SYDNEY, Australia, March 15 (UPI)--Finding Earth-like planets with advanced life forms could be easier than we once thought, says one astrophysicist.

Charles Lineweaver, of the University of New South Wales, says the probability of a solar system harboring life-sustaining planets is directly proportional to the amount of metal in that system's star.

"The ability to produce "Earths" is zero at low metallicity and increases linearly with metallicity of the host star," Lineweaver said.

While too little metal content in a star precludes formation of Earth-like planets, too much will lead to massive "Jupiters," incompatible with life and liable to collide with planets similar to ours. Earth-like planets only form when the right amount of metal is present. Lineweaver terms this the "Goldilocks" planetary selection effect.

"These are reasonable assumptions," said Guillermo Gonzalez, Professor of Astronomy at the University of Washington, and an earth-origins researcher. "These ideas are not original to Lineweaver, but his is the best quantitative treatment to date," Gonzalez told United Press International.

Stars are critical to Earth-like planetary formation, because they create the metals the planets use as raw material, Lineweaver said.

"These heavy elements, or 'metals', were not produced in the Big Bang. They result from fusion inside stars and have been gradually building up over the lifetime of the universe," Lineweaver said. "The star formation rate plays a dual role in this analysis since stars make planets and stars make metals."

Using metal content, Lineweaver said he obtained the probability for a solar system to harbor an Earth-like planet. Combining this probability with current estimates of star formation rate and metal buildup in the Universe, he worked out an estimate of how old possible Earth-like planets could be.

"Three-quarters of the Earth-like planets in the Universe have an average age 1.8 billion years older than the Earth," Lineweaver said.

"I think the average age he quotes is a bit large, since most other galaxies are less massive than the Milky Way, and, hence, more metal-poor," Gonzalez told UPI.

Lineweaver said his analysis might reveal clues about the nature of life on other planets. "If life forms readily on Earth-like planets, this analysis gives us a rare clue about how we compare to other life that may inhabit the Universe," he says.

That clue indicates highly advanced aliens may not be so far-fetched. "The average Earth in the Universe is 1.8 billion years older than ours. And, if life exists on some of these Earths, it will have evolved, on average, 1.8 billion years longer than we have."

"Theoretical constructs, like this one by Lineweaver, are valuable and timely," says John Delano, distinguished teaching professor of Earth and Atmospheric Sciences at the State University of New York. "The implications of these ideas are profound."

So how might we find life on other planets? According to Lineweaver, it's as simple as using metal detectors in outer space.

"To maximize the chances of finding Earth-like planets, NASA's terrestrial planet finder (TPF) should look at stars with peak metal content," Lineweaver said.

Gonzalez concurs, but with a simple caveat. "This sounds reasonable, but the TPF should search a broader parameter range," Gonzalez told UPI. "There are other factors that make a planet truly 'Earth-like.'"

Lineweaver's paper will appear in a future issue of the journal Icarus.

(Reported by Mike Martin in Columbia, Missouri)

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