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| Artist's concept of an extrasolar planet and moon | |
WASHINGTON (AP) -- In a discovery that takes astronomers a step closer to finding other Earths orbiting other suns, researchers have found two objects slightly less massive than Saturn circling distant stars.
A team of planet-hunting astronomers announced Tuesday that they have found evidence of the smallest planets yet discovered to be in orbit of stars other than the sun.
Both planets are smaller in mass than Saturn, a solar system planet that is about nine times wider than Earth.
About 30 planet-sized objects have been found orbiting other stars, but all those were the size of Jupiter or bigger. The newly discovered planets are about a third the size of Jupiter.
Geoff Marcy of the University of California, Berkeley, said in a statement that searching for planets orbiting distant stars is "like looking at a beach from a distance."
"Previously we only saw the large boulders, which were Jupiter-sized planets or larger," he said. "Now we are seeing the 'rocks,' Saturn-sized planets or smaller."
Seeing Earth-sized objects, said Marcy, would be like seeing pebbles on that beach and astronomers are not yet able to do that.
Discovery of the Saturn-sized planets, however, supports a theory that planets, such as those in the solar system, formed around many stars in the universe. It also supports the theory that most planets are relatively small, such as Earth, Mars and Venus.
"This a trend the researchers are beginning to see in their data," according to a NASA statement.
Marcy and his colleagues, Paul Butler of the Carnegie Institution of Washington, and Steve Vogt of the University of California, Santa Cruz, used the Keck telescope in Hawaii to make the discoveries.
They found a planet about 80 percent the mass of Saturn orbiting around a star called HD46375 some 109 light-years from Earth. A planet 70 percent the mass of Saturn was found around star 79 Ceti, which is 117 light-years away in the constellation Cetus.
Both planets are close to their stars, which means they have short orbits. The HD46375 planet circles its star in just three days, while the 79 Ceti planet takes 75 days to orbit. Both are thought to be gas planets, like Saturn and Jupiter. Both are so close to their stars that they would reach temperatures of more than 1,500 degrees Fahrenheit, far too hot to sustain ordinary forms of life, the researchers say.
The planets are not actually seen by the astronomers. Instead, the researchers measure the gravitational effect of the planets on the central stars. As the planets orbit, they cause the stars to wobble very slightly. By measuring this wobble, the astronomers can detect the presence and size of a planet.
Marcy and his team have used this technique to catalog 21 extrasolar planets. The group is searching some 1,100 stars within 300 light-years of Earth to find evidence of planets. Other astronomer groups are also searching and have found about 10 extrasolar planets. A light-year is the distant light travels in a year in a vacuum, about 6 trillion miles.