October 6, 1995

51 Pegasi b

Mayor and Queloz announce evidence for 51 Pegasi b in Florence

The planet's entire year lasted 4.23 Earth days — a result so unexpected that many astronomers initially doubted the planetary interpretation.

6 October 1995, at a conference in Florence

Michel Mayor and Didier Queloz told an astronomy conference in Florence on 6 October 1995 that periodic motion of the Sun-like star 51 Pegasi indicated an orbiting planetary companion. Their evidence came from radial-velocity observations with the ELODIE spectrograph at France's Haute-Provence Observatory.

The star's spectral lines shifted with a period of 4.2293 ± 0.0011 days and a semi-amplitude of 59 ± 3 metres per second. That implied a minimum companion mass of 0.47 ± 0.02 Jupiter masses at a separation the discovery paper put at about 8 million kilometres.

An announcement, not yet a confirmation

Eighteen months of ELODIE observations had produced the repeatable four-day Doppler signal, and checks of stellar rotation, pulsation and activity made non-orbital explanations unlikely enough for the pair to announce a planetary interpretation. Nothing was seen directly; the planet was inferred from the star's wobble.

The claim met with scepticism. Geoffrey Marcy and R. Paul Butler redirected an already scheduled Lick Observatory run to 51 Pegasi and, over four nights from 11 to 15 October, independently reproduced the signal — then posted their velocity plot on the still-new World Wide Web.

What was, and was not, a first

This became the first reliably identified exoplanet orbiting a normal, solar-type main-sequence star. Planets around the pulsar PSR B1257+12 had already been announced in 1992.

The peer-reviewed paper did not appear that day: Nature published it on 23 November 1995. Because the orbital inclination was unknown, the radial-velocity method established only a minimum mass.

Sources

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What the sources leave uncertain

NASA, the Nobel Committee and the NCCR PlanetS history independently give 6 October. A few secondary timelines give 5 October, apparently locating the announcement one day earlier within the 3–6 October conference; the stronger institutional record supports 6 October.
On 6 October the object was an announced planetary interpretation, not yet an independently confirmed planet. Independent confirmation followed roughly nine days later.
Radial velocity originally established only a minimum mass because the orbital inclination was unknown.