Each giant planet in the solar system has two main types of moon. "Regular" moons are typically large and follow circular orbits. They also rotate in the same direction as their planet in "prograde" orbits. Irregular moons, on the other hand, are smaller, have highly elliptical and inclined orbits, and rotate in "retrograde" orbits.

Until now the two known irregular moons of Neptune -- Triton and Nereid -- appeared to be different to the irregular moons of other planets. Triton, which was discovered in 1846, is as big as Pluto and traces a circular but retrograde orbit. Nereid, discovered in 1949, is small with a highly eccentric prograde orbit. However, the discovery of the five new irregular moons -- two with prograde and three with retrograde orbits -- means that Neptune's moons are more like those of the other giant planets after all.

A team of astronomers led by Matthew Holman of the Harvard-Smithsonian Center for Astrophysics discovered the moons using wide-field mosaic CCD cameras on the 4-meter telescope at the Cerro Tololo Inter-American Observatory in Chile and the 3.6-meter Canada-France-Hawaii telescope in Hawaii (M Holman et al. 2004 Nature 430 865). These cameras are capable of detecting objects that are more than a hundred million times dimmer than a bright star. The astronomers took long series of images, which they then combined using software, to look for small, unresolved moving objects that were too faint to see in single exposures.

The team investigated the entire stable region of space around Neptune in which satellites are able to orbit. This region, known as the "Hill Sphere", is where the planet's own gravitational forces outweigh the tidal forces of the Sun. For Neptune, the Hill Sphere radius extends out to about 1.15 x 1011 kilometres.

Holman and co-workers calculated that all the new moons have relatively small diameters -- between 30 and 50 kilometres. According to Holman, this suggests that they could be fragments left over from collisions between larger progenitors that were later captured by Neptune. Most irregular moons around the giant planets are thought to form in this way.

The two new moons found around Saturn are only three and four kilometres across, which makes them the smallest bodies seen around the ringed planet so far. Their discovery raises Saturn's satellite count to 33.