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Earth's moon rare: astronomers

books Updated: Nov 23, 2007 12:52 IST

IANS
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Though moons are common enough in the universe, ours is rather uncommon, according to a new study by US astronomers.

The Earth's moon, the subject of much art, myth and poetry, was formed out of a tremendous collision, a rare event seen in less than 10 per cent of moon formations, Sciencedaily.com reported.

The study, based on new observations by NASA's Spitzer Space Telescope, was undertaken by researchers at the University of Florida and appears in the latest edition of the Astrophysical Journal.

Scientists say the moon was created between 30 million and 50 million years after the sun was born, and after other rocky planets had begun to take shape.

A body as big as Mars is thought to have smacked into the infant Earth, breaking off a piece of its mantle. Some of the resulting debris fell into orbit, eventually coalescing into the moon we see today.

The other moons in our solar system either formed simultaneously with their planet or were captured by the planet's gravity.

"When a moon forms from a violent collision, dust should be blasted everywhere," said Nadya Gorlova, lead author of the study. "If there were lots of moons forming, we would have seen dust around lots of stars - but we didn't."

Gorlova and her colleagues looked for the dusty signs of similar smash-ups around 400 stars that are all about 30 million years old.

They found only one out of the 400 stars immersed in the telltale dust. Taking into consideration the amount of time the dust should stick around, and the age range at which moon-forming collisions can occur, the scientists calculated the probability of a solar system making a moon like Earth's to be at most five to 10 per cent.

The astronomers said that the planet-building process itself winds down by 30 million years after a star is born. Like our moon, rocky planets are built up through messy collisions that spray dust all around.

Current thinking holds that this process lasts from about ten million to 50 million years after a star forms.

The fact that Gorlova and her team found only one star out of 400 with collision-generated dust indicates that the 30-million-year-old stars in the study have, for the most part, finished making their planets.