Doppler Shift

Precise measurement of the velocity or change of position of stars tells us the extent of the star's movement induced by a planet's gravitational tug. From that information, scientists can deduce the planet's mass and orbit.

Why does a planet cause a star to sway? If a star has a single companion, both move in nearly circular orbits around their common center of mass. Even if one body is much smaller, the laws of physics dictate that both will orbit the center of the combined star and planet system. The center of mass is the point at which the two bodies balance each other.

The radial velocity method measures slight changes in a star's velocity as the star and the planet move about their common center of mass. In this case, however, the motion
detected is toward the observer and away from the observer. Astronomers can detect these variances by analyzing the spectrum of starlight. In an effect known as Doppler shift, light waves from a star moving toward us are shifted toward the blue end of the spectrum. If the star is moving away, the light waves shift toward the red end of the spectrum.

This happens because the waves become compressed when the star is approaching the observer and spread out when the star is receding. The effect is similar to the change in pitch we hear in a train's whistle as it approaches and passes.

The larger the planet and the closer it is to the host star, the faster the star moves about the center of mass, causing a larger color shift in the spectrum of starlight. That's why many of the first planets discovered are Jupiter-class (300 times as massive as Earth), with orbits very close to their parent stars.