Saturn's rings
Why are Saturn's rings so spectacular? It could be that the planet managed to cling onto a moon when all the other gas giants in our solar system had already lost theirs. Today's rings formed when the moon was smashed up.

Sebastien Charnoz and colleagues at the University of Diderot, Paris, suggest it was during the "late heavy bombardment", 700 million years after Saturn formed, that a chunk of debris collided with one of the planet's moons. Because the moon was orbiting at just the right distance from Saturn when it shattered - within the so-called Roche limit - the tiny pieces formed the rings instead of dispersing.

This could explain why other planets don't have rings like Saturn's. Even if other planets had moons within their Roche limits at the birth of the solar system, the team's calculations show that the moons would soon been dragged down into the planet or unshackled from their orbits. Yet Saturn's rapid rotation meant it could hold a satellite within its Roche limit until the bombardment. The work will appear in the journal Icarus.

Ken Rice from the Institute of Astronomy in Edinburgh, UK, says the team's way of showing Saturn's uniqueness among gas giants is interesting, but that their hypothesis cannot be proved until we have better ways of modelling the evolution of the solar system.