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Last Updated: Wednesday, 21 November 2007, 18:05 GMT
Earth's Moon is 'cosmic rarity'
By Paul Rincon
Science reporter, BBC News

Moon. Image: Nasa
Only 5-10% of systems host moons like ours
Moons like the Earth's - which are formed in catastrophic collisions - are rare in the Universe, a study by US astronomers suggests.

The Moon was created when an object as big as the planet Mars smacked into the Earth billions of years ago.

The impact hurled debris into orbit, some of which eventually consolidated to form our Moon.

The Astrophysical Journal reports that just 5-10% of planetary systems in the Universe have moons created this way.

"When a moon forms from a violent collision, dust should be blasted everywhere," said lead author Nadya Gorlova of the University of Florida in Gainesville, US.

"If there were lots of moons forming, we would have seen dust around lots of stars - but we didn't."

Cosmic dust-up

Using Nasa's Spitzer Space Telescope, Dr Gorlova and her colleagues searched for the dusty signs of similar collisions around 400 stars that were all about 30 million years old.

This is roughly the age our Sun was when the Moon was formed in a collision.

Collision between planetesimals. Image: Nasa.
Collisions between rocky objects are key to planet formation

Rocky planets form from a porridge of dusty debris that surrounds young stars. They are gradually built up into bigger and bigger objects in messy collisions that spray lots of dust around.

Astronomers think this process lasts between 10 and 50 million years after a star forms. It is common to find dust swirling around stars at the young end of this range.

But by the time the Earth's moon formed, when the Sun was 30 million years old, the planet formation process in our Solar System should have been approaching its end.

Moon buggy. Image: Nasa.

In the latest study, Dr Gorlova's team looked at the heat signature of stars using the infrared. This allows astronomers to predict how much of that heat comes from the star itself and how much is re-emitted by dusty material encircling it.

"We found about two to four [dusty] objects, but only one fits all the characteristics of a moon collision. The dust is at the right temperature and at the right distance," Dr Gorlova told BBC News.

'Rare event'

Taking account of the amount of time that dust should stay around, and the age range when collisions like this should occur, the team calculated that moons like our own should form in only 5-10% of planetary systems.

Nasa's Spitzer infrared telescope

"We don't know that the collision we witnessed around the one star is definitely going to produce a moon, so moon-forming events could be much less frequent than our calculation suggests," said co-author George Rieke from the University of Arizona in Tucson, US.

But Scott J Kenyon, from the Center for Astrophysics at Harvard University, who studies the formation and evolution of stars and planets, was cautious about drawing firm conclusions.

He told BBC News: "What we're seeing in all of our discoveries is that the amount of debris that we see doesn't fit in detail with the theory.

"We see the debris that we expect, but it doesn't behave the way we'd like it to. We should see more debris among younger stars. But at least for terrestrial bodies, we're sometimes seeing more debris around older stars."

Dr Kenyon cited a study by Joseph Rhee, from the University of California, Los Angeles, and colleagues which found two stars with ages of 100 million years and 400 million years that are orbited by warm dust particles.

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