(Xinhua)
Updated: 2007-11-21 14:11

Los Angeles -- The moon came into existence after a body as big as Mars crashed into the then-infant planet Earth, knocking loose part of its mantle, the Jet Propulsion Laboratory (JPL) reported Tuesday.

Debris from the collision is believed to have coalesced into the familiar white satellite we see today, said the JPL, based in Pasadena, Los Angeles.

A moon comes into existence in only 5-10 percent of planetary systems at most, said the JPL in a report in the Astrophysical Journal.

The finding is based on observations from NASA's Spitzer Space Telescope, which is managed by JPL.

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

Scientists used the Spitzer Space Telescope to look for the telltale dust around 400 stars that are about 30 million years old -- roughly the age of the sun when Earth's moon formed. Only one of those stars was immersed in dust, JPL reported.

"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 then calculated the probability of a solar system making a moon like Earth's to be at most 5 to 10 percent," according to JPL.

Still, astronomers believe that there are billions of rocky planets in the universe and "5-10 percent of billions is still a lot of moons," JPL reported.