Last week, astronomers reported they'd found a star system where planetary "embryos" seemed to be smashing into one another, much like the bang-up believed to have created our own Moon.
Now another team, working with NASA's Spitzer Space Telescope, say that an actual moon-creating smash-up seems in fact to be quite rare indeed.
Led by Nadya Gorlova of the University of Florida, Gainesville, the team studied 400 stars around the universe estimated to be between 30 million and 50 million years old. Scientists think that was about the age of our Sun when a Mars-sized object collided with the Earth, breaking off debris that ultimately coalesced into the Moon.
Any such massively destructive encounter would naturally throw up a lot of dust -- but the team found only one instance out of the 400 where a considerable amount of telltale dust was found. From this, they conclude that the chances of a moon-forming collision happening in a solar system are only between five percent and 10 percent.
"When a moon forms from a violent collision, dust should be blasted everywhere," Gorlova said in a statement. "If there were lots of moons forming, we would have seen dust around lots of stars - but we didn't."
Of course that doesn't mean that moons themselves are necessarily uncommon. Most moons in our own solar system – and there are plenty of them – were formed instead at the same time as their mother planet, or were captured by that planet's gravity. The same would presumably hold true in many other solar systems.
The observations are also sparking new thoughts about the age at which rocky planets tend to be formed. The prevailing view is that Earth-like planets, which are also formed through dust-spewing collisions, tend to be built up between 10 million and 50 million years after a star's birth.
However, the very limited amount of dust around the 30 million-year-old stars seems to indicate that they are mostly through with the planet-making process, the team said.
A paper on the team's results is being published in the Nov. 20 issue of the *Astrophysical Journal. *
Astronomers Say Moons Like Ours Are Uncommon [NASA JPL]
(Image: The Earth and the Moon, paired in a 1992 view from the Galileo spacecraft. Credit: NASA JPL)
