The Copernican Revolution spawned the notions that the stars might be suns in their own right and that planetary worlds might orbit such suns. The idea that stars are suns gained plausibility through the investigations by Wollaston and Fraunhofer who showed that the Sun and many stars exhibit similar spectral signatures. It became established scientific fact in 1838 when Bessel, Struve, and Henderson determined, respectively, the distances to 61 Cygni, Vega, and Alpha Centauri, and calculated that they have absolute brightness similar to the Sun. The idea that planets orbit other normal stars remained in the realm of philosophical speculation until 1995, when Michel Mayor and Didier Queloz found, and Geoffrey Marcy and Paul Butler confirmed, the existence of the first planet around a sunlike star, 51 Pegasi. Since that exciting result a decade ago, Marcy and Mayor have led the two most productive and successful research groups searching for extrasolar planets.

Both groups detect extrasolar planets by finding small periodic variations in the radial velocity of the host star. Along with a number of other groups, they worked in relative obscurity for many years, in large part because most astronomers believed that these radial-velocity oscillations would be too small to detect in plausible planetary systems. This belief was based on the analogy with our own solar system, resulting in the theoretical prejudice that massive planets could not form close to their host star, where they would generate the largest reflex motion in the central body.

These prejudices were demolished by the discovery in 1995 of a 0.47 Jupiter-mass object orbiting the star 51 Pegasi with an orbital period of 4.23 days and therefore having an inferred distance from the central star that is one percent of the orbital distance of Jupiter from the Sun. Since that watershed finding, nearly two hundred planets have been discovered, the majority by the radial-velocity techniques used by Marcy and Mayor. The discoveries have revealed a remarkable variety of important and unexpected features of extrasolar planet systems, such as: (1) giant planets are found in orbits that lie very close to the host star; (2) the frequency of planets is a strong function of the abundance of heavy elements such as iron in the star; (3) many multiple-planet systems lie in orbital resonances; (4) the orbital eccentricities of extrasolar planets are far larger than the eccentricities of the planets in our own solar system; and (5) the distribution of planet masses is roughly constant per logarithmic interval, with a sharp cutoff at about 10 Jupiter masses. None of these results is well-understood theoretically. All came as surprises. All provide fundamental yet mysterious clues to the processes of planet formation that theorists worldwide are scrambling to decipher.

The discovery of extrasolar planets is thus one of the most dramatic and important scientific results of the last several decades. Marcy and Mayor stand at the forefront of these developments, an accomplishment for which they richly deserve the award of the Shaw Prize in Astronomy for 2005.

Astronomy Selection Committee
The Shaw Prize

2 September 2005, Hong Kong