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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]

51 Pegasi b is the giant exoplanet whose 1995 discovery established a planet around a Sun-like star. It is a hot Jupiter: its roughly 4.23-day orbital period and about 0.05 astronomical unit separation differ markedly from Jupiter's distant orbit. Earlier confirmed exoplanets had already been found around a pulsar; the host category is part of this discovery's significance.^[ref]

Michel Mayor and Didier Queloz used the ELODIE spectrograph to measure periodic Doppler shifts in the host star's spectrum. The star's motion around the shared centre of mass produces alternating approaching and receding velocities even though the planet is unresolved. For a nearly circular orbit and a planet much less massive than its host,The measured velocity amplitude and period therefore constrain $M_p\sin i$, originally about half Jupiter's mass, rather than independently determining inclination and true mass. Checking spectral-line changes also helps distinguish orbital motion from changing stellar line profiles.^[ref]