The ancient catastrophe that gave birth to the Moon may have produced additional satellites that lingered in Earth's skies for tens of millions of years.
A new model suggests moonlets may have once occupied the two Earth-Moon Lagrangian points, regions in space where the gravitational tug of the Earth and the Moon exactly cancel each other out. Objects trapped in these points are called Trojans and can remain stationary forever if left undisturbed.
Scientists think the Moon was created when Earth was struck by a Mars-sized object some 4.5 billion years ago.
"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," says study team member Jack Lissauer of NASA Ames Research Center in California, US.
Once captured, the Trojan satellites likely remained in their orbits for up to 100 million years, Lissauer and co-author John Chambers of the Carnegie Institution of Washington say. Then, gravitational tugs from the planets would have triggered changes in the Earth's orbit, ultimately causing the moons to become unmoored and drift away or crash into the Moon or Earth.
"The perturbations from the other planets are very, very tiny," Lissauer said. But they change the shape of Earth's orbit, which changes the effect that the Sun's gravity has on the moons. "[That] is what ultimately destabilises the Trojans," Lissauer told New Scientist.
Separate modelling work by Matija Cuk, an astrophysicist at the University of British Columbia in Canada, suggests small, asteroid-sized objects a few tens of kilometres across would have lasted the longest as Trojan satellites. Cuk estimates these 'lost moons' might have circled Earth for a billion years or more after the Moon's formation."They would have looked more like Jupiter or Venus in the sky than a satellite," Cuk told New Scientist. "They would have resembled very bright stars."