An international team of astronomers has described five new moons around Neptune.

The satellites may be captured asteroids, the researchers say in Thursday's issue of the journal Nature.

Planetary scientist J.J. Kavelaars of the National Research Council of Canada in Victoria, B.C., and his colleagues first announced the discovery in 2003.

Now the researchers have described the orbit of the five outer satellites.

Scientists say the moons orbiting giant planets fall into two classes:

  • Large moons, orbiting close to the parent planet and in a prograde manner, meaning they follow the rotational direction of the planet.
  • Irregular, often small moons, distant from the planet and orbiting in inclined, eccentric and often retrograde or opposite direction to the planet.

Neptune's largest moon, Triton, was discovered in 1846 and is as large as Pluto. Triton follows a retrograde orbit.

Nereid, discovered in 1949, is small with an eccentric orbit.

The Voyageur 2 spacecraft identified six more tiny moons in 1989.

Since then, moons were found orbiting other giant planets, but the absence of discoveries for Neptune had led scientists to theorize Triton's violent capture destroyed the planet's outer satellite system.

The addition of five new moons of Neptune may cause scientists to cast aside the theory.

"Here we report the discovery of five irregular moons of Neptune, two with prograde and three with retrograde orbits," the researchers wrote. "These exceedingly faint ... moons, with diameters of 30 to 50 km, were presumably captured by Neptune."

The researchers theorize the moons may have formed from collisions.

The team said the next step is to study the colour of the faint satellites and look for more satellites to determine the origin of the moons.