Nobody has seen them yet, but scientists now speculate that there are tens of billions of planets the general size and mass of Earth in the Milky Way galaxy alone - a startling conclusion based on four years of viewing a small section of the nighttime sky.
The estimate, made by astronomers Andrew Howard and Geoffrey Marcy of the University of California at Berkeley, flows from the simple premise that the number of small but detectable exoplanets - planets outside Earth's solar system - is substantially larger than the number of big exoplanets in distant solar systems.
In a paper released Oct. 28 by the journal Science, the two report that based on this galactic preferential option for smaller planets, they can predict that almost one quarter of the stars similar to our sun will have Earth-size planets orbiting them.
"This is the first estimate based on actual measurements of the fraction of stars that have Earth-size planets," said Marcy, who did his observing with Howard at the Keck Observatory in Hawaii.
Their observations and speculations say nothing about whether all these Earth-size planets will actually have the same characteristics of Earth: its density, its just-right distance from the sun, the fact that it is a rocky structure rather than a gaseous ball.
But Marcy said that with so many Earth-size planets now expected to be orbiting distant suns - something on the order of 50,000,000,000,000,000,000,000 across the universe - the likelihood is high that many are in "habitable zones" where life can theoretically exist.
"It's tantalizing, without a doubt, to think some of those Earths are in habitable zones," Marcy said. "And based on what we know, really, why wouldn't they be?"
Current planet-hunting technology allows astronomers to find exoplanets down to the size of so-called super-Earths that are three times the size of our planet. The new conclusion that billions of planets similar in mass (or bulk) to Earth exist in the Milky Way is based on extrapolations of the number of these super-Earths compared with the number of larger exoplanets. Because the finding is not based on firm measurements, Marcy said "it's a very exciting set of numbers that we have confidence in, but there are yellow flags."