An artist's impression of 2003-UB313 (Xena) and her companion, Gabrielle. That's the Sun at upper left, some 8.5 billion kilometres away.

Credit: NASA

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On the first day of the 19^th century, an Italian astronomer named Giuseppe Piazzi stumbled across a new planet. He hadn't exactly been searching for it - the task in hand had been to test a French star catalogue - but it turned up where many astronomers thought an undiscovered planet might just be lurking, in the vacant region between the orbits of Mars and Jupiter. No doubt Piazzi's fellow sky-watchers were a tad miffed when he beat them to it, but his discovery was nevertheless welcomed as a major breakthrough.

The new object was hailed as the eighth planet of the Solar System, and was named 'Ceres' after the Roman goddess of fertility. As time went by, however, it soon became apparent Ceres wasn't quite what had been expected. It was a tiny speck, barely 950 km across. Then, the following year, another small object turned up in the same neck of the Solar System; and a couple of years after that, yet another one. By 1807, no fewer than four of these little misfits had become known, and the astronomers who had trumpeted the discovery of the eighth planet could only look at the floor and shuffle their feet in embarrassment. It was left to the elder statesman of British astronomy, William Herschel, to sort out the whole embarrassing mess: he coined the term 'asteroid' for these rocky new worlds, giving them an identity.

Sounds like a familiar story, doesn't it? That's because what qualifies as a planet is again in dispute, and you can blame it all on the ninth planet, Pluto.

In 1930, when Pluto was discovered (after a lengthy search prompted by gravitational irregularities in the orbit of Uranus), there was jubilation in astronomical circles. Surely Pluto was the planet they had been seeking.

Once again, however, that quickly turned to consternation when it was realised that Pluto was too small to have any noticeable effect on the gas giant - indeed it is now known to be only two thirds the size of our own Moon.

Moreover, Pluto's orbit was quite different from other planets: very elongated, with a 17-degree tilt to the plane of the Solar System.

The disappointment provoked a renewed search for the hypothetical 10th planet that was supposedly upsetting the equilibrium of the outer Solar System. Indeed, the idea of 'Planet X' didn't evaporate completely until the 1980s when the mass of Neptune - and its effect on Uranus - was re-determined from the trajectory of the Voyager 2 spacecraft.

Meanwhile, in 1950, Kenneth Edgeworth and Gerard Kuiper independently postulated a second, unseen asteroid belt in the freezing outer reaches of the Solar System beyond the orbit of Neptune. They imagined that this disc of debris would constitute the leftovers of the formation of the Solar System some 4.5 billion years ago, and predicted the existence of tens of thousands of small objects that had never coalesced to build planets. Such a history would also mean that these objects had to be very different in composition from those of the main asteroid belt. Never having been processed by heat, they would be icy agglomerates, resembling comets rather than the rocky bodies of the inner Solar System.

That speculation was reinforced during the 1980s when this Edgeworth-Kuiper belt was identified as the probable source of short-period comets - those whose orbits bring it around our Sun and back every 200 years and which tend to have orbits close to the plane of the Solar System.

Then, in 1992, the first confirmed member of this new Kuiper belt zone was identified: a remote object with the pretty uninspiring name of 1992 QB1. More such discoveries followed, and today about 1,000 of these so-called Kuiper belt objects have been catalogued, mostly with sizes ranging from 100 km to 500 km (Kuiper belt objects, or KBOs, are also known as trans-Neptunian objects, or TNOs). The lower size limit is probably a selection effect imposed by their huge distances (they're more than 5 billion km from the Sun), which render the smaller KBOs undetectable from Earth.

Recent discoveries, however, have included significantly larger ones, with estimated diameters (calculated from their brightness) of 1,000 km or more. First, in 2000, was Varuna, then Sedna, Quaoar and, most recently, the celebrated object 2003 UB313, popularly known by its discoverers' nickname of Xena, from the cult T.V. series, Xena: Warrior Princess. And there's the rub. Because Xena is bigger in diameter than Pluto's 2,300 km, logic demands that it should be recognised as the 10th planet. But we know that it really is a KBO. And if astronomers were compelled to admit it, all the signs are that Pluto is a KBO too. So, has the Solar System got 10 planets, or eight? Just as Ceres was demoted from planethood to asteroidhood, shouldn't Pluto be demoted to Kuiperhood? The important difference, of course, is that while Ceres was mistaken as a planet for a scant few months, Pluto has been recognised as a planet for more than three-quarters of a century. And this, together with a number of other new discoveries in astronomy, has prompted the current vigorous debate on how exactly we should define a planet.