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.

FROM THE REMOTE fringes of the Solar System, more information is steadily trickling in about the nature of these intriguing objects.

With orbital speeds around the Sun of less than 5 km/second (compared with Earth's 30 km/second) and surface temperatures lower than -230°C, they are definitely among the sluggish members of the Sun's family. But there are signs that past interactions between them may have been quite violent.

Pluto has been known since 1978 to have a large moon, Charon (pronounced 'Care-on', not - if you please - 'Sharyn'). Its diameter of 1,209 km was recently measured to within a couple of kilometres by observing its passage in front of a distant star - an occultation, which took place in July 2005.

The same observations revealed that Charon has no atmosphere to speak of, refuting earlier speculations that a thin mantle of gas could be continuously drawn from Pluto in a unique case of celestial infant feeding.

Pluto and Charon are, like the Earth and Moon, sometimes described as a 'double planet' because their relative sizes are much closer to one another than those of the other planets and their satellites. And that gives a clue to the possible origin of Charon. The Earth-Moon system is thought to have arisen as a result of a collision between the proto-Earth and a Mars-sized object back in the Solar System's turbulent youth, with the Moon forming from the shards of planets.

Could Pluto and Charon have formed in a similarly violent way? Computer simulations have revealed that this is indeed possible, but there is at present no way of distinguishing that scenario from those in which Charon was simply captured from the Kuiper belt.

A further tantalising clue turned up late in 2005 in the shape of two more moons of Pluto: tiny objects no bigger than 150 km across, known as Nix and Hydra. These little worlds nestle close to Pluto, and orbit in the same plane and the same direction as Charon - suggesting that they may have formed as by-products of the collision event. A neat and tidy theory, but only a closer look by a passing spacecraft will provide the information needed to confirm it - data such as crater number counts and surface compositions.

Pluto and Charon are in synchronous rotation, a cold celestial dance that forces the two bodies always to have the same faces turned to one another as Charon trundles along in its 6.4-day orbit around its parent planet. The mechanism by which this has arisen is exactly what keeps the same face of the Moon turned towards Earth: tidal friction. It is not yet known whether anything similar has happened to Xena - for yes, it, too, has a moon, discovered in September 2005 with one of the two 10-metre Keck Telescopes in Hawaii. It will come as no surprise to TV buffs that this object has been nicknamed Gabrielle after Xena's offsider; but its official name remains - in the poetic tradition of newly catalogued astronomical objects - the less delightful 'S/2005 (2003 UB313) 1'. If having a moon was a prerequisite of planethood, then both Xena and Pluto would be in - but so would about 10 per cent of all KBOs and an even bigger fraction of asteroids. Obviously, a comprehensive review is needed.

WHY HAVE KBOS in general, and Pluto in particular, become such celebrities in the astronomy of the early 21st century? The answer lies in what they might tell us about the formation of the Solar System, and perhaps even about the origins of life on Earth. If the typical KBO is a frozen remnant of the protoplanetary disc that surrounded the infant Sun, its chemistry could be nothing less than the Rosetta Stone of our corner of the Universe, with pristine dust grains that have been forever cold, and organic ices that may contain the progenitors of living cells.

We already know KBOs can be sorted into at least two varieties: some having a neutral grey colour and others, such as Sedna, being decidedly red. This may speak of different cosmic histories throughout the age of the Solar System, the red ones perhaps having a surface layer that has been modified by effects such as cosmic ray bombardment. Either way, any one of these objects that strayed close to the Sun would quickly develop features characteristic of a comet: a coma (or microatmosphere formed by the out-gassing of volatile materials and the release of dust) and a prominent tail. There is a recognised class of just such objects in unstable orbits that may eventually fall into the inner Solar System as short-period comets; they are called Centaurs, for the mythical creatures that were half man, half beast. Hence, half KBO, half comet. Who says astronomers have no soul? The importance of this is that comets striking the early Earth are thought to have been a significant source of ices such as water ice, methane and ammonia. It is highly likely that more complex organic molecules that led to life on our planet were included in the same package, and a handful of scientists think that even life itself may have arrived in this way. Hence the extraordinary interest in investigating the icy materials contained in comets and Kuiper belt objects.

Large KBOs, Pluto and Xena among them, may have a different story to tell. Here, the process of planet formation seems to have been interrupted in mid-stream, resulting in half-finished worlds that have nevertheless become big enough for gravity to pull the solid material to the middle. This process of 'differentiation' is likely to have given Pluto, and perhaps Charon, a rocky core with an icy mantle. It would have been greatly enhanced by melting if a collision did, indeed, give rise to Charon. Pluto's surface is known to consist of frozen nitrogen, with methane, carbon dioxide and ethane also present. But the bulk of Pluto's icy mantle is likely to consist of water ice, buried beneath the more volatile surface ices. The planet's tenuous atmosphere, whose existence was confirmed during an occultation in 1988, is probably mostly gaseous nitrogen.

Why do we think Pluto and Xena might be half-finished planets? The evidence is mainly from computer simulations of planet formation performed at such institutions as the Southwest Research Institute in Boulder, Colorado. They demonstrate that Earth-sized objects could indeed have formed in the outer regions of the Solar System. Why the process stopped is a mystery. But if Pluto is a half-built world, a close look at it would be a unique opportunity to see the process in freeze-frame, giving real insight into our understanding of the process.

This is an opportunity too good to miss, and is just one of the imperatives that have driven New Horizons - the first space mission targeted on Pluto and the Kuiper belt - which lifted off successfully in January 2006 from Cape Canaveral.

The spacecraft's brief encounter with Pluto and Charon is scheduled to take place in July 2015 after a slingshot rendezvous with Jupiter in February 2007. A comprehensive array of onboard instruments will allow detailed surface maps to be made, as well as tell-tale data to be collected on surface composition and atmospheric constituents.

There was some urgency in setting up this mission, because Pluto's elongated orbit means that the energy it receives from the Sun falls by a factor of three as it moves from perihelion (closest point to the Sun) to aphelion (furthest point) in its ponderous 248-year orbit. Perihelion last occurred in September 1989, so the planet is already well on its way towards the zone in which its atmosphere will simply freeze out onto the surface. The sooner we can get to Pluto, the more interesting it will be. It's hard to overstate the importance of New Horizons, since our first-hand knowledge of Pluto and its environment is so sparse. The results are almost certain to be the most surprising of any deep space mission yet - and as the spacecraft will then go on to selected KBOs beyond Pluto, the excitement may continue well into the century.

Meanwhile, as New Horizons sets out on its long, cold journey, the Kuiper belt throws up more surprises. In 2005, an announcement was made of the discovery of a largish (500 km to 1,000 km) KBO whose official name sounds like the latest in sports sedans: 2004 XR 190.

Its Canadian discoverers, however, nicknamed it 'Buffy', after the TV vampire-slayer of the same name. Why? Because they think it'll be "a bit of a theory-slayer".

Buffy orbits in an unusual circular path the edge of the main KBO region, 8.5 billion km from the Sun. What is really strange is the tilt of its orbit to the plane of the Solar System: 47 degrees. Explaining its unique combination of circularity and tilt is difficult and involves ideas such as changes in Neptune's orbit, and encounters with nearby stars. It may even cause a rethink of the Kuiper belt's shape.

No doubt, in the decade that we have to wait for New Horizons' encounter with Pluto, there will be more surprises from the watchers of the Solar System's twilight zone.

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