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
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.
Fred Watson is astronomer-in-charge of the Anglo-Australian Observatory, Coonabarabran, New South Wales, and author of Stargazer: The Life and Times of the Telescope.
