Once they were three

In its youth our Moon may have once been bracketed by two asteroidal companions, says a new study. From Earth, these tiny moons – up to 100 km wide – would have appeared as two extremely bright stars.

The mathematical modelling study, detailed in the journal Icarus, predicts that the moonlets could have existed for hundreds of millions of years in gravitationally stable zones of equilibrium know as Lagrange points. These are where the gravitational pull of the Earth and Moon is cancelled out, allowing objects (such as research probes) to remain in a long-term stationary orbit.

Lone wolf

Until now, the accepted view had been that our anomalously large Moon had always been a lone wolf. It is believed to have formed some 4.4 billion years ago in the aftermath of a chance encounter between a Mars-sized planetary embryo and our early Earth.

But the paper's co-authors, planetary scientists Jack Lissauer, at NASA Ames Research Centre in Mountain View, California, USA and John Chambers at the Carnegie Institution in Washington DC, believe the moonlets (named 'Trojans' after similar captured satellites of Jupiter) may have formed simultaneously with the larger impact.

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