Earth may once have had more than one moon, most probably two more, NASA scientists think.
Prevailing scientific consensus holds that the existing Moon was formed when a Mars-sized planet collided with the Earth 4.5 billion years ago, when the solar system was very young.
So much matter would have been thrown up into space that it recollected under its own gravity to form the Moon, which for millions of years would have glowed red-hot as the molten rock from the planetary collision cooled.
However, researchers Jack J. Lissauer of NASA's Ames Research Center near Mountain View, Calif., and John E. Chambers of the Carnegie Institution of Washington figure quite a bit of that ejected matter would have recollected into two other small moons at the so-called "Lagrangian points" or "Trojan points."
Those are fixed places in the Moon's orbit around the Earth where the gravity of both large bodies would keep smaller objects in stable positions.
"The giant impact that likely led to the formation of the Moon launched a lot of material into Earth orbit, and some could well have been caught in the Lagrangian points," Lissauer tells New Scientist magazine.
For example, there are two groups of asteroids in the Lagrangian points of Jupiter's orbit around the Sun — one called the "Greek camp" leads the giant planet by 60 degrees in its circular orbit, while the other group, dubbed the "Trojan camp," follows 60 degrees behind.
Lissauer and Chambers figure the two small moons would have orbited Earth for about 100 million years before the slight gravitational tugs from the solar system's other planets sent them spiraling off course, either to crash into the Moon or Earth or to drift off into space.
In a separate study, astrophysicist Matija Cuk of the University of British Columbia thinks the smallest possible moons, those only a few dozen miles across, could have lasted for up to a billion years at the Lagrangian points in the Moon's orbit.
"They would have looked more like Jupiter or Venus in the sky than a satellite," Cuk said to New Scientist. "They would have resembled very bright stars."