By BBC News Online science editor Dr David Whitehouse

Astronomers have found a new member of the Solar System that orbits the Sun beyond Neptune.

It could be half the diameter of Pluto, which in 1930 was the last planet to be discovered.

The object, temporarily named 2000 WR106, was spotted on 28 November. It was picked up by the University of Arizona's Spacewatch team which searches the sky for potentially hazardous objects.

Astronomers say that after Pluto, 2000 WR106 is the brightest object of its kind in this particular region of the Solar System.

Data uncertainties

The object was imaged by the Spacewatch survey telescope. After 12 observations made over three days, researchers reported the existence of the body to the International Astronomical Union's Minor Planet Center (MPC) in Cambridge, Massachusetts, who issued an alert.

Using the available data, the MPC calculated that the new object was 43 times further from the Sun than the Earth - a distance of about 6.5 billion kilometres (4bn miles). This puts it in a class known as the Trans-Neptunian Objects.

At the moment, astronomers say it is not possible to make a definitive statement about the object's diameter. This is because of uncertainties about its reflectivity.

However, if it is similar to other Trans-Neptunian Objects, it could be between 650 and 1,300 km (400 - 800 miles) across.

Asteroids and comets

By comparison, Pluto has a diameter of 2,370 km (1,470 miles).

Further observations of the object's position will be made in the coming weeks and months to improve knowledge of its orbital parameters and physical properties.

Astronomers will want to obtain a spectrum so that they can determine of what materials the body is made.

The Spacewatch Project is a search for new objects in space throughout the whole Solar System. It was started in 1980 by Tom Gehrels and Robert McMillan at the Lunar and Planetary Laboratory of the University of Arizona.

This project is one several around the world that automatically scan the sky for asteroids and comets.