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How do radial velocity measurements help astronomers discover extrasolar planets?

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Habitable zone The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.

The radial velocity method is a primary technique used by astronomers to discover extrasolar planets by measuring the subtle motion of a host star caused by the gravitational influence of its orbiting planets.12

As a planet orbits a star, both bodies revolve around their common center of mass. This causes the star to "wobble" slightly, moving toward and away from Earth. Astronomers detect this motion by observing a Doppler shift in the star's spectral lines: the light shifts toward shorter wavelengths (blueshift) as the star moves toward the observer and toward longer wavelengths (redshift) as it moves away.13

By analyzing these periodic changes in radial velocity, astronomers can determine the orbital period and the size and shape of the planet's orbit. Furthermore, this method allows scientists to calculate the minimum mass of the planet. While this technique is most sensitive to massive planets orbiting close to their stars, it remains a vital tool in astrobiology for identifying planetary systems, including those that may contain worlds within a star's habitable zone where liquid water could potentially exist.12