Scientists estimate tens of billions of Earth-size planets in Milky Way

According to scientists, there are tens of billions of Earth-size planets in the galaxy.
By Marc Kaufman
Washington Post Staff Writer
Thursday, October 28, 2010; 2:02 PM

Nobody has seen them yet, but scientists now believe there are tens of billions of planets the general size and bulk 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 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 observations and extrapolations say nothing about whether all these Earth-size planets will actually 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 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."

Exoplanet hunters, who found the first planet outside our solar system in 1995, are entering a period of especially heightened and excited discovery. The new assessment from Howard and Marcy, funded by NASA and Keck Observatory, comes only weeks after two other astronomers published a paper saying they had detected an apparently rocky planet in a habitable zone around a star relatively close to Earth called Gliese 581G.

(READ ARTICLE: First 'habitable zone' planet found outside solar system)

That conclusion by Steven Vogt of the University of California at Santa Cruz and Paul Butler of the Carnegie Institution in Washington, has not been confirmed, and some have challenged the discovery, especially a Swiss team that has been a leader in exoplanet research. But very few now doubt that Earth-size and Earth-like exoplanets in habitable zones will be found in the months and years ahead.

The assessment that Earth-size planets are ubiquitous in distant solar systems is expected to get additional support in February when the scientists operating NASA's Kepler Mission, which is searching for Earth-size and habitable planets, report on what they have been finding.

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