Astronomers have reached another milestone in their search for life on other worlds with the discovery of two more planets around distant stars.
The planets may be smaller than Saturn - the second largest planet in the solar system - and their discovery marks the first time objects smaller than Jupiter, the biggest-known planet, have been detected.
Of the 30 extra-solar planets discovered over the past eight years, all have been about the size of Jupiter, a gas giant that cannot support life.
Detection of smaller planets demonstrates it may soon be possible to find Earth-sized planets some 300 times smaller than Jupiter that might be capable of supporting life.
Using the giant Keck telescope on Mauna Kea in Hawaii, the astronomers were able to find the planets from the gravitational wobble they caused as they disturbed the stars around which they orbit. The discovery was made by a team of three planet sleuths, Geoff Marcy of the University of California, Berkeley, Paul Butler of the Carnegie Institution of Washington and Steve Voigt of the University of California, Santa Cruz.
The existence of planets the size of Saturn, which is three times smaller than Jupiter, confirms a 20-year-old theory of how planets form by a snowball effect, growing as a proto-planet collects debris from a star-encircling disk of dust.
Dr Macy said that searching for smaller planets is like looking at a beach from a distance. "Previously we only saw the large boulders, which were Jupiter-sized planets or larger. Now we are seeing the rocks, Saturn-sized planets or smaller," he said. "We still don't have the capability of detecting Earth-like planets, which would be equivalent to seeing pebbles on the beach."
One of the planets is at least 80 per cent of the mass of Saturn and orbits at a distance of 3.8 million miles from the star HD46375, which lies 109 light years away in the constellation Monoceros. The other is slightly smaller and orbits at 32.5 million miles from the star 79 Ceti, which is 117 light-years away in the constellation Cetus.
Both planets have short orbits, taking just 3 days and 75 days to make a full circle around their respective stars.
America hopes to launch a planet-hunting space telescope in 2006 to photograph planets up to five times bigger than Earth at 50 light-years.