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
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.
