Astronomers have observed an object orbiting the Sun that is more distant than anything yet discovered.

It has been designated 1999 DG8. It is an example of what researchers call a Scattered Kuiper Belt Object (SKBO).

It is probably about 100 kilometres in size and made of ice and rock. It is believed to a leftover from the formation of the planets some four and a half billion years ago.

Since the first Kuiper Belt Object (KBO) was discovered in 1992, astronomers have found over 100 such objects that orbit the Sun beyond the most distant planet.

Neptune encounter

There could be over a hundred thousand of them whose mass in total would equal about 10% of the Earth's mass.

KBO's have been found in three types of orbit. Main belt KBO's, KBO's in a so-called resonant orbit with the planet Neptune, and SKBO's that have been flung into deep space after a gravitational encounter with Neptune.

A mission to the Kuiper belt is being planned

1999 DG8 was found by a team led by Dr Brett Gladman of the Canadian Institute of Theoretical Physics. He is currently working at the Observatorie de la Cote d'Azur in France.

It is the most distant solar system object ever photographed, being 60 times further away from the Sun than the Earth.

Because it was only seen on two consecutive nights earlier this year, astronomers do not have enough measurements to calculate an orbit for it. This means that researchers do not know how much further out it will go.

Accurate orbits

More SKO's were detected earlier this year. Chadwick Trujillo and David Jewitt of the Institute of Astronomy in Hawaii and Jane Luu of the Leiden Observatory in the Netherlands are about to report the discovery of three more.

Each one was tracked for three months allowing fairly accurate orbits to be calculated.

One of them, designated 1999 CF119, has the largest known orbit of anything orbiting the Sun. Its furthest point from the Sun is 200 times the Earth-Sun distance, which is known as an Astronomical Unit (AU).