The asteroid, designated 2001 KX76, has a diameter of at least 1,200 kilometers (750 miles) and as much as 1,400 kms (875 miles), the European Southern Observatory (ESO) said in a press statement.

That makes it far bigger than Ceres, the first asteroid ever to be discovered, which was detected by the Italian astronomer Giuseppe Piazzi on January 1, 1801.

Ceres is about 950 kms (600 miles) across and has held the size record ever since its discovery.

The discovery of 2001 KX76 was announced on July 2 by a team led by Robert Millis at Lowell Observatory in Flagstaff, Arizona, but the first observations were so sparse that little more could be said beyond this.

Estimates of the asteroid's size and orbit have now been calculated in a unique project combining the ESO's telescope in La Silla, Chile, and a brand-new computer model, Astrovirtel, which simulates the actions of a telescope.

Astrovirtel was used to scan archived photographic plates obtained from various telescopes as well as recent observations made by La Silla in the same quadrant where the US team found the asteroid.

It was able to locate several plates in which faint images of 2001 KX76 could be identified, some of which dated way back to 1982.

This enabled astronomers to calculate its orbit, which when combined with an estimate of the asteroid's light reflection, or albedo, gave an idea of its size.

2001 KX76 is located about 6.5 billion kms (4.06 billion miles) from the Earth in a distant region of the Solar System called the Kuiper Belt, ESO, based in Garching, Germany, said.

So far, more than 400 bodies have been discovered in this region, but 2001 KX76 "is definitely the largest" discovered so far, it said.

Kuiper Belt objects are believed to be debris from the formation of the Solar System and intrigue astronomers as they are probably the most primitive objects available for close study from the Earth, relatively untouched by such forces as gravity, heat and light.

More and more of these enigmatic objects are coming to light as telescopes and computer power improves, and the discoveries are raising nagging questions about the prevailing view about the cosmos.

One of the challenged concepts is that Pluto, on the Kuiper Belt's fringe, is the Solar System's ninth planet.

Some astronomers contend that Pluto should be downgraded to an asteroid, although the International Astronomical Union (IAU), in a lively debate in February 1999, agreed to let it remain a planet after all.

But the ESO suggests the debate could be reopened.

The orbit of 2001 KX76 is "just outside" that of Pluto, it noted.

In addition, the asteroid is half the size of Pluto, whose diameter is about 2,300 kms (1,400 miles), and bigger than Pluto's moon, Charon, which has a diameter of 1,150 kms (720 miles).

"This increases the likelihood that there are other bodies still to be discovered in the outer Solar System that are similar in size to Pluto," ESO said.

Under astronomical tradition, objects in the Kuiper Belt are conferred with a name from mythology. The Millis team have the right to make their choice, which must then be ratified by the IAU's Committee on Small Body Nomenclature.

All rights reserved. � 2000 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.

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