New observations of the planet Neptune by the Hubble Space Telescope and a ground-based telescope in Hawaii have reawakened an old puzzle about the broken rings that encircle the planet, according to two reports in the 19 August issue of Nature.

Although Saturn's broad, bright rings make it the most instantly recognizable of the huge gas-giant planets of the outer Solar System, in fact all four of these worlds – Jupiter, Saturn, Uranus and Neptune – are circumscribed by dust rings. But those of the Jupiter, Uranus and Neptune are narrow, and some are gossamer-thin; so they are not easy to spot from Earth. Uranus's rings were discovered in 1977, prompting a search for similar structures around the other planets. In 1984, they were seen around Neptune as it passed in front of a particularly bright star.

But Neptune's rings were different. They were, in fact, not rings at all, but arcs: mere fragments of rings, clumps of dust like raised eyebrows above the planet's surface. This was profoundly surprising, because any such material in orbit around the planet would be expected to become stretched out into a continuous ring in a matter of months. Yet Neptune's arcs are still there today. The three main arcs are clustered in just a small segment of a complete ring, and are named Liberté, Egalité and Fraternité.

The explanation favoured so far for this revolutionary trinity invokes the gravitational effects of one or more of Neptune's moons. In addition to one large moon, Triton, Neptune has at least seven smaller satellites ranging in size from about 30 to about 200 kilometres across. Researchers have believed that these 'shepherd' the dust around Neptune into the arcs, preventing them from spreading into true rings. (In fact, Neptune does also possess no fewer than five 'true' rings. The arcs lie embedded in one of these rings – the Adams ring – which is even more tenuous.)

The arcs lie more or less along the orbit of a satellite called Galatea, a moon about 80 kilometres across. It has been widely believed that Galatea is the sole shepherd, ushering the dust into a 'resonance' point along its orbit which rotates in synchrony with the satellite itself.

But Christophe Dumas of the Jet Propulsion Laboratory in Pasadena, California and colleagues have now used the Hubble Space Telescope to take snapshots of Neptune's arcs, enabling the researchers to locate them more accurately than before. And Bruno Sicardy of the Observatoire de Paris, France and colleagues have taken pictures from the Canada-France-Hawaii telescope on the Pacific island, using so-called adaptive optics to eliminate the distorting effects of the Earth's atmosphere. The ground-based observations support the conclusions from the Hubble pictures: the arcs are not quite in the right place to be shepherded by Galatea.

They are indeed very close to the resonance point, however, and so it is possible that Galatea does play a major role in confining the dust. But that can't be the whole story – the dust would be expected gradually to leak out of its present positions and spread around the ring if Galatea alone was holding it there. Both groups speculate that perhaps some additional, smaller (and so unseen) satellite might be assisting, or even dominating, the shepherding. A two-moon theory for confinement of the arcs was in fact suggested by Jack Lissauer of the University of California in 1985. A body just six kilometres wide might be sufficient – but would be very hard to see with current telescopes.