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Earth's Moon: Once in a Blue Moon

Irene Klotz, Discovery News

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Our Rare Moon

Nov. 26, 2007 -- The huge impact that catapulted chunks of baby Earth into space and which ultimately formed our moon is probably rare, scientists have found in research that suggests the chances that complex life evolved on other planets are equally rare.

University of Florida researcher Nayda Gorlova used infrared imagery from the Spitzer Space Telescope to look for telltale dust clouds enveloping suns that are the right age to have young planetary systems in tow.

Her findings suggest the key conditions that formed our moon occur in no more than 10 percent of evolving planetary systems.

"When a moon forms from a violent collision, dust should be blasted everywhere," said Gorlova, the lead author of a study that appears in the current issue of Astrophysical Journal.

She and her colleagues searched around more than 400 stars that are about 30 million years old -- roughly the age that rocky planets began to take shape around our sun.

Sometime during Earth's formative years, a body about the size of Mars is believed to have smashed into it, breaking off a piece of the planet's mantle. Some of the debris fell into orbit around Earth and coalesced into the moon.

None of the other moons in our solar system formed from an impact with their mother world. Some were captured by their planet's gravity. Others formed simultaneously with the planets they circle.

Of the 400 stars studied, only one was shrouded in dust, the researchers reported. Extrapolating from the data, the team calculated the probability of another solar system making an Earth-like moon to be, at most, five to 10 percent.

The finding decreases the odds of complex life evolving on another planet because the collision that created our moon helped form a uniquely large body that is housed within the inner solar system. The moon's size and its proximity to the sun was key to life's rise, providing tidal surges that likely left primordial life forms on dry land for periods of time.

As Earth evolved, the moon also provided a stabilizing influence on the planet's spin, which makes climatic changes more gradual and less dramatic.