Michel Mayor

2000 Balzan Prize for Instrumentation and Techniques in Astronomy and Astrophysics

For his achievements that made it possible to detect the first planet around a star other than the Sun.

Born in 1942 in Lausanne, Michel Mayor is Professor in the Department of Astronomy at the University of Geneva. Director of the Geneva Observatory since 1998, he is the discoverer of the first extraterrestrial satellite orbiting around the star 51 Pegasi. This discovery – made public in early October of five years ago – was achieved thanks to the improvements made in spectrography instrumentation capable of measuring the velocity of a star and the perturbations in its motion, this being indirect proof of the presence of a planet. Using the same systems, since then a further twenty or so planets have been detected, some of them by Mayor himself.

Over several years Professor Mayor has developed a spectrograph for measurements of velocities of stars along the line of sight. It is thanks to this instrument, and its successive improvements, that Mayor has attained the extraordinary precision of a few meters per second in these velocities. This has led to the first detection of a planet outside our solar system by Mayor and his postgraduate student Didier Queloz in the year 1995 (*).

The discovery of a planet around the star 51 Pegasi (number 51 in the constellation of Pegasus) is the result of a long and patient development of observational techniques regarding the precise measurement of stellar velocities. Over more than twenty years Mayor and his colleagues have continuously refined Doppler-shift measurements of spectral lines by “cross correlation” techniques, giving line of sight components of stellar velocities. A first instrument called “CORAVEL”, with numerable applications in stellar and galactic kinematics, has enabled Mayor to publish a study of the properties of double stars in the solar neighbourhood (**). This study is the basic reference describing the orbital properties of double stars, where the more massive component is similar to the Sun.

This study already revealed the existence of bodies in orbit around stars with masses as small as some ten times the mass of the heaviest planet in our solar system, Jupiter – the hypothetical “brown dwarfs”. An object with a mass equivalent to eleven times that of Jupiter was discovered in collaboration with researchers at Harvard: and the road was open for search for companions even less massive – the giant planets.

In the early 1990s a new instrument for Doppler spectroscopy was conceived and developed by Mayor in collaboration with instrumentalists at the Observatoire de Haute-Provence. With the spectrograph “ELODIE” the precision was improved by a factor 20 and variations in stellar velocities of a few meters per second could be detected. The fantastic contrast between the luminosities of stars and planets (about one thousand million for the contrast between the Sun and Jupiter) combined with the small angular distance between the bodies has not yet permitted an image of an extrasolar planet. The road chosen by Mayor for detecting the hypothetical planets is that of an indirect detection – i.e. the perturbations introduced by the planet in the movement of the star. It is this road that led to the discovery of the first extrasolar planet. Within the months that followed the discovery of the planet (named 51 Peg b), other new planets have been discovered by various groups of researchers using similar techniques. The final proof that the velocity variations observed are due to perturbations from an orbiting planet was given when the dimming of the light from such a star due to the transit of a planet in front of it was observed (***).

The discovery of a planet around the star 51 Pegasi not only carries proof of the existence of extrasolar planets, but it obliges the astronomers to put into question the presently adopted scenarios for the formation of planetary systems, in particular our own. The discovery of 51 Peg b with its not foreseen characteristics (mass less than 0,45 of Jupiter’s mass, orbital period = 4,2 days, orbital radius 0,05 that of the Earth) has immediately stimulated research concerning the mechanisms able to explain an orbit with such a small radius.

At the beginning of this year about thirty extrasolar planets had been discovered, of which one third by Professor Mayor’s group. The special case of the planet 51 Peg b stands as the prototype for a class of planets, the “warm Jupiters”, where the planets have short periods and orbital radii less than 0,1 of the Earth’s orbit. The discovery of the first extrasolar planet will indeed be recorded in the history of astronomy.

(*) A Jupiter-mass companion to a solar-type star. M. Mayor, D. Queloz, Nature 378, 355-359, 1995

(**) A number of articles by Mayor and collaborators published in Astronomy and Astrophysics and Astronomy and Astrophysics Supplement

(***) Detection of Planetary Transits Across a Sun-like Star. D. Charbonneau, T.M. Brown, D.W. Latham, M. Mayor, Astrophysical Journal 529, Letter p. 45-48, 2000

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