Figure 8-17: Discovering Planets from a Star’s Wobble: 51 Pegasi As a star wobbles, it moves toward us and away from us, with a varying radial velocity that is measured through the Doppler shift. The shape of the curve indicates the planet’s semimajor axis and also the planet’s mass. However, the planetary mass is actually a minimum mass because an orbital plane tilted out of our line of sight would require an even larger mass to produce the observed Doppler shift.