More than 30 planets have been found in orbit of stars outside the solar system, but the two planets announced Wednesday are the first to be smaller than Jupiter, astronomers said at a news conference.
"We have discovered the first Saturn-sized planets outside of the solar system," said Geoffrey W. Marcy of the University of California, Berkeley.
The new planets are thought to be hot, gaseous giants, rapidly orbiting their stars and are probably devoid of life, he said.
"This is an important milepost" in the search for Earth-sized planets that orbit stars other than the sun, said R. Paul Butler, a staff scientist at the Carnegie Institute in Washington. "This suggests there will be many more small planets out there."
A team led by Marcy, Butler and Steve Vogt of the University of California, Santa Cruz, found the new, smaller planets in orbit of stars more than 100 light years away. Both of the new planets are slightly smaller than Saturn, a gas giant that is the second largest planet in the solar system after Jupiter, with 95 times the mass of Earth.
All previously discovered extrasolar planets, including 20 others found by the Marcy team, have been as big or bigger than Jupiter, which is 318 times more massive than Earth.
Butler said a new instrument attached to the Keck Telescope in Hawaii enabled the team to detect the smaller planets. The team is five years into a survey of about 2,000 stars and Marcy said there are six to 10 other star systems that "look promising" for planets.
"The discovery of Saturn-sized object is an extremely critical step toward finding terrestrial-sized planets orbiting other stars," said Heidi B. Hammel, a scientist at the Space Science Institute in Boulder, Colo. The terrestrial planets are Earth, Venus, Mars and Mercury.
The Marcy team finds extrasolar planets by observing and measuring the gravitational wobble that orbiting planets impart to the host star.
Marcy said that once the team knows the size of the wobble, they use basic Newtonian formulae to calculate the mass of the planet and how near it is to the host star.
Using the new instrument on Keck, the team is able to detect a wobble motion speed as slight as 6.7 miles an hour across more than 100 light years. This is not fine enough to detect an Earth-sized planet orbiting distant stars, they said. The Earth imposes a wobble motion on the sun of only about 0.22 miles per hour.
One of the new planets is about 80 percent the mass of Saturn and orbits a star, called HD46375, that is 109 light years away. The other new planet is 70 percent the mass of Saturn and orbts 79 Ceti, a star in the constellation Cetus, 117 light years away. A light year is about 6 trillion miles.
Both of the new planets orbit very close to their host stars, with one, HD46375, making a complete orbit in just three days and the other in only 75 days. Marcy and Butler said both planets are probably gas balls with temperatures well over 1,500 degrees Fahrenheit and unlikely to have life.
By PAUL RECER