The rock lies in the Kuiper Belt, the name for the flock of objects beyond Neptune's orbit that are believed to be leftover rubble from the Solar System's building phase and are the source for many comets, the Canada-France Ecliptic Plane Survey (CFEPS) said.

Measuring between 500 and 1,000 kilometers (300 to 600 miles) across and taking about 440 years to make just one circuit of the Sun, Buffy is remarkable not for its size -- around half a dozen identified Kuiper Belt objects are bigger -- but for its location and orbital tilt.

"This new object challenges current theories about the history of the early Solar System," CFEPS said in a press release.

"(...) This new discovery is exciting because it causes us to rethink our understanding about how the Kuiper Belt formed."

Buffy has an almost perfect circular orbit and encircles the Sun at an extreme tilt, at 47 degrees to the orbital plane of the planets as they swing around the Sun.

But it lies in a curious outer region of the Kuiper Belt, on the dark, bone-freezing fringes of the Solar System.

The theory is that, billions of years ago, this remote community of rocks, the so-called "extended scattered disk" of the Kuiper Belt, got their extremely eccentric orbits because of a passing star.

The star's gravitational pull was enough to give the objects a tug, pulling them out of a circular orbit but not enough to coax them away from enslavement to the Sun.

But Buffy is the odd one out -- its almost-perfect orbit and tilt circular puts a dent in the "star" theory.

One possibility, the discoverers say, is that, in the infant days of the Solar System, the nascent Neptune lay much closer to the Sun.

It eventually migrated outwards, causing at least some members of the Belt's "extended scattered disk" to develop more circular and tilted orbits, they speculate.

Buffy is the temporary name given by the team for the object, whose official designation by the Paris-based International Astronomical Union (IAU) is 2004 XR 190.

Its orbit is in a relatively narrow range of between 52 and and 62 astronomical units (AU) from the Sun (an AU is a standard measurement, being that of the distance between the Earth and the Sun, of approximately 150 million kilometers, or 93 million miles).

By comparison, another "extended scattered disk" member called Sedna swings out to as far as 900 AU before coming as close to the Sun as 76 AU.

The Kuiper Belt was first recognised in 1992.

Most of its objects lie in a region that extends from 30 to 50 AU where there are "at least" 70,000 rocks with a diameter of 100 kms (60 miles) or more, according to David Jewitt, a specialist at the University of Hawaii.

Buffy was first noted by Lynne Allen of the University of British Columbia, Canada, in December last year as she pored over data from powerful computers that sift through telescopic images in search of new celestial sightings.

Because Kuiper Belt objects take so long to go around the Sun, it takes between one and two years of additional observations to calculate their orbits precisely.

Further measurements are needed over the next three months for a fine-tuning of Buffy's orbit.

related report

Discovery of a large Kuiper Belt object with an unusual orbit

A team of astronomers working in Canada, France and the United States have discovered an unusual small body orbiting the Sun beyond Neptune, in the region astronomers call the Kuiper belt. This new object is twice as far from the Sun as Neptune and is roughly half the size of Pluto. The body, temporarily code-named "Buffy", has a highly unusual orbit which is difficult to explain using previous theories of the formation of the outer Solar System.

Currently 58 astronomical units from the Sun (1 astronomical unit, or AU, is the distance between the Earth and the Sun), the new object never approaches closer than 50 AU, because its orbit is close to circular.

Almost all Kuiper belt objects discovered beyond Neptune are between 30 AU and 50 AU away. Beyond 50 AU, the main Kuiper belt appears to end, and what few objects have been discovered beyond this distance have all been on very high eccentricity (non-circular) orbits.

Most of these high-eccentricity orbits are the result of Neptune "flinging" the object outward by a gravitational slingshot. However, because this new object does not approach closer than 50 AU, a different theory is needed to explain its orbit. Complicating the problem, the object's orbit also has an extreme tilt, being inclined (tilted) at 47 degrees to the rest of the Solar System.

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