Earth-sized planets could be ten a penny, say UC Berkeley astronomers, who estimate they may orbit as many as a quarter of all Sun-like stars.
"One of astronomy's goals is to find eta-Earth, the fraction of Sun-like stars that have an earth," says Berkeley's Andrew Howard. "This is a first estimate, and the real number could be one in eight instead of one in four. But it's not one in 100, which is glorious news."
Over a five-year period, Howard and Geoffrey Marcy examined 166 G and K stars within 80 light years of Earth using the Keck telescope and found large numbers of planets.
"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," said Howard.
"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."
Because the researchers only detected only planets close to their star, there could be even more Earth-sized planets further out, such as within the habitable zone in which the Earth is found.
The researchers' results conflict with current models of planet formation and migration. After their birth in a protoplanetary disk, planets had been thought to spiral inward because of interactions with the gas in the disk. Such models predict a 'planet desert' in the inner region of solar systems.
The astronomers used the 10-meter Keck telescopes in Hawaii to measure the minute wobble of each star. Current techniques allow detection of planets massive enough and near enough to their stars to cause a wobble of about one meter per second.
That means they saw only planets that were at least 15 to 30 times the mass of the Earth at up to a quarter of one astronomical unit from their star.
Only 22 of the stars had detectable planets – 33 planets in all – within this range. After accounting statistically for the fact that some stars were observed more often than others, the researchers estimate that about 1.6 percent of the Sun-like stars in their sample had Jupiter-size planets, and 12 percent had super-Earths of three to ten Earth masses.
If the trend of increasing numbers of smaller planets continues, they conclude, 23 percent of the stars would have Earth-sized planets.
This would imply that there are 120-260 "plausibly terrestrial worlds" orbiting some 10,000 nearby G and K dwarf stars with orbital periods less than 50 days.
Twelve further possible planets were also detected, but need further confirmation.
"What this means," Howard added, "is that, as NASA develops new techniques over the next decade to find truly Earth-size planets, it won't have to look too far."
The full report is in Science.