Michel Mayor of the Geneva Observatory in Switzerland and his colleague Didier Queloz discovered the first world around another star in 1995. Marcy and Butler quickly confirmed the result from the Lick Observatory in California, then went on to make the bulk of planet discoveries for years thereafter. Other teams have since joined the hunt.

For the past five years, Marcy's growing team has relied primarily on the larger Keck Observatory in Hawaii. They also use the smaller Anglo-Australian Observatory.

Of roughly 3,500 relatively nearby, Sun-like stars -- all within about 160 light-years -- the researchers selected about 1,200 as being the most amenable to harboring planets. Young stars and tight-orbiting pairs of stars were not included.

To understand the technique's capabilities and limitations, Marcy explained that if alien astronomers on any of the stars in his survey were using the same method to explore our own solar system, they would detect Jupiter and possibly even Saturn, but not Earth or any of the other planets. Neptune and Uranus, though large, are too far from the Sun and induce too little wobble to be detected.

The wobble method will never find planets smaller than 10 Earth masses, he said, and even doing that would require very long, detailed observations. Yet somewhere in their souls, all planet hunters want to find other "Earths". In addition, the data tells them they are very close.

Marcy said it is a bad idea to extrapolate the existing extrasolar planet data to estimate the possible number of Earth-sized planets that he would expect to find. "But that didn't stop me," he said. "I did it anyway." He figures there ought to be at least three Earth-sized planets for every 100 stars that his team is studying.

While there is no evidence for the existence of solar systems exactly like ours, complete with Earth-mass planets, few experts doubt the likelihood. Some astronomers argue that such situations will be rare, yet even that would be remarkable.

"Even if it's rare, it doesn't mean it's unique," said Mario Livio, who heads up the Institute Science Division at STScI. "Surely there will be other solar systems like ours, and maybe there will be many."

Only a space-based mission will find Earth-like planets, often referred to as "terrestrials," most researchers believe. Such missions are only in the planning stages.

"We're a long way from finding terrestrial planets," Marcy said. "There is no technique in the next few years capable of detecting Earth-like planets."

In general, astronomers say Kepler will have the capability to detect terrestrial planets in comfortable orbits around Sun-like stars. However, the stars it will survey are much farther away than the stars searched by Marcy's team. Examining the atmospheres and other characteristics of planets detected by Kepler will prove daunting.

"The good news is we'll detect Earth-like planets," Marcy said, adding that even this task will be a challenge for Kepler. "The bad news is that they will be about a thousand light-years away."