward shift in frequency with respect to an observer.
In Astronomy, the Doppler effect is best used for detecting large extrasolar planets that are orbiting close to their stars, especially by observing the Doppler shifts in the spectrum of its parent star.
Furthermore, orbital period of a planetary body can be determined when the Doppler method is used to measure the period with which a star alternately moves toward and away (radial-velocity) from planet Earth (us) due to an orbiting planet.
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