The discovery of extrasolar planets is one of the most dramatic and important findings in the history of Astronomy. For their roles in this development, Professor Michel Mayor of the University of Geneva and Professor Geoffrey Marcy of the University of California at Berkeley are awarded the Shaw Prize in Astronomy for 2005. Professor Mayor is being recognized for his discovery of the first planet around a normal star other than our own Sun. Professor Marcy is being recognized for his characterization of the masses and orbital properties of a large and statistically meaningful number of extrasolar planets and planetary systems.

The process of explaining the motions of the planets in our own Solar System spawned the Copernican Revolution. The later realization that stars are suns in their own right led to speculation that other planetary worlds might orbit distant suns. These speculations gained concrete foundation in 1995 when Mayor and Didier Queloz found a planet to orbit about the sunlike star, 51 Pegasi, with a 4-day period. The short period came as a shock because it meant that this giant planet, with about half of the mass of Jupiter, orbits its host star at a radius that is only 1% of the distance of Jupiter about our own Sun. Existing theories of planet formation held that giant planets should form in a rotating nebular disk surrounding a newly born star only beyond a so-called ˇ§frost lineˇ¨ where water vapor freezes and condenses out as solid ice. The exact location of the ˇ§frost lineˇ¨ at the time of giant planet formation is uncertain, but it should not have been much smaller than the present location of Jupiter, which is the innermost of the giant planets. The discovery of Mayor and Queloz electrified the field and spurred a huge increase in research activity on extrasolar planets, which has not abated to this day, ten years later.

Together with Paul Butler, Marcy quickly confirmed the reality of the 51 Pegasi planet using the method of precise Doppler measurement that he had developed for the purpose of extrasolar planet detection over many years. Seventy of the next 100 extrasolar planets discovered were found by his team. Many of these planets had distinctly non-circular orbits, in contrast with the orbits of known planets in our own Solar System. Several reside in systems where there is more than one giant planet, with multiple planets in a given system often having orbital periods that bear integer relationships to one another. As the observational sensitivity and the understanding of systematic errors steadily improve, planets with masses more like Saturn and Neptune are being found, instead of only the Jupiter-like bodies that characterized the initial discoveries. Significantly, there are no bodies that have masses appreciably in excess of 10 Jupiter masses. In a few cases, the orbital plane lies sufficiently edge-on to the observational line of sight that the giant planet transits in front of the host star, blocking out a small part of the starˇ¦s light, which allows an estimate of the radius of the transiting planet. A major surprise occurred in the latest such case where the combination of mass and radius implies that the planet has a dense rock-ice core that is seventy times the mass of the Earth, a much larger value than holds for any of the giant planets in our own solar system.

As the baseline for good orbital-period determinations passes the one decade mark, giant planets are being discovered with orbital distances from their host stars more akin to the case of our own Solar System. The stage is now being set for answering one of the most intriguing questions ever posed in science: how typical or atypical is our own Solar System? For moving this question from the dreamy realm of philosophical speculation to the concrete field of empirical fact, Michel Mayor and Geoff Marcy are justly honored tonight with the award of the Shaw Prize in Astronomy for 2005.