Though the discovery gave astronomers hope, as the first evidence of planets beyond the solar system, this was nothing like the solar system, nothing to encourage speculation about extraterrestrial life. No one could conceive of the conditions for life, if they existed, surviving the cataclysm of a stellar explosion.
The first detection of a planet around a normal shining star, 51 Pegasi, was made in 1995 by Dr. Michel Mayor of the Geneva Observatory in Switzerland, and was soon confirmed by Dr. Marcy.
This and subsequent solitary planet discoveries prepared astronomers for the possibility that the first multiple-planet system they observed would differ considerably from the solar system. Most of the planets discovered were larger than Jupiter. The ones traveling circular orbits, like solar planets, hugged close to the searing heat of the star, closer than Mercury comes to the Sun. The others follow eccentric, oval-shaped orbits, unlike solar planets, but they remain at distances from their stars corresponding roughly to those of Venus and Earth from the Sun.
The problem has become even more complex for theorists now that they have encountered three such huge planets in one system, one planet in a close circular orbit and two in elliptical orbits. It had been generally assumed that such large bodies, presumably gas spheres like Jupiter, could have coalesced only in the cold regions far from their star.
In an article prepared for publication later this year, Dr. Marcy, Dr. Mayor and Dr. William D. Cochran of the University of Texas at Austin wrote, ''This 'pile-up' of planets near stars suggests that inward orbital migration occurred after formation,'' probably through gravitational interactions with other planets that, in reaction, were nudged outward into distant orbits where they are not yet detectable.
The astronomers also concluded that the eccentric orbits ''arise from gravitational interactions with other planets, stars, or the protoplanetary disk'' in their early history.
In the nascent field of comparative planetary systems, the new discovery has already given scientists a few insights. They are more certain than ever that planetary systems are ubiquitous. They think they are observing what is probably close to the upper mass limitations for planets. And they feel sure that what they are detecting are, indeed, planets.
A few scientists had argued that many of the discoveries over the last four years might not be planets but some new class of astronomical bodies or possibly substellar objects known as brown dwarfs. These are stars that failed to attain sufficient mass to ignite the thermonuclear fusion that makes stars shine. Dr. Robert Noyes of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., leader of the other team that independently made the new discovery, said the configuration of the three Upsilon Andromedae objects should quiet those critics.
Astronomers said they could not rule out a habitable planet or moon in the Upsilon Andromedae system. But the unsettling gravitational forces from such large planets in close proximity could have cleared the neighborhood of smaller bodies.
In any event, Earth-size bodies will probably fall below the detection limits of today's telescopes for many years. Astronomers will probably find several more systems similar to Upsilon Andromedae's before they find something with some of the familiar hallmarks of the solar system. The next major milestone, they said, would be finding a planet about the size of Jupiter at about the same distance from another star as Jupiter is from the Sun.
In the next few years, the Hubble Space Telescope will try to examine nearby stars for the slightest flickering change in their light. These could reveal the transit of a Jupiter-size planet at Jupiter distances across the face of the star. The French space mission Corot, to be launched in 2001, will in this way monitor the light of 25,000 stars in search of extra-solar planets.
The American Space Interferometry Mission, planned for launching in 2005, will use an array of telescopes to look for smaller extra-solar planets, possibly as small as Saturn or Neptune. It will probably be less than 10 years before the Terrestrial Planet Finder begins looking for Earthlike planets, and 20 before the Life Finder spacecraft begins its work. That could usher in another new field of science, comparative planetary biology.