The most distant object ever seen in the Solar System appears to be even stranger than first suggested, after astronomers revealed full details of the discovery.
Nicknamed Sedna, for an Inuit goddess of the sea, the object lies three times as far from the Sun as Pluto and appears to be about three-quarters Pluto's size.
But orbital observations suggest it strays much further - more than 10 times its current distance - on an elliptical orbit that takes more than 10,500 years to complete.
That extreme distance makes Sedna's discoverers believe it may be the first ever sighting of an object orbiting in the remote Oort Cloud. This is a theoretical collection of icy bodies that surrounds the Solar System in a spherical shell from an unknown distance beyond Pluto to as far as several thousand times Pluto's distance from the Sun.
The Oort Cloud objects are thought to have formed around Jupiter's current orbit when the Solar System condensed from a dense gas cloud 4.6 billion years ago.
But the newborn gas giant planets are thought to have disturbed the remaining objects sometime in the Solar System's first 100 million years. Some were pushed into the Sun, others into interstellar space, and the rest into the current Oort Cloud.
The cloud is thought to contain as many as 10 trillion comets, some of which, like Halley, occasionally get nudged toward the Sun by passing stars. Sedna actually lies about 10 times closer than the expected inner bounds of the Oort Cloud, so the discovery team believes a star moving near the Sun a few billion years ago pushed it into its observed orbit.
"This is the first good direct evidence that the Sun formed in a cluster of stars," said co-discoverer Michael Brown of the California Institute of Technology.
But other astronomers are not so sure. Sedna may be the same type of object as Pluto, which is the largest known Kuiper-Belt Object, or KBO. The Kuiper Belt is a flat ring of ice and rock also left over from the birth of the Solar System that stretches outwards from Neptune to just beyond Pluto.
About 800 Kuiper-Belt Objects (KBOs) have been discovered since 1992, some with predicted orbits that would take them as far as 150 billion kilometres from the Sun, beyond Sedna.
"My belief is this object belongs to the Kuiper Belt," says astronomer Kathrin Altwegg of the University of Bern in Switzerland. She cites the fact that the object lies in the orbital plane of the planets - the same as all KBOs. In contrast, Oort Cloud comets come from any direction in the sky.
Sedna's 1700-kilometre diameter also argues against an Oort Cloud origin, she says - comets arriving from the cloud are usually less than 100 km in size. "Small objects are kicked out farther than large objects," she explains.
Sedna's discovery highlights the gaping holes in astronomers' understanding of where the Kuiper Belt ends and the Oort Cloud begins. "In principle, the Kuiper Belt and the Oort Cloud are the same thing," Altwegg says.
The Oort Cloud objects, which formed near giant planets Jupiter and Saturn, were simply pushed further out during the Solar System's tumultuous early years.
Sedna was discovered on 14 November 2003 using a 1.2-metre, wide-field telescope on Palomar Mountain near San Diego, California, US. Other telescopes around the world soon confirmed the observation, but NASA's Spitzer Space Telescope failed to detect heat from the object.
That told astronomers Sedna must be under 1700 km wide, about three-quarters Pluto's size. This is not big enough to be called a planet, Brown says, preferring to use the term planetoid.
The new object may be trailed by a tiny moon and has a temperature of -240ýC - the coldest known object in the Solar System.