Nereid, a distant and wayward satellite of the planet Neptune, has water – or rather ice – on its surface. This finding, reported by Michael E. Brown and colleagues from the California Institute of Technology in Pasadena, California in Astrophysical Journal Letters, may shed light on the origin of this strange and tiny satellite, as well as on other icy and rocky inhabitants of the outer regions of the Solar System.

Neptune has eight known satellites. Six are small – each no more than 400 km across and mostly much smaller – and orbit close to the planet. They were all discovered by the spacecraft Voyager 2 on its visit to Neptune in 1989. Another, Triton, is 2,700 km in diameter, the size of a small planet. Triton was discovered in 1846, a month after the discovery of Neptune itself. Triton is unusual in that it is the only large satellite in the Solar System to orbit a planet in the 'retrograde' sense – that is, in the direction opposite to the rotation of Neptune.

The last, Nereid, was discovered in 1949 by a pioneer of the outer Solar System, Gerard Kuiper. At about 350 km in diameter, Nereid has a highly elliptical orbit, which is also tilted at an angle relative to Neptune's equator, and its distance from Neptune varies from 1,353,000 to 5,980,200 km.

Small satellites with distant, elliptical and inclined orbits around giant planets are nothing unusual in the Solar System. They are usually assumed to be asteroids captured by the gravitational field of the planet concerned. But is this true for Nereid? Brown and colleagues suspect not.

Their evidence comes from spectroscopy. They obtained near-infra-red images of Nereid using the Keck telescope in Hawaii. A spectrum contains a great deal of information about the chemical composition of an object's surface. The spectrum of Nereid is similar to those of two of the larger satellites of Uranus, Oberon and Umbriel. Like these two, much larger bodies (each is more than 1,000 km in diameter), Nereid appears to have a surface that consists in part of water ice.

Crucially, Neried's spectrum differs from those of the darker, redder 'Centaurs' – a family of comet- or asteroid-like bodies that may have originated in the 'Kuiper Belt' beyond Neptune, and currently careen among the giant planets on chaotic orbits. The Centaurs are good models for prosepctive 'captured' satellites of giant planets.

Brown and colleagues think that Nereid began its career as a conventional satellite. So what explains its strange orbit? The answer may be Triton. Triton's retrograde orbit, and details of its composition, suggest that it – not Nereid – was a 'captured' object, a Pluto-like planet trapped by Neptune's gravity early in the history of the Solar System. This capture may have forced Nereid, perhaps in a neat, circular orbit closer to Neptune, into its present unusual track.