http://news.bbc.co.uk/2/hi/science/nature/7104558.stm
*Earth's Moon is 'cosmic rarity' *
By Paul Rincon
Science reporter, BBC News
*Moons like the Earth's - which are formed in catastrophic collisions - are
extremely 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.
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.
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.
"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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