Another moon has been found orbiting the planet Uranus - 13 years after the crucial photographs were taken.

It's the 18th moon discovered orbiting this distant gas giant planet. Until now, Saturn was the only planet in our solar system known to have as many as 18 satellites.

Erich Karkoschka, a researcher at the Lunar and Planetary Lab of the University of Arizona in Tucson, made the discovery.

Only 25 miles wide

"This discovery is very unusual," he said. "Typically, satellites are found within days after the discovery image has been taken. In this case, the discovery image is more than 13 years old."

The Voyager 2 took seven images of the new satellite when it flew past Uranus in January, 1986. However, nobody recognized the satellite until Karkoschka investigated these images recently.

He has studied the Uranian satellites based on images taken with the NASA/European Space Agency Hubble Space Telescope (HST) and found the new satellite when he compared his HST results with images taken by Voyager 2.

The new moon, designated S/1986 U 10, passes close to the previously known moon, Belinda. They pass each other once a month. This is the first example of two satellites in nearby orbits passing each other so slowly.

Soon after England's William Herschel discovered Uranus in 1781, he found its two largest satellites, which are about half the size of our moon. In 1851, English astronomer William Lassell detected two more Uranian satellites. In 1951, Gerard Kuiper of the University of Chicago discovered Uranus' fifth satellite.

The Voyager team found 10 more Uranian satellites in 1985-86.

Two years ago astronomers discovered the 16th and 17th satellites. These two satellites are some 100 times farther away from Uranus than are the satellites discovered by the Voyager team.

The new satellite is only about 40 kilometres (25 miles) in diameter.

Based on the detection in seven images, astronomers conclude that the satellite orbits Uranus once every 15 hours and 18 minutes. This is similar to the rotation period of Uranus.