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# What is the Radial Velocity Method?

## Also known as the Doppler-shift Method)

The radial velocity method, or RV method, for detecting exoplanets involves measurements of the star's velocity by means of monitoring variations in the Doppler shift of specific signatures in the light of the parent star. In the majority of cases, neither the planet or the star can be directly imaged (meaning that the star's motion cannot be spatially observed because the system appears to be too small). The method is based on the fact that the star wobbles as the planet orbits.

There are no “pillars” in space (or anywhere else for that matter) that hold things in a fixed position. Everything is relative. So a planet that is orbiting a star does not actually orbit a star that is stationary because there is no such thing as stationary without reference to another object. What happens is that the planet and star are in motion around their common “center of mass” point. The motion (with respect to the center of mass) of each of the objects in the orbit is commensurate with its mass. The higher the mass of the star, the smaller is the magnitude of the motion (or “wobble”) of the star. On the other hand, a more massive planet would cause a larger magnitude of motion compared to a less massive planet. A planet that is closer to the star would also produce stronger variations than one that is situated further from the star.

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The star's velocity can be measured at various points in time and if the star is hosting an orbiting planet, the variations in velocity with time will show a specific form that depends on the various orbital parameters and properties of the planet and star. A model can be constructed to model what is called the velocity amplitude function, with adjustable parameters. The parameters are adjusted to fit the data. This may not however, give completely unambiguous or unique solutions and the resulting uncertainties are limitations of the method, which nevertheless is very powerful. In general the method yields only a lower limit on the planet's mass because the inclincation of the plane of the orbit with respect to the observer is an unknown. If this angle can be constrained by independent means, then the planet's mass can be better constrained. The RV method is biased towards finding the most massive exoplanets. The method does not yield the size (radius) of the exoplanet.

File under: Explanation of the radial velocity (RV) method of detection of exoplanets. Also known as the Doppler-shift method.