The star moved by a human-scale speed
In 1995, Michel Mayor and Didier Queloz found a repeating change in the light of 51 Pegasi, a star broadly similar to the Sun. The star approached and receded with a velocity half-amplitude of about 59 metres per second, around two hundred kilometres per hour. The speed was familiar; the precision needed to measure it across interstellar space was extraordinary. A cycle lasted only 4.23 days.
Their interpretation was an unseen companion pulling the star around their common centre of mass. The companion’s minimum mass was about half Jupiter’s, yet it orbited far closer to its star than Mercury does to the Sun. That combination made the discovery surprising. A giant planet had turned up where the familiar architecture of our Solar System gave little reason to expect one.
The team announced the result in October. The discovery paper, “A Jupiter-mass companion to a solar-type star,” appeared in Nature on 23 November 1995. Its result concerned the first confirmed planet around a Sun-like star; planets around a pulsar had already been reported in 1992.
Thousands of dark lines moved together
The observation used ELODIE, a fibre-fed spectrograph at the Haute-Provence Observatory in France. A spectrograph spreads starlight by wavelength. At many wavelengths, atoms in the stellar atmosphere absorb light, leaving dark lines. Motion along our line of sight shifts the whole pattern: toward shorter wavelengths when the star approaches and longer wavelengths when it recedes.
This is the Doppler effect, the same broad idea that changes the pitch of an approaching siren. Here it was measured in light. The shift was so small that relying on one line would waste much of the available information. The analysis combined the positions of roughly five thousand absorption lines into one averaged feature.
A calibration lamp monitored instrumental changes during an exposure. The fibres also helped stabilize how light entered the spectrograph. These details mattered because movement within the instrument can mimic movement of the star. The discovery paper compared stars in the survey that appeared constant and found no instrumental drift large enough to explain 51 Pegasi’s behaviour.
The instrument measured the component of motion toward or away from Earth. It did not have to resolve the planet next to the bright star. In effect, the star supplied both the light and the visible response to its companion’s gravity.
Sources: [1]
The same cycle returned on later nights
The first observations of 51 Pegasi began in September 1994. A short-period orbit was fitted in January 1995. Intensive sequences in July and September followed the motion on consecutive nights. When the observations were placed at corresponding points in a 4.23-day cycle, the same rising and falling velocity pattern emerged.
Table 1 of the original paper gives a circular-orbit solution with a period of 4.2293 days and a half-amplitude of 0.059 kilometres per second, each with a quoted fitting uncertainty. It used 35 measurements, with a residual scatter of 13 metres per second. The full rise from the most negative velocity to the most positive is twice the half-amplitude.
A star can also shift its apparent velocity when dark spots rotate into view or its atmosphere moves. Mayor and Queloz examined the star’s low activity and the expected rotation timescale when judging those alternatives. Later researchers tested the shapes of the spectral lines as well as their positions. The planetary interpretation survived that continuing scrutiny.
The final version of the discovery paper also noted independent confirmations from other observatories after the October announcement. A repeating pattern measured with independent instruments is much harder to attribute to one spectrograph’s quirks.
The orbit gave a minimum mass
Both bodies move around their common centre of mass. A massive star travels around a small orbit; a lighter planet travels around a larger one. Measure the star’s speed and the period, adopt a stellar mass, and Newtonian gravity connects that motion to the companion’s mass.
One piece of geometry remains hidden in radial velocity alone. An edge-on orbit sends the largest part of the star’s motion toward and away from us. A more face-on orbit hides more of the same motion in the plane of the sky. To produce a given measured velocity from a more face-on orbit, the companion has to be heavier.
The original solution gave a minimum mass of 0.47 Jupiter masses, with a quoted uncertainty of 0.02 Jupiter masses. “Minimum” is essential: it corresponds to the most favourable viewing inclination. The paper also discussed additional assumptions about stellar rotation when estimating how much larger the mass might be. Those assumptions supply extra information beyond the measured velocity cycle.
Sources: [1]
A close giant changed the search
The inferred orbital separation was about 0.05 astronomical units, roughly eight million kilometres. An astronomical unit is the mean Earth–Sun distance. At that separation a year lasts four days, and the companion is intensely heated by its star. Such close gas giants became known as hot Jupiters.
The observation encouraged astronomers to search without insisting that every planetary system resemble ours. It also posed a formation question: how does a giant planet come to occupy such a tight orbit? The velocity curve measured the present orbit; explanations involving formation and later orbital evolution needed further theory and observations.
The 2019 Nobel Prize recognized Mayor and Queloz for discovering an exoplanet orbiting a solar-type star. The discovery’s most durable lesson sits in the measurement itself. A planet too faint to see separately can still reveal its mass and orbit by making the star move.