When is a planet not a planet? That's what astronomers will be asking after yesterday's announcement of the biggest planetary body discovered in the solar system in more than 70 years.
Quaoar is a round world that orbits the sun every 288 years, at a distance greater than any of the nine planets. At 1250 kilometres wide, it is bigger than any of the asteroids. In fact, it's bigger than all the asteroids put together.
Quaoar (pronounced Kwah-o-ar) is just over half the size of the smallest planet of the solar system, Pluto, which already struggles to be classified as a planet because of its size. For that matter, it is just over a third the size of our moon. But, unlike a moon, it does not orbit a planet.
What is it? An "ice dwarf", apparently, and, according to its discoverers, Mike Brown and Chad Trujilo of the California Institute of Technology, Quaoar (and Pluto) should be referred to as "Kuiper-belt objects". The Kuiper belt is like a second asteroid belt, but while most asteroids orbit between Mars and Jupiter the Kuiper belt is at the icy fringes of the solar system, far beyond the eighth planet, Neptune.
"Quaoar definitely hurts the case for Pluto being a planet," said Professor Brown. "If Pluto were discovered today, no one would even consider calling it a planet because it's clearly a Kuiper-belt object."
Quaoar - named after the creation force of the Tongva Indian tribe, the original inhabitants of the Los Angeles area where the Caltech campus is located - was discovered in June, but it was not announced until yesterday. Its discoverers are still determining how quickly Quaoar spins, so they can work out the length of its "day". They are also trying to determine what precisely it is made of and even if it has a thin atmosphere.
Ross Taylor, of the Australian National University's geology department, said the Kuiper belt was the left-over remains of the massive cloud of gas that coalesced into the sun and planets billions of years ago.
He agreed that neither Pluto nor Quaoar should be known as planets, but instead as "ice-dwarfs". Both bodies, and even the innermost planet Mercury, were all smaller than one of Jupiter's moons and one of Saturn's. But such labels were a grey area, he said.
"It's arbitrary. You get into a real semantic sort of swamp. It's like trying to define life," he said.
But Professor Taylor said Quaoar was definitely not the mysterious "Planet X" for which astronomers have vainly searched the sky for decades. When Neptune was discovered in 1846, its orbit differed markedly from astronomers' calculations. It was assumed another planet was pulling it off course.
But when Pluto was discovered in 1930, it was too small to exert such a pull. The search continued.
Eventually, said Professor Taylor, scientists realised the discrepancy could be explained because they had wrongly calculated Neptune's mass.
He predicted many more Kuiper belt objects would be discovered because only a fraction of the sky had been searched. "I think it's probable that we'll pick some up to Pluto's size," he said.
Quaoar's finders, who are continuing their search for Kuiper-belt bodies, agree. "Right now, I'd say they get as big as Pluto," said Professor Brown.
None of them are likely to be labelled planets and even Pluto could lose that distinction. "I think the International Astronomical Union are trying to get their nerve up on this (Pluto)," said Professor Taylor.
