By Dr David Whitehouse BBC News Online science editor

The rare body is a type of asteroid known as a Trojan.

Clusters of Trojans are known to share the same orbit as Jupiter.

Projections of its trajectory into the future reveal that it can co-orbit with Neptune for at least billions of years Dr Eugene Chiang

The object, catalogued as 2001 QR322, is the first to be found in association with Neptune.

"Neptunian Trojans were long suspected to exist and it is gratifying to finally know that they do," said US team member Eugene Chiang of the University of California at Berkeley.

The first Trojan associated with Jupiter was discovered in 1906 and about 1,600 such objects are now known.

Pristine object

2001 QR322 was found during a survey of the outer Solar System using telescopes in the US and Chile funded by the US space agency (Nasa).

It is estimated to be about 230 km (140 miles) in diameter and, like Neptune, requires about 166 years to orbit the Sun.

Researchers originally found it on 21 August 2001 in deep digital images taken with the four-metre Blanco Telescope at Cerro Tololo.

However, it took over a year of observations along with orbital calculations to prove that 2001 QR322 is a Neptune Trojan.

Astronomer Eugene Chiang said: "The orbit of 2001 QR322 is remarkably stable; projections of its trajectory into the future reveal that it can co-orbit with Neptune for at least billions of years.

"It is likely that 2001 QR322 is a dynamically pristine object whose orbital eccentricity and inclination have been largely unaltered by processes that afflicted the majority of bodies in the outer Solar System."