New world rivals Pluto

Veruna: 43 times further away from the Sun than Earth

By BBC News Online science editor Dr David Whitehouse

Detailed observations of a recently discovered large object is prompting a reappraisal of the myriads of icy worlds that live in the cold, outer reaches of our Solar System - the Kuiper Belt.

The observations are of Varuna, which was discovered in November 2000. It was immediately recognised as a very large object, probably the largest in the Kuiper Belt except for Pluto and its moon Charon.

Combining data obtained from two different types of telescope, the researchers have calculated Varuna's diameter to be 900 km (550 miles).

Varuna's large size threatens Pluto's status as a fully-fledged planet as it now seems to be merely the largest of a swarm of similar large worlds in deep space.

More reflective

Writing in the journal Nature, David Jewitt of the Institute for Astronomy, Honolulu, Hawaii, US, and colleagues report simultaneous optical and sub-millimetre wavelength measurements of Varuna, determining its size and reflectivity for the first time.

Veruna was seen in 1953 but not recognised at the time

Until now, Pluto and its moon Charon were the only members of this ancient ring of icy bodies for which accurate sizes were known.

At 900 km (550 miles) across, Varuna is only slightly smaller than Charon (1,200 km or 750 miles in diameter), the tiny moon that orbits Pluto (2,400 km or 1500 miles in diameter), the most distant of the Sun's planets.

The data also indicates that Varuna is more reflective than most other small worlds for which accurate measurements are available - though it is less reflective than Pluto or Charon.

Shuttle telescope

Scientists say that Varuna goes some way to vindicate the views held by the late US astronomer Clyde Tombaugh. He discovered Pluto in 1930 looking for what he called Planet X. He continued his search after its discovery believing there were other worlds out there waiting to be discovered.

Stephen Tegler, of Northern Arizona University, said: "This work raises the possibility that Pluto is not the only Planet X, but perhaps one of several.

"We can now imagine that bodies even larger and more distant than Pluto will be found. Such objects have so far escaped detection because of their extreme faintness, due in part to the feeble illuminating light from the Sun and in part to their very dark surfaces."

Astronomers are hopeful that further discoveries could be made in the Kuiper Belt, following the launch of a telescope attached to the cargo bay of the space shuttle.

The Shuttle Infrared Telescope Facility will be deployed in 2002 and is expected to measure the diameters and reflectivities of dozens of Kuiper Belt objects.