WASHINGTON, April 15 - Building on 11 years of observations, astronomers say three giant planets have been detected around a sunlike star 265 trillion miles away - representing the first planetary system that scientists think could be like our own.         THE TRIPLE DETECTION, announced Thursday by a team of seasoned planet-hunters, adds new glimmers of hope to the search for Earthlike planets and perhaps even extraterrestrial life. It firms up the argument that these worlds are indeed planets rather than brown dwarfs or captured stars. But it also poses new puzzles.         "Today, with the discovery of the first planetary system beyond our own, we are witnessing the emergence of a new era in human exploration, " declared Geoffrey Marcy, an astronomer at San Francisco State University and one of the system's discoverers. Just as past cultures looked from their own lands across seas and skies, earthlings were now beginning "a reconnaissance, if you will, of planets around other stars," he said.         Marcy and his longtime colleague in the search for extrasolar planets, R. Paul Butler of the Anglo-Australian Observatory, detected the first planet around Upsilon Andromedae in 1996, using what has become a standard method: They tracked a pattern of Doppler shifts in the spectrum of light from the star, which hints at a wobble caused by the gravitational pull of the circling planet. About 20 distant worlds have been detected in this manner.         In the case of Upsilon Andromedae, there was an extra wobble, even after the first planet's gravitational pull was taken into effect. Butler, Marcy and other researchers from San Francisco State, the Harvard-Smithsonian Center for Astrophysics and the High Altitude Observatory in Boulder, Col., determined that the extra wobble could only be explained by the presence of two additional planets. A computer simulation confirmed that the orbits could be stable.         The researchers said they have submitted a paper on the subject to the Astrophysical Journal, drawing on observations made from the Lick Observatory in California and the Whipple Observatory in Arizona.         Some skeptics have wondered whether the solitary objects detected using the Doppler method might actually be brown dwarfs or failed stars rather than planets. But the fact that three such objects were detected circling the same star should ease that second-guessing, said Charles Beichman, chief scientist for the Origins program at NASA's Jet Propulsion Laboratory.         "It's just so spectacular, because in science we say, 'Well, we think it's like this. 'But there is no substitute for really demonstrating that you're right," he told MSNBC.         The researchers said the new findings suggest that planetary systems like our own are abundant among the 200 billion stars in our galaxy alone. Butler said this was just the start of what he expected would become a huge database of distant solar systems.         "Maybe what we have here is a Rosetta Stone that will eventually explain how these planets are formed," he said.         Upsilon Andromedae is 44 light-years away, with each light-year equal to about 6 trillion miles. It is in the same class as our sun, roughly two-thirds as old, and is visible to the naked eye.         The innermost of the three planets was the first one detected: It's at least three-quarters the mass of Jupiter and completes an orbit every 4.6 Earth days, circling only 6 million miles away from the star.         That would make the innermost planet far too hot to support life. But the middle planet, with at least twice the mass of Jupiter, is about as far away as Venus is from our own sun, following a 242-day elliptical orbit. Under the right conditions, Beichman figures the temperature on a moon of that planet could vary between 60 and 140 degrees Fahrenheit (15 to 60 degrees C), with the potential for liquid water. "That's not bad," he said.         The outermost planet is at least four times as massive as Jupiter and completes one orbit every 3.5 to four years, at an average distance of about 250 million miles. The temperature there could be a chilly 112 degrees below zero (minus 80 degrees C), Beichman said.         Planetary scientists once thought such giant planets had to form relatively far out from their parent star, but the researchers said these three giants must have either formed closer in or migrated toward the center in a game of planetary billiards.         They said they couldn't completely rule out the possibility that the system also harbored Earthlike planets too small to show up in the Doppler data. But Marcy said the giant planets' eccentric orbits create gravitational perturbations that would have flung any Earths completely out of the solar system.         The fact that so many of the giant planets found so far have eccentric orbits is bad news in the search for Earthlike planets, Marcy said. The good news is that such "marauding Jupiters" had been detected around only 5 percent of the stars surveyed so far.         "Ironically, it's our non-detection that gives us the best hope for finding other habitable Earthlike planets in our own galaxy," he said.         Beichman said Upsilon Andromedae would be a "prime target" for the Space Interferometry Mission, an observatory to be launched in 2005 or 2006 that could detect planets down to Neptune's size -- or down to Earth's size for nearer stars.         Eventually, more sensitive instruments should be able to detect Earth-size planets and even analyze their atmospheres from afar.