Nobody has seen them yet, but scientists now think there are tens of billions of planets the general size and bulk of Earth in the Milky Way galaxy alone - a startling conclusion based on four years of viewing a small section of the nighttime sky.

The estimate, made by astronomers Andrew Howard and Geoffrey Marcy of the University of California at Berkeley, flows from the logic that the number of small but detectable exoplanets - planets outside Earth's solar system - is substantially larger than the number of big exoplanets in distant solar systems.

In a paper released Thursday by the journal Science, the two report that based on this galactic preference for smaller planets, they think that almost one-quarter of the stars similar to our sun have Earth-size planets orbiting them.

"This is the first estimate based on actual measurements of the fraction of stars that have Earth-size planets," said Marcy, who did his observing with Howard at the Keck Observatory in Hawaii.

Their observations and extrapolations say nothing about whether these Earth-size planets will have the characteristics of Earth: its density, a distance from the sun that is just right for liquid water, the fact that it is a rocky structure rather than a gaseous ball.

But Marcy said that with so many Earth-size planets now expected to be orbiting distant suns - something on the order of 50,000,000,000,000,000,000,000 across the universe - the likelihood is high that many are in "habitable zones" where life can theoretically exist.

"It's tantalizing, without a doubt, to think some of those Earths are in habitable zones," Marcy said. "And based on what we know, really, why wouldn't they be?"

Planet-hunting technology allows astronomers to find exoplanets down to the size of super-Earths that are three times the size of our planet. The new conclusion that billions of planets similar in mass (or bulk) to Earth exist in the Milky Way is based on extrapolations of the number of these super-Earths compared with the number of larger exoplanets. Because the finding is not based on firm measurements, Marcy said, "it's a very exciting set of numbers that we have confidence in, but there are yellow flags."

Exoplanet hunters, who found the first planet outside our solar system in 1995, are entering a period of especially heightened and excited discovery. The new assessment from Howard and Marcy, funded by NASA and the Keck Observatory, comes only weeks after two other astronomers published a paper saying they had detected an apparently rocky planet in a habitable zone around a star relatively close to Earth called Gliese 581g.

That conclusion, by Steven Vogt of the University of California at Santa Cruz and Paul Butler of the Carnegie Institution in Washington, has not been confirmed, and some have challenged the discovery, especially a Swiss team that has been a leader in exoplanet research.

But the assessment that Earth-size planets are ubiquitous in distant solar systems is expected to get additional support in February when the scientists operating NASA's Kepler Mission, which is searching for Earth-size and habitable planets, report on what they have been finding.

In a previous paper, the Kepler team reported finding about 350 new candidate planets that they are confirming. Since the first exoplanet was identified, about 500 more, large and small, have been discovered and confirmed.

Most were detected by measuring the minute wobbles of stars caused by the gravitational pull of the exoplanets that orbit them. This technique has been dramatically refined in recent years, and astronomers have been given the long telescope observation times needed for the measurements. The best planet hunters can now, with the use of spectrographs, detect solar wobbles of as little as one meter a second.

The field has also been revolutionized by other increasingly sophisticated methods of detecting exoplanets, especially using the "transit" method, which looks for tiny reductions in the light coming from the star and planets being observed - usually on the order of 100 parts per million. If these one-to-16-hour reductions are observed in a regular pattern over time, then astronomers know a planet is orbiting its sun.

The Kepler Mission is using similar "transiting" to search for Earth-size planets in one small section of the sky. Marcy, who is a member of the Kepler science team, said the orbiting telescope will survey thousands of stars to determine with unprecedented precision how many are circled by exoplanets, and especially by Earth-size exoplanets that might be in habitable zones. The size of the exoplanet can be determined by the amount the brightness of the star decreases when it transits its sun.

"This is an extraordinarily exciting time in exoplanets and distant Earths," Marcy said. "Think of it: We know that Aristotle was once at a cafe outside Athens drinking ouzo and speculating about whether there are other Earths in the universe. That's a question we're getting much closer to answering."

The 166 solar systems reported on Thursday by Howard and Marcy are within 80 light-years of Earth - the very great distance light travels in 80 years, but a short distance by astronomical measures. "What this means is that as NASA develops new techniques over the next decade to find truly Earth-size planets, it won't have to look too far," Howard said.

Based on their census of those 166 solar systems with sun-like stars at their centers, only 2 percent of exoplanets will be the size of Jupiter, 6 percent the size of Neptune and 12 percent "super-Earths." This progression led to the conclusion that 100 sun-like stars would be orbited by 23 planets sized between half of an Earth and two Earths.