51 Pegasi is a Sun-like star whose radial-velocity variations led Michel Mayor and Didier Queloz to infer a close planetary companion in 1995. The discovery of 51 Pegasi b showed that a giant planet can orbit much closer to a normal star than the giant planets of the Solar System do.
The measured signal was the star's changing line-of-sight velocity, inferred from shifts in its spectrum. The planet was inferred through the star's reflex motion, rather than seen as a resolved bright point. Orbital inclination enters the mass inference, so a radial-velocity amplitude alone generally constrains a minimum mass.[ref]
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