When is a planet not a planet?

When is a planet not a planet? When it’s a warrior princess. On 29 July, astronomers at Caltech’s Palomar Observatory announced the discovery of an object larger than Pluto in the outer reaches of the Solar System. It’s currently known only as 2003 UB313, because it was first seen – or, rather, ‘the data from which the object was discovered were obtained’, as one of its discoverers, Michael Brown, carefully puts it – in the second half of October 2003 (the 21st fortnight of the year; U is the 21st letter of the alphabet). Brown and his colleagues, Chad Trujillo and David Rabinowitz, have submitted a name to the International Astronomical Union, but it will remain secret until it gets the IAU’s stamp of approval. Unofficially, they’re calling it ‘Xena’, after the character played by Lucy Lawless in the camp fantasy adventure Xena: Warrior Princess that ran on TV in the late 1990s.

The IAU, which was founded in 1919, has strict rules about the names that can be given to new discoveries. Objects in the Kuiper Belt – a flurry of icy rocks swirling round the Sun on the far side of Neptune, of which 2003 UB313 and Pluto are the biggest known – have to be named after figures from creation myths: other discoveries by Brown and Co include Quaoar (spotted in June 2002), named for the creator in Tongva mythology (the Tongva people lived in the Los Angeles area before the arrival of Europeans), and Sedna (spotted in November 2003), named for an Inuit sea goddess. They say that their ‘new proposed name expands to different traditions still’: is it worth noting that there is as yet no astronomical body known as Adam?

The most recent batch of names to be officially endorsed by the IAU, in July 2004, were those of 35 new planetary satellites. The moons of Jupiter used to be named for the god’s favourites (a polite term for the many people and nymphs he tried to have sex with: Europa, Io, Ganymede, Callisto etc) but the rules have had to be relaxed to include his descendants, as it turns out that the planet is even more promiscuous than the god was: according to the last count, it has at least 38 moons. ‘At some time in the future,’ the IAU warns, ‘it may be advisable to stop naming very small satellites.’ Most of Uranus’s satellites are named after characters in Shakespeare – Trinculo is the latest addition – though two, Belinda and Umbriel, take their names from The Rape of the Lock. This inconsistency can be blamed on 19th-century amateurism: the IAU surely wouldn’t have tolerated it.

Victorian enthusiasm also deprived 2003 UB313 of more than one potentially suitable name. It spends half of its 560-year orbit close to Pluto, and half of it a lot further away, much as Proserpina, who was abducted by Pluto, spent six months with him in the Underworld and six months with her mother above ground. Astronomically speaking, however, Proserpina is an asteroid, discovered in 1853. And Persephone, the Greek version of the name, is an asteroid too, the 399th to be discovered, in 1895. The time may yet come when the only mythologies left to draw on are those made for television.

Whatever 2003 UB313 ends up being called, that won’t answer the question of whether or not it counts as a planet. The ‘main stumbling block’, as Brown puts it, is that, ‘scientifically, it is quite clear that Pluto should certainly not be put in the same category as the other planets.’ It’s considerably smaller than the other eight, for a start, and has a much more eccentric orbit. On the other hand, everyone knows that there are nine planets, from Mercury to Pluto. Brown takes the sensible line that it’s a cultural question as much as a scientific one. (After all, there’s a school of thought according to which the Earth and the Moon form a ‘binary planet’: to call the Earth alone a planet and the Moon its satellite is unconscionably Earth-centric. Then again, a certain amount of Earth-centricity is inevitable, even necessary, since the Earth is where we’re looking from.) But if Pluto counts as a planet, then 2003 UB313 – which is made up of much the same stuff as Pluto: an interior of rock and ice, with a crust of solid methane – must too, because it’s bigger, though the extreme eccentricity of its orbit could count against it.

The discoveries of two other Kuiper Belt objects were revealed in the same week as 2003 UB313. The first to be announced, on 28 July, was 2003 EL61, discovered by Jose-Luis Ortiz and his team at the Sierra Nevada Observatory in Spain. The astronomers at the Palomar Observatory had seen it too, but weren’t yet ready to go public. Though the Spaniards had found 2003 EL61 entirely independently, ‘some in the community privately expressed their concerns’ to Brown that it might be possible for an unscrupulous person to hack into Palomar’s telescopes, steal the data and dishonestly claim for themselves one of the objects that Brown and Co had been painstakingly observing for several months.

On the morning of 29 July, Brown got in touch with Brian Marsden at the IAU’s Minor Planet Center, and learned that such an attempt was not only possible, but underway: ‘Someone’ – it’s not clear who – ‘had already used a web service of the MPC to use past observations of an object to predict locations for tonight. The past observations were precisely the logs from the telescope we had used.’ Brown, Trujillo and Rabinowitz called a press conference that afternoon. They were thus, as they deserved to be (unless that mysterious ‘someone’ turns out to have been a disgruntled resident of the Kuiper Belt), the first to raise their metaphorical flags on 2005 FY9 and 2003 UB313. Meanwhile, nine billion miles away, 2003 UB313 continued oblivious on its orbit.