A major discovery announced last week has given birth to a new field of science, the comparative study of planetary systems.

At last, after centuries of speculation, often more fanciful than scientific, and recent years of expectant searching, astronomers have found a family of planets to compare the solar system with. They detected three large planets -- two of them with masses much greater than Jupiter's -- orbiting another solar-type star, Upsilon Andromedae, 44 light-years away.

Scientists confidently predict that this will be the first of many similar discoveries. Dr. Geoffrey W. Marcy of San Francisco State University, leader of one of the discovery teams, has his eye on another star, 55 Cancri, where there is mounting evidence of multiple planetary companions. Observers are tracking other nearby stars for planetary systems but are cautiously refraining from making any claims.

They are further encouraged by other discoveries over the last four years. Examining the telltale motions of almost 20 other stars, astronomers have inferred the presence of at least one Jupiter-class planet close to each of them, exerting slight gravitational disturbances. Smaller or more distant planets may also accompany these stars, but their detection would require more prolonged observations and may be beyond current technology.

If there were astronomers with comparable technology in the Upsilon Andromedae system, they would be unable to detect any of the Sun's planets, not even mighty Jupiter, because they lack sufficient mass or are too distant from the Sun. So it is possible, scientists said, that observers have yet to take the full measure and census of the Upsilon Andromedae system or others nearby in the Milky Way galaxy.

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Spacecraft being planned for flight in the next couple of decades are expected to find many more planetary systems, including any Earth-size objects that might harbor some forms of life.

The Terrestrial Planet Finder, a mission planned by 2010, should be able to detect and photograph the full retinue of planets around other stars. Its instruments will also be designed to determine the constituents of atmospheres, which could reveal signs that the planets may be habitable.

About 10 years later, a spacecraft called Life Finder, which is being planned by the Jet Propulsion Laboratory in Pasadena, Calif., would carry remote-sensing instruments for a more detailed study of planet atmospheres. The presence of methane and ozone together, for example, would be a sure sign of some kind of life on the planet.

Theoretical astrophysicists have already seen enough, though, to suspect that the Sun and its family may not be a typical planetary system. They have been surprised to find so many examples, at Upsilon Andromedae and also among the solitary planet detections around other stars, of Jupiter-class objects existing much closer to their stars than Jupiter is to the Sun. And none of the current theories predicted what they discovered at Upsilon Andromedae: that three such giant worlds could form and survive around a single star.

''True giant planets are not that easy to form,'' said Dr. Douglas N. C. Lin, a theorist at the University of California at Santa Cruz. ''Finding three around one star, and extremely close to it, is quite significant and puts more constraints, limits, on any theory of planetary formation and evolution.''

Dr. Alan P. Boss, a theoretical planetary scientist at the Carnegie Institution of Washington, noted that 10 times more mass is locked up in the three Upsilon Andromedae giants than in all the planets of the solar system. The innermost of these has a mass three-quarters that of Jupiter, and the middle one a mass at least twice Jupiter's. The outermost, at more than four times Jupiter's mass, is not much farther from its star than Mars is from the Sun.

''I'm worried that our conventional ways of explaining giant planets is inadequate,'' Dr. Boss said.

About the only thing that has not astonished astronomers is the discovery itself of another planetary system. As early as the classical Greeks, philosophers have contemplated the possibility of other worlds. In the 17th century, writers began entertaining ideas about ''a plenitude of worlds.'' In recent decades, scientists had said the laws of probability favored the existence of other planetary systems.

Because the Sun is a common and ordinary star, scientists reasoned, groups of planets probably formed around some other stars like it. Because there are 200 billion stars in the Milky Way alone, planetary systems are probably abundant, in many cases a natural result of the process of star formation. Recent observations by the Hubble Space Telescope of disks of dust around many young stars, which appeared to be an early stage of planetary formation, supported this line of reasoning, astronomers said.

The first breakthrough, in 1992, was both encouraging and disappointing. Radio astronomers detected two or three planets orbiting a stellar object, but it was not a normal star. It was a pulsar, the spinning relic of an exploded star.

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.

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