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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]

André Baranne designed precision astronomical spectrographs, including the ELODIE spectrograph, which was used for Michel Mayor and Didier Queloz's discovery of the planetary companion to 51 Pegasi. The instrument combined broad spectral coverage with precise radial velocity measurements.

The scientific goal was to distinguish tiny stellar Doppler shifts from instrumental changes. Its design and calibration supported long-term velocity stability sufficient for detecting giant planetary companions. The original instrument paper reports the construction, data treatment and measured performance, making the observing hardware part of the explanation of the discovery.^[ref]

Didier Queloz and Michel Mayor detected a planetary companion to the solar-type star 51 Pegasi through periodic changes in its radial velocity. Their 1995 observations used the ELODIE spectrograph to measure the stellar Doppler shift.

The signal repeated in about four days, implying a close companion rather than a distant analogue of Jupiter. The inferred minimum companion mass was planetary. Because radial velocity measures only the line-of-sight component of the reflex motion, the orbital inclination remains part of the mass inference.^[ref]

Michel Mayor develops precise stellar radial velocity measurements and their use in searches for unseen companions. With Didier Queloz, he reported a planetary companion to 51 Pegasi in 1995.

The host star's regularly changing Doppler shift revealed its reflex motion around a shared center of mass. The approximately four-day orbital period showed that a giant planet could exist much closer to its host star than the giant planets of the Solar system. The discovery made this previously unexpected orbital arrangement an observational constraint on planet formation.^[ref]