A giant asteroid has been spotted between the outer planets Pluto and Neptune. It is probably bigger than any other known asteroid in the Solar System apart from the huge Ceres, discovered in 1801.
The huge chunk of rock and ice is around a quarter the size of Pluto itself. And it may not be alone: astronomers say they expect more than 100 heavyweight neighbours of the new asteroid to turn up over the next few years.
David Hughes, an astronomer at the University of Sheffield, calls the discovery "absolutely fascinating". He says it's not very surprising that such a large asteroid could go undiscovered till now. Asteroids appear as tiny specks of light that are easily mistaken for distant stars unless someone takes the trouble to track their motions very precisely.
"People have probably seen this asteroid before, but the trouble is that it would look like a star," says Hughes. "There are lots of little 'stars' out there that aren't little stars at all."
Hughes adds that objects of this size would bridge the gap between Pluto-sized objects and much smaller icy objects in the outskirts of the Solar System. "This has always been slightly worrying," he says. "The feeling is that lurking out there, there must be objects this size."
This latest addition to the asteroid catalogue is called 2000 EB173. It lies nearly 40 times as far from the Sun as the Earth, and is one of the so-called "plutinos", a gang of asteroids that have stable orbits between Pluto and Neptune.
The team that made the discovery was led by Gustavo Bruzual of the Centre for Astronomical Research in M?ida, Venezuela, and Charles Baltay of Yale University in New Haven, Connecticut. They made their observations on 15 March, using a telescope in Venezuela, then used computers to look for objects that appeared to be moving fast relative to the distant stars behind them.
From the apparent brightness and distance of the asteroid, the scientists estimate that it is about 600 kilometres across. Some known asteroids might be bigger than both Ceres and EB173, but these do not have stable orbits around the Sun. Finding the monster plutino on their one-night search suggests that there may more than 100 similar heavyweights waiting to be discovered.
If EB173 is confirmed as the second largest asteroid in the Solar System - bigger than the "minor planets" Juno, Pallas and Vesta - it may later be given a name worthy of the big sister of three goddesses.
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