By NATALIE ANGIER

The recent announcement that many Sun-size stars in our galaxy have Earth-size planets of their own has given astronomers and alien lifeseekers heart .
In the past decade, astronomers have found some 250 extrasolar planets, but most have been forbiddingly huge. They are made of gas, presumed to have no solid surface and to be hundreds of times the mass of Earth.
But in the new report, Michel Mayor of the Geneva Observatory and his colleagues said they had found 45 planets that were only a few times as massive as Earth, which means that they, like Earth, are probably built of rock.
The tally is proportionally impressive: roughly one in three stars surveyed showed signs of harboring stony planets, and other researchers performing similar studies said the figure might be more like one in two.
And though the 45 planets on the Geneva list are all “star-huggers,” as one astronomer put it, with orbital periods of 2 to 50 days - even Mercury needs nearly three months to circumnavigate the Sun - researchers are confident that other rocky planets remain to be found at distances from their suns that are comparable to Earth’s.
Sara Seager, a planetary theorist at the Massachusetts Institute of Technology, said astronomers hunt for planets by detecting the telltale wobbles they induce in their host stars, a method that selectively nets the too big or too near. Nevertheless, she said, the fact is, as soon as astronomers started looking for low-mass planets, they found a whole bunch, and that’s a real breakthrough.”
To some theorists, the new results virtually guarantee the existence of other Earthlike worlds.
“Suppose you have a tribe, and the most noticeable members are the warriors, because they’re adventuresome, they roam around, they’re the first to be spotted,” said Douglas N. C. Lin, a professor of astronomy and astrophysics at the University of California, Santa Cruz. “But you know that for every warrior, there’s a family behind the warrior.”
Dr. Lin continued, “Just as you can extrapolate from the warriors you see what the size of the larger population deep in the woods may be, so the presence of these short-period, super Earths implies that there are clusters of other planets farther out.” Potentially pleasant planets at that. “I would imagine that a significant fraction of ordinary Sunlike stars, maybe more than 10 percent, have habitable planets around them,” Dr. Lin said.
Habitable or not, planets are inescapable. “You make a star, you’re probably going to get planets,” said Seth Shostak, a senior scientist at the SETI Institute in Mountain View, California. When a cloud of dust and gas collapses to make a new star, spinning faster and faster as it shrinks, competing forces of gravity, pressure and rotation cause some of its midriff to flatten into a disk, rather as the skirt of a skater flies into a circle as she pulls in her arms for a spin. The planets in turn condense from the dust, gas and ice of that central disk, in sequences that researchers have just begun to model.
If planets abound, scientists suspect that life abounds, too, at least of the microbial kind. After all, they said, life arose here relatively quickly, maybe 800 million years after Earth’s condensational birth - and then stayed unicellular for the next three billionplus years.
Astronomers have high hopes for the Kepler spacecraft to be launched in February. Kepler will take a different approach in its planetary scan, Dr. Seager said, searching not for stellar wobbles but for “tiny drops in brightness,” possible signs of a planet transiting across the distant Sun’s face. Kepler will track 100,000 stars for four years, enough to detect the occasional crossing of any planets with leisurely orbits like ours.
“It will be akin to the great age of exploration, the explorers of the 16th century,” Dr. Shostak said.
“We will nail down what fraction of stars have planets,” and more important, “what fraction of those planets are small, terrestrial planets.”
With that comprehensive planetary atlas in hand, we can pick out the places most worthy of follow-up probes: planets that are relatively close, and closest in kind to the one we know best. We can look for rocky planets that follow stable paths, and are laced with clouds of water vapor that hint at liquid oceans below, and atmospheric oxygen, the voice of a biosphere. “Oxygen is so reactive that it shouldn’t be in the atmosphere unless it’s being produced by something like photosynthesis,” Dr. Seager said. “It’s a huge indicator of life.”