PLANETS similar to the Earth could be orbiting one in every four stars like the Sun, according to research that suggests potential homes for life could be common elsewhere in our galaxy.
A new survey of 166 Sun-like stars in our cosmic neighbourhood has indicated that 23pc of them are likely to have a small rocky planet that is of about the same mass as the Earth.
They have just not been detected yet because of the technical difficulties involved, scientists believe.
Many of these worlds may occupy the fabled "Goldilocks zone" -- the orbital path where conditions are not too hot or cold but "just right" to support liquid surface water, and possibly life.
Astronomers in the US came to the conclusion after spending five years studying 166 Sun-like stars within 80 light years of Earth. They found increasing numbers of smaller planets, with the highest proportion being so-called "super-Earths", which are the smallest size detectable today.
Dr Andrew Howard, from the University of California at Berkeley, said: "Of about 100 typical Sun-like stars, one or two have planets the size of Jupiter, roughly six have a planet the size of Neptune, and about 12 have super-Earths between three and 10 Earth masses.
"If we extrapolate down to Earth-size planets -- between one-half and two times the mass of Earth -- we predict that you'd find about 23 for every 100 stars."
The technique the astronomers used could only detect planets close to their stars. Taking this into account, there could be an even higher proportion of Earth-size planets at greater distances, including ones within the habitable "Goldilocks zone".
Over the next decade, new methods of planet detection and more powerful telescopes could soon be uncovering true Earth-like worlds orbiting distant stars, the scientists said.
The findings, reported yesterday in the journal 'Science', conflict with current models of planet formation which predict empty "deserts" in the inner regions of solar systems. In reality, the inner zones appeared to contain the greatest numbers of planets.
"These results will transform astronomers' views of how planets form," said Professor Geoffrey Marcy, another member of the Berkeley team.
The astronomers used the twin 10-metre Keck telescopes in Hawaii to carry out their research. They relied on the well-established "wobble" method of planet detection, based on measuring the effect of a planet's gravity tugging on its parent star.
Using this technique they were able to spot large Jupiter-like gas-giant planets, Neptune-like planets and "super-Earths" at close orbital distances.
The stars chosen were yellow stars like the Sun or slightly smaller orange-red stars known as K-type dwarfs.
In total, 33 planets circling 22 stars were detected within the restricted range of masses and orbital distances the astronomers were able to study.
The scientists estimated that about 1.6pc of the stars in their sample had Jupiter-size planets and 12pc had super-Earths.
Prof Marcy is also a member of the US space agency Nasa's Kepler mission to survey 156,000 faint stars using a newer "transiting" method of planet-spotting.
This involves measuring the minute dimming of starlight as a planet passes in front of its star.