IT'S HARD TO IMAGINE TWO more undesirable pieces of extraterrestrial real estate. The first, a planet orbiting a star known as 47 Ursae Majoris, 200 trillion miles from Earth in the Big Dipper, is about twice the size of Jupiter. Like our own largest p lanet, it probably consists mostly of such noxious gases as hydrogen sulfide, ammonia and methane. Fierce jet streams blow unceasingly at hundreds of miles per hour, sometimes spiraling into mammoth hurricanes that last for centuries and are big enough to swallow the Earth. And if this harsh world has any solid surface at all, it's buried under an atmosphere thousands of miles deep, crushed by pressures a thousandfold greater than those at the bottom of the deepest terrestrial sea. A second planet, circli ng the star 70 Virginis, in the constellation Virgo, is probably even less inviting: because it has more than six times the mass of Jupiter, weather conditions there could be even more extreme.

Yet inhospitable as both these worlds seem, their discovery, announced two weeks ago by San Francisco State University astronomers Geoffrey Marcy and Paul Butler, has thrown an almost wholly speculative area of study solidly into the realm of tangible fact. Despite years of searching with the most powerful telescopes, despite decades of listening for the faint crackle of radio signals from distant civilizations, despite endless theorizing about how life might or might not arise, nobody had ever found c oncrete evidence to suggest that our planet, our civilization, our life-forms were anything but unique in the cosmos.

Now, suddenly, everything has changed. Not only do these still unnamed planets triple the number of worlds known to orbit stars like the sun--the only other example having been found just four months ago--but they have an even more profound significanc e. Both of them are temperate enough to allow water to exist in liquid form. And whatever else is necessary for life as we know it, say biologists, liquid water is an absolute prerequisite.

It isn't that life necessarily exists on either of the new planets. The question is impossible to settle with today's technology, and if organisms do inhabit these distant worlds they would be a bizarre sort of life, proceeding from birth to death, gen eration after generation, without ever touching solid ground.

Even if the new planets are sterile, though, their very existence is a powerful piece of astronomical news. If our solar system is any indication, giant, unpleasant planets are likely to be accompanied by small, friendly ones. Giant planets also tend t o be attended by giant moons, small worlds in their own right, and these too could be hospitable to life. Perhaps most important, the discovery of planets around three relatively nearby sunlike stars implies that the Milky Way, 100 billion stars strong, m ust be bursting with other worlds. Unless the chances are literally 100 billion to one against the emergence of life--and recent advances in biology suggest that the chances are considerably better--there is life out there, somewhere.

If so, it may not be long before we find out. Energized by the latest discoveries, astronomers are racing back to their telescopes for more observations and to their computers to analyze years' worth of data still sitting in their disk drives. Everyone wants to be the next to find a distant world. The scientists are eagerly awaiting the results from the Infrared Space Observatory (iso), a newly orbiting European satellite that can detect the faint heat from distant planets. They're looking forward to t he 1997 installation of a new infrared camera on the Hubble Space Telescope, which could take a picture of at least one of the newly discovered worlds.

Most promising of all, they're buoyed by a newly unveiled NASA initiative, known as the Origins project, that will build a generation of space telescopes to search for new worlds. Says NASA administrator Daniel Goldin: "We are restructuring the ag ency to focus on our customer, the American people." And the public excitement about this field, he says, "is beyond belief."

It shouldn't be. Man's fascination with other worlds is as old as Western civilization. Galileo's discovery that they actually existed--that at least some of the pinpoints of light that wandered throughout the night sky had mountains and moons--set off a centuries-long quest to discover new planets. The first great success came in 1781, when William Herschel found Uranus. Then came the discovery of Neptune by Johann Galle in 1846. Eventually, the notion of otherworldly life made the transition out of t he pages of philosophy and fiction: in 1894, the wealthy astronomer Percival Lowell built his own observatory in Arizona to try to detect the life he believed existed on Mars. He never found it, but in 1930 Clyde Tombaugh, then an assistant at Lowell Obse rvatory and now a professor emeritus at New Mexico State University, found Pluto. It was the last planet that would be discovered until the 1990s.

Not that astronomers ever stopped looking--at first, within the solar system, for the mysterious Planet X (now considered very unlikely to exist), and then, as powerful instruments like the 200-in. Hale telescope came online, around other stars as well . But picking out a planet against the glare of a star is like trying to spot a 100-watt light bulb next to a 100-billion-watt searchlight. Astronomers find it much easier to look for the subtle influence a planet might have on its parent star.

An orbiting world's gravity should, for example, tug faintly on the star that is its sun, pulling it first this way, then that. If the plane of the planet's orbit is such that a star is being pulled first toward and then away from the Earth, the motion will cause light waves coming from the star to be squeezed together, then stretched apart--making the light look first a little bit bluer than it really is, then a little bit redder, then bluer again, and so on. These subtle color changes--examples of th e so-called Doppler shift--can be precisely measured, and the magnitude of the wobble pinned down, with a device called a spectrometer.

While some astronomers searched for planets, others tried to detect intelligent alien life directly. In 1960, astronomer Frank Drake started Project Ozma, an attempt to look seriously for radio signals from alien civilizations. It was the first in what became a series of experiments in seti, the Search for Extra-Terrestrial Intelligence. Several are still going on, but so far without success.

The race to discover planets around sunlike stars proved similarly fruitless until about 18 months ago. At the time, Marcy and Butler were sure they had the inside track on finding them. The telescope they use, at Lick Observatory in the mountains abov e California's Silicon Valley, has an excellent view of the heavens. It also has one of the world's finest spectrometers. After a major refurbishment in November 1994, the device was even better. In principle, says Marcy, "we could detect not just Ju piters but Saturns."

In principle, maybe, but in practice there are dozens of factors that can confuse matters. For example, stars, including the sun, pulse rhythmically with waves generated deep in their interiors, making the surface bulge toward and sink away from the Ea rth just as though the whole star were wobbling. Stars can also have huge blotches--sunspots, in essence--that change the mix of colors as they rotate into and out of view. And spectrometers are subject to all sorts of errors that come from changes in tem perature and electronic glitches. Thus, Marcy and Butler had to run their observations through a sophisticated computer program they'd written to sort useful from useless information--a piece of software so complex and so demanding of computer time that t heir colleagues kidded them that it would never work.

They were wrong. As it happened, Butler was in the middle of rewriting the software last October to accommodate the spectrometer's newly heightened sensitivity when a disconcerting flood of E-mail started pouring in. Michel Mayor and Didier Queloz, of the Geneva Observatory in Switzerland, had just detected a planet circling the star 51 Pegasi, lying 45 light-years away in the constellation Pegasus. Says Queloz: "We first thought that our instrument was faulty, but repeated verifications and compu tations finally convinced us that we had bagged a planet."

More than one astronomical discovery has disappeared on a closer look, though, so Marcy and Butler headed for the telescope, determined either to debunk or verify the Swiss team's claims. Sure enough, says Marcy, after four nights at Lick and many hour s of computer time, "everything they'd said about the planet was confirmed." (Butler and Marcy did, however, show that hints the Swiss team had found a second planet around the same star were mistaken.)

The object turned out to be peculiar. It's half as massive as Jupiter, but orbiting closer to 51 Pegasi than Mercury is to the sun. That means its surface temperature is 1300šC, hotter than a blast furnace. Still, it is a planet. "I was a little s chizophrenic about it," says Marcy. "On the one hand, we had been scooped. But I also felt euphoric that humanity had entered a new era in which new worlds were going to be subject to exploration."

Marcy didn't rest on his euphoria. He and Butler went into high gear, determined to be at least the second team to find planets around a sunlike star. They begged telescope time from their colleagues and borrowed more than $100,000 worth of computer eq uipment to crunch gigabytes' worth of data from observations stretching back over eight years. "We knew," says Butler, "that we could get scooped again and again by the Swiss team."

After two months, they had analyzed 60 of the 120 stars in their survey. On the morning of Dec. 30, Butler went to the office to check on the computer's progress. "When I saw the data come up, I was completely blown away," he says. It was the telltale signature of the object orbiting around 70 Virginis. Recalls Butler: "It knocked me off the chair." His colleagues at the American Astronomical Society's winter meeting in San Antonio, Texas, where Marcy and Butler announced their find ings two weeks ago, were no less excited. "What we are seeing," said Robert Brown, an astronomer with the Space Telescope Science Institute, after Marcy sat down, "is the culmination of intellectual history that began with Copernicus 500 ye ars ago."

It is also a beginning of sorts. Faced with the existence of these planets, astronomers must now revise their theories to fit the new facts. To begin with, theorists have to scramble to explain how the 51 Pegasi planet could have formed and survived in tact so close to its parent star. The planet around 70 Virginis is also problematic: its orbit is egg-shaped rather than circular, which suggests to some astronomers that it formed more like a star than like a planet. Indeed, many experts think it is tech nically a brown dwarf--a star that never got big enough to ignite--rather than a planet. Only the third object matches what astronomers expected. Says Brown cautiously: "I can't say for certain that it is a planet. But so far, it looks like a planet, it walks like a planet, it quacks like one."

What the data may be saying is simply that the dividing line between stars and planets may be less distinct than astronomers had believed. "Everything found so far poses challenging questions for planetary formation theory," says astronomer R obert Stefanik, of the Harvard-Smithsonian Center for Astrophysics. That was underscored last week when, after weeks of government shutdown, results were released from a NASA experiment much closer to home. The probe's plunge from the Galileo spacecraft i nto Jupiter's atmosphere showed that the planet has higher winds, less lightning, less water, helium and neon, and--at the point of impact at least--fewer clouds than the experts had been expecting.

More surprises are almost certain to follow if astronomers find more and more planets circling other stars. But while finding new planets of any sort is terrifically exciting, says Alan Boss, an astronomer at the Carnegie Institution of Washington, &qu ot;the Holy Grail is to find an extrasolar planet that is capable of supporting life."

THAT IS EXACTLY WHAT DANIEL Goldin has challenged NASA to do in the Origins program: locate and even photograph Earthlike planets outside our solar system. The initiative is headed by Edward Weiler, an agency veteran who also serves as chief scientist for the Hubble telescope. "That's a pretty lofty goal," says Weiler of Goldin's challenge. "You're talking about objects that are millions to billions of times fainter than the stars around which they're revolving." The only way to do it, he and other astronomers agree, would be to use a space telescope with a mirror as wide as a football field is long.

Such a gigantic scope is utterly beyond current technology, and beyond anything engineers can imagine for the next century as well. But astronomers know they can simulate a huge telescope by orbiting several smaller ones, widely separated, and combinin g their light electronically. This multimirror device is known as an interferometer, because rather than gathering light directly, it measures interference patterns created when light waves from several mirrors overlap each other.

Unlike traditional NASA projects, which tend to be expensive and complex, this one is relatively modest. "We really don't want to start out building the Battlestar Galactica," says Weiler. Instead he will start with a demonstration model by t he turn of the century, a device consisting of four to six mirrors a foot or two across. Even at that size, the interim interferometer should be able to spot objects the size of Neptune around nearby stars.

Finally, by about 2010, NASA hopes to launch what it calls the Planet Finder: an interferometer with five 3-ft.-to-6-ft. mirrors spread over 300 ft., orbiting out by Jupiter, where the solar-system dust begins to thin out. The Planet Finder should allo w scientists to identify Earthlike planets, which should show up as pale blue dots in images beamed back to ground controllers, and analyze their atmosphere for signatures of life like ozone, oxygen or carbon dioxide.

That is not ambitious enough for Goldin, though. He wants engineers to create a device so powerful that it could take pictures of Earthlike planets in such detail that we might see clouds, continents and oceans. "That's mighty tough," caution s Weiler, careful to say Goldin's vision isn't impossible. "It sounds way out, but when Kennedy said, 'We're going to be on the Moon in nine years,' a lot of people thought that was way out. On the other hand," he adds wistfully, "they got $25 billion."

Origins will receive nothing close to that. Goldin expects his scientists and engineers to do it for several hundred million dollars, drawing from existing programs and saving money by technological innovations as yet unspecified. And if his ambitious goals are not met, will he consider Origins a failure? "No!" Goldin fairly shouts. "I'm trying to say, let's expand our minds and let's see if we can answer some basic questions. It may be there's not a terrestrial-size planet out there.&qu ot;

One point in his favor: by keeping the budget low, proceeding in small steps and refusing to make firm promises about what the program will produce, Goldin may be improving his chances of keeping Congress on his side. Boss, for one, thinks Origins will endure even after Goldin is gone. "The momentum is building," he says. "The scientific imperative is there."

Despite Goldin's caution about assuming the existence of Earthlike planets, few astronomers doubt they are out there. If other solar systems do contain Earthlike worlds, says NASA exobiologist Michael Meyer, at least some should fall into the "hab itable zone"--the region, governed by a planet's distance from its star, where water is liquid rather than solid or gaseous. "The good news," he says, "is that if our solar system is typical, there's a 50% chance that a planet will be in the right zone."

That is crucial, observes David Des Marais, a NASA biogeochemist. Liquid water is an ideal medium in which carbon-based organic chemicals can dissolve and react with one another in myriad ways. Why carbon, necessarily? Because, says Des Marais, "i t is such a versatile chemical. It makes so many different and complex compounds. And it's the fourth most abundant element in the universe." Carbon compounds literally litter the cosmos, drifting through interstellar space in giant molecular clouds and making up a significant percentage, by mass, of comets and asteroids. Some scientists are convinced that the basic building blocks of life fell to Earth from space and that the same could easily happen anywhere.

Whether life would inevitably arise from those building blocks is still an open question. With only one example, it is impossible to say whether life on Earth was a fluke or a foregone conclusion. But most biologists cautiously lean toward the latter. Life on this planet emerged surprisingly quickly--as early as a few hundred million years after Earth formed. At the time, the planet was intensely volcanic, with the occasional leftover asteroid screaming in every few million years--yet primitive life fo rms persisted and flourished.

Until a few years ago, biologists were at a loss to understand how life could have arisen under such conditions. But laboratory experiments have convinced them that self-replicating molecules are relatively easy to assemble. And the discovery of hot-wa ter volcanic vents deep in the ocean, surrounded by rich ecosystems of exotic life, implies that a hot, young, volcanic planet might in fact be an ideal incubator.

That suggests, though it does not prove, that biology will take hold if it possibly can, even under hostile conditions. In fact, biologists have not quite given up on our own solar system yet. They think Mars may have had a brief fling with one-cell li fe that could have left fossil evidence behind. Some even hold out the hope that microorganisms are still surviving somewhere under the Martian surface. Attention is also turning to Europa, one of Jupiter's moons; its icy white surface could conceal ocean s of liquid water, and perhaps some sorts of living organism. Both possibilities are likely targets of future NASA investigation.

Alien life of any sort would make biologists ecstatic, of course, but it is the prospect of intelligent life that fires most people's imagination. "That final step from life to intelligent life is probably the longest shot of all," observes D es Marais. Even so, the small band of astronomers devoted to the search for broadcasts from high-tech extraterrestrials is encouraged: their 35-year quest has always rested on the assumption that planets exist outside Earth's solar system, and the fact th at they have been proved right makes the search seem considerably less quixotic.

The astronomers who are looking for planets, meanwhile, are sounding downright cocky. Butler says that he and Marcy are "close, real close" to finishing the analysis of their remaining 60 stars and that they would not be surprised to find two or more additional planets popping out of the data--perhaps in a matter of weeks. The pair will soon be heading for the Keck Telescope in Hawaii, the world's largest, to continue the search with even more powerful equipment. Mayor and Queloz, meanwhile, are back at their telescope in Europe. At the same time, dozens of other groups, using instruments ranging from the high-flying Hubble to relatively small scopes, are stepping up their activities. Predicts Marcy: "We are going to find, between us and the Swiss, 10 more planets in the next two years." Concurs Butler: "Very shortly, there could be more planets known outside the solar system than inside." Whether or not they are right, the human race has already moved closer to answering the most enduring question about its true place in the cosmos.

--Reported by Hannah Bloch/Washington and Sylvester Monroe/San Francisco, with other bureaus