Large world found near Pluto

2001KX76 shows up as two coloured dots against the stars

By BBC News Online science editor Dr David Whitehouse

Astronomers have found one of the largest objects ever detected orbiting the Sun.

When we spotted it we just wrote 'wow' on the image

Lawrence Wasserman, Lowell Observatory

It was seen in a deep space survey looking for bodies circling our star out near Pluto, the most distant planet. Only planets are larger than this new object, dubbed 2001 KX76.

The icy, reddish world is over a thousand kilometres across and astronomers say there may be even larger objects, bigger than planet Pluto itself, awaiting discovery.

"What we have seen may be only the tip of the iceberg," co-discoverer Dr Lawrence Wasserman told BBC News Online.

Big surprise

The world - it is big enough to be called a world - has a typical reddish hue and is probably covered in ice. It orbits the Sun beyond Neptune in the so-called Kuiper Belt - a region that extends far beyond the known planets.

Since 1992, over 400 Kuiper Belt Objects (KBOs) have been detected. Their discovery has revolutionised our view of the distant reaches of our Solar System. It is the sheer size of 2001 KX76 that is exciting astronomers.

"When we spotted it, we just wrote 'wow' on the image," Lawrence Wasserman, of the Lowell Observatory in Arizona, US, said. "We knew right away it was a big one."

Lowell Observatory Director, Robert Millis, added: "This object is intrinsically the brightest Kuiper Belt Object found so far."

"The exact diameter of 2001 KX76 depends on assumptions that astronomers make about how its brightness relates to its size. Traditional assumptions make it the biggest by a significant amount, while others make it larger by at least 5%."

Uncertain orbit

2001 KX76 could be as large as 1,270 km (788 miles) across, bigger than Ceres, the largest known asteroid (an object that orbits the Sun between Mars and Jupiter). It is even larger than Pluto's moon Charon, which has an estimated diameter of 1,200 km (744 miles).

It is just a matter of time until we see Pluto 2, Pluto 3, and so on

Dr David Jewitt, University of Hawaii

2001 KX76 was discovered in the course of the Deep Ecliptic Survey, a Nasa-funded search for KBOs. It was seen on 22 May in deep digital images of the southern sky taken with the 4-metre Blanco Telescope at Cerro Tololo in Chile.

Astronomers estimate that 2001 KX76 is currently at a distance of just over 6.4 billion km (4 billion miles) from the Sun. Its orbit is inclined by approximately 20 degrees with respect to the major planets, but the detailed shape of its orbit remains uncertain.

Available evidence suggests that the newly discovered KBO may be in an orbital dance with Neptune, orbiting the Sun three times for each time that Neptune completes four orbits.

"2001 KX76 is so exciting because it demonstrates that significant bodies remain to be discovered in the Kuiper Belt," Robert Millis explains.

True extent

Lawrence Wasserman agrees: "We have every reason to believe that objects ranging up to planets as large or larger than Pluto are out there waiting to be found."

Dr David Jewitt of the University of Hawaii who has discovered many KBOs, including the first one ever seen, told BBC News Online: "We're inching up to Pluto. It is just a matter of time until we see Pluto 2, Pluto 3, and so on."

Robert Millis said: "Until the Kuiper Belt has been thoroughly explored, we cannot pretend to know the extent or the content of the Solar System."

The researchers hope that other astronomers who have access to large telescopes over the next few weeks will be able to turn them on 2001 KX76 in the hope of gathering enough light to get a spectrum of the object.