More important, the planets were found around a variety of stars, suggesting that planets are common in our galaxy.

Gas giant planets were found orbiting "metal-poor" stars - those lacking in elements other than hydrogen and helium - which until now had been considered unlikely places for planets to form.

The first exoplanet was found in 1995. With this latest batch, the census tops 400. The lowest-mass planet announced yesterday has a mass about five times that of Earth.

Astronomers hope someday to find signs of an Earth-mass planet in an Earth-like orbit - circling a star at a distance that allows for the chance that water may be liquid at the planet's surface.

The astronomers from the European Southern Observatory who made yesterday's announcement used a spectrograph to study about 2,000 relatively nearby stars. The instrument, known as the High Accuracy Radial Velocity Planet Searcher (HARPS), measures the slight change in starlight caused by an orbiting planet, which cannot be seen directly.

"These findings consolidate the results of simulations of planet formation predicting a large population of super-Earths," said astronomer Stephane Udry of Switzerland's Geneva University by e-mail.

"The formation models furthermore predict an even larger population of Earth-mass planets, providing solid scientific justifications for the development of ambitious programs (in space and on the ground) to look for those Earth-type planets."

He said that a new instrument under development, known as ESPRESSO (Echelle Spectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations), "should allow us to detect Earth twins around solar-type stars, within 5 to 10 years."

"Personally, I am convinced that planets are everywhere," Udry said. "Nature does not like void and is especially efficient to fill up the 'holes.' "