'Mini-Pluto' spotted orbiting the Sun

Object EB 173 seen against a crowded star-field

By BBC News Online science editor Dr David Whitehouse

Astronomers have discovered the largest object orbiting the Sun since they found Pluto in 1930.

It is the second largest so-called minor planet, or asteroid, to be spotted - about a quarter of the size of Pluto.

Designated 2000 EB 173, the object was detected in March from an observatory in Venezuela.

It circles the Sun every 240 years between the orbits of Uranus and Pluto, and is bright enough to be seen by amateur astronomers armed with a 30-cm (12-inch) telescope.

The object is estimated to be between 300 and 700 km (185 to 435 miles) in size by the International Astronomical Union's Minor Planet Center. It is second in size only to the asteroid Ceres which orbits between Mars and Jupiter.

Dark and distant

The closest EB 173 ever gets to the Sun is 4.3bn km (2.7bn miles). Most of the time, it is more than 5.6bn km (3.5bn miles) away. Astronomers are classifying it as a Plutino or Trans-Neptunian object.

Since 1992, more than 300 such objects have been found orbiting the Solar System's cold and dark outer reaches beyond Neptune. But EB 173 is the brightest seen so far.

Astronomers believe the giant rock has been orbiting the Sun for over four billion years and is made of material that has been unchanged since the Solar System's birth.

The surface of EB 173 appears to be slightly reddish in colour, which is typical of such objects. It was possible to calculate a detailed orbit for it after follow-up observations were made at the Klet Observatory in the Czech republic.

Precise observations of the object's spectrum have been made at the Keck Observatory in Hawaii but they failed to provide much detail.

Pluto controversy

Astronomers had hoped to detect signs of frozen water on the object but so far they have not seen the distinctive spectral signature of water-ice.

The team, led by Professor Michael Brown of the California Institute of Technology, said that EB 173 could be very large or very bright.

The discovery of EB 173 will renew the debate about how large an object has to be to be classified as a planet.

Pluto has maintained its official planetary status despite some criticism from those who believe it is too small and unusual to be regarded as a true planet.

The question some astronomers will now be asking is whether EB 173 is large enough to be called a planet. But others will point out that the definition of a planet is an arbitrary one anyway.