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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]

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 and Didier Queloz began observing 51 Pegasi in September 1994 and revisited its repeating Doppler spectroscopy signal in 1995. Their circular fit used 35 measurements: orbital period $4.2293\pm 0.0011$ days, stellar radial-velocity semi-amplitude $59\pm 3\mathrm{m}{\mathrm{s}}^{-1}$, and residual scatter $13\mathrm{m}{\mathrm{s}}^{-1}$ after epoch-dependent velocity offsets. Apparently stable comparison stars checked the instrumental zero point. Low chromospheric stellar activity and an inferred roughly 30-day stellar rotation period argued against starspots; light curves and the spectral line bisector tested stellar pulsation. These checks favoured orbital motion but had finite sensitivity. The reported $M_p\sin i=0.47\pm 0.02$ Jupiter masses was not a true mass. Stronger upper bounds used stellar-spin information and assumed spin–orbit alignment, limiting a nearly face-on binary star interpretation conditionally.^[ref]

The hot Jupiter matters both for planetary migration and for how exoplanets are found. At fixed small companion mass, orbital inclination and orbital eccentricity, the stellar radial-velocity semi-amplitude scales as $K\propto P^{-1/3}$. Comparing chosen orbital periods of $4$ days and $12$ years gives an amplitude ratio of about $10.3$: a close companion can be much easier to detect and complete many orbits during one observing season. Finding a giant planet close to its star also asks how its present orbit relates to where it formed. Doppler spectroscopy supplies a dynamical constraint; it does not by itself establish a formation pathway or measure the exoplanet's radius.

Built by André Baranne, Michel Mayor, Didier Queloz and colleagues, the ELODIE fiber-fed spectrograph carried light from Haute-Provence's 1.93-metre telescope through optical fibers. Its echelle grating dispersed light in high diffraction orders; an optical prism and grism cross-disperser separated them perpendicularly on a charge-coupled device. This cross-dispersed echelle spectrograph achieved spectral resolving power $R=42000$. A simultaneous thorium reference tracked instrumental drift. Wavelength calibration mapped detector positions to wavelengths; spectral cross-correlation combined many spectral lines into a fitted profile. A barycentric velocity correction removed the observer's motion.^[ref]

Why can a spectrograph measure a Doppler shift much smaller than one spectral resolution element? Resolution concerns separating neighbouring features, whereas locating the centre of a known profile uses information across that profile and across many spectral lines. For a chosen example, at $\lambda =500\mathrm{nm}$ a recession velocity of $60\mathrm{m}{\mathrm{s}}^{-1}$ givesAt $R=42000$, one spectral resolution element corresponds to $c/R\simeq 7.14\mathrm{km}{\mathrm{s}}^{-1}$, about $119$ times larger. A well measured centre can move by a small fraction of that width. More photons and more useful spectral lines improve the estimate, but do not remove wavelength calibration errors or stellar activity.

A simple mathematical picture of spectral cross-correlation makes the readout concrete. Define $x=c\log (\lambda /{\lambda }_0)$, with fixed reference wavelength ${\lambda }_0$, and let $d(x)$ be continuum-normalized line absorption. For small velocities, a common Doppler shift approximately translates $d(x)$. A positive absorption mask $m(x)$ then givesIf $d(x)=m(x-v_r)$, the maximum occurs at $u=v_r$; a convention correlating the received observed radiative flux instead produces a dip. A fitted correlation centre is an effective collective line position, not a claim that each individual spectral line has been resolved into a narrower feature. The lamp tracks instrumental drift, while the barycentric velocity correction removes the observer's changing line-of-sight velocity; neither removes changes in the star itself.

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]

One half went to James Peebles for theoretical discoveries in physical cosmology. One quarter each went to Michel Mayor and Didier Queloz for discovering an exoplanet orbiting a solar-type star.^[ref]