AUSTIN, Texas (AP) _ Two additional planets orbiting distant stars have been discovered by astronomers, bringing to 17 the number of worlds known to exist beyond Earth's solar system.

Experts at the national meeting of the American Astronomical Society estimated Saturday that about 2 percent of the billions of sun-like stars near and in the Milky Way galaxy may host planets. Astronomers are mounting an increasingly active campaign to find them.

None of the 17 distant planets are Earth-like or apt to harbor life. The planets more closely resemble Jupiter, the gaseous giant that is the largest planet in the solar system. They range in size from about half that of Jupiter to about 11 times bigger.

Debra Fischer, a member of the San Francisco State University team that has found most of the extrasolar planets, said the new discoveries are both larger than Jupiter and both orbit very close to their host star.

Because of the proximity, they orbit at an extremely fast pace. One is only 13 million miles from its host star, giving the planet a ``year'' that lasts only 18 days. The Earth completes one orbit of the sun every 12 months.

Fischer said her team's planet-finding skills have improved and the group is now systematically searching a list of 200 candidate stars.

``With a sample of suitable stars and enough telescope time, we expect to find planets around about two percent of the sun-like stars within a few months,'' she said.

Geoffrey Marcy, leader of the San Francisco team, said that nine of the new planets are in egg-shaped, eccentric orbits. This finding means it is unlikely that their parent stars could also host smaller planets.

The oval-shaped orbits of the gas giants, he said, would doom any Earth-like planet.

As the huge planets swing through their eccentric orbit, said Marcy, any smaller planet ``would be yanked out of its orbit.'' Such instability dampens the expectations of finding life in such star systems, he said.

In the solar system, the Earth and its sister planets are in stable, circular orbits, which means they are not as susceptible to gravitational gyrations that could be caused by a family of planets in oval orbits.

Virtually all of the new planets have been discovered by measuring the wobble of the parent star. As planets orbit, they cause the host star to slightly alter its motion. Marcy and his team determine the presence of a planet by measuring this wobble. The characteristics of the motion tell how big the planet is, how close it is to the star and the shape of its orbit.

This technique can discover only large planets that have a strong gravitational tug against the star.

Other techniques, however, also are finding evidence of extrasolar planets. Two astronomers announced Friday that the Hubble Space Telescope had photographed dust rings about two stars. Such rings are thought to signal the presence of planets.

Another group announced Saturday that they had found the suggestion of a planet orbiting a star that passed between the Earth and a more distant star. This passage caused a lensing effect, magnifying light from the farthest star. An analysis of the light hinted that the nearest star hosted a planet, but more study was needed, said Sun H. Rhie of the University of Notre Dame.

William D. Cochran of the University of Texas said even better techniques for finding extrasolar planets are in development. Astronomers have proposed keen-sighted space telescopes that would be able to spot Earth-sized planets, either directly, or by measuring the drop in light caused by the shadow of a planet passing between its host star and the Earth.