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Billions and Billions of Earth-Size Planets Estimated

Billions and Billions of Earth-Size Planets Estimated

Source: Seattle Times Newspaper (http://www.seattletimes.com)

Nobody has seen them, but scientists think there are tens of billions of planets the general size and bulk of Earth in the Milky Way galaxy alone, a conclusion based on four years of viewing a small section of the night sky.

By Marc Kaufman

The Washington Post

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Nobody has seen them, but scientists think there are tens of billions of planets the general size and bulk of Earth in the Milky Way galaxy alone, a conclusion based on four years of viewing a small section of the night sky.

The estimate, by astronomers Andrew Howard and Geoffrey Marcy of the University of California, Berkeley, flows from the logic 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 Thursday by the journal Science, the two report that based on this galactic preference 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 estimate means that the Milky Way galaxy, with its roughly 200 billion sunlike stars, has at least 46 billion Earth-size planets orbiting close to the stars, and perhaps billions more circling farther out in what astronomers call the habitable zone, Howard said.

The team's observations and extrapolations say nothing about whether all these Earth-size planets will have the characteristics of Earth: its density, its just-right distance from the sun, the fact that it is a rocky structure rather than gaseous ball.

But Marcy said that with so many Earth-size planets expected to be orbiting distant suns, 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."

The new assessment from Howard and Marcy, funded by NASA and Keck Observatory, comes weeks after two other astronomers published a paper saying they had detected an apparently rocky planet, Gliese 581G, in a habitable zone around a star relatively close to Earth.

That conclusion by Steven Vogt of the University of California, Santa Cruz, and Paul Butler of the Carnegie Institution in Washington, D.C., has not been confirmed.

The roughly 200 billion sunlike stars reported Thursday are all within 80 light-years of Earth, a short distance by astronomical measures. "What this means is that, as NASA develops new techniques ... it won't have to look too far" for Earth-size planets, Howard said.

Material from the Los Angeles Times is included in this report.

 

 

 

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