The world’s in Pluto frenzy this month. NASA’s SOFIA observatory aircraft has been operating out of Christchurch, New Zealand, to capture data on Pluto’s atmosphere via star transit spectrometry – and on 14 July, some 3662 days after leaving Earth, the New Horizons probe will storm past Pluto and its family of moons.
It’s our first visit to that world – and last, for the foreseeable future. And what an achievement! That probe is the fastest object ever built by humanity, and it’s already returned new data about the Pluto system. In the weeks after the encounter, as it transmits its hoard of information back – New Horizons will revolutionise everything we know about that remote world and its moons. Always assuming it doesn’t bang into anything, of course. At 51,500 km/h, an encounter with a grain of sand would do serious mischief. The fact that Pluto has one giant moon – Charon – and four smaller ones suggests the system might have been formed by an ancient collision, and there could be debris along the encounter path.
On the other hand, JPL officials are fairly sure the risk is minimal. The NASA team under Alan Stern used New Horizons’ long-range imager (LORRI) to look ahead for debris on 22, 23 and 26 June, concluding that the intended path ahead was safe. The Pluto system is in a state of gravitational resonance, which means any debris is expected to be clustered in discrete positions. Mostly.
You’ll notice I haven’t mentioned the word ‘dwarf planet’. That’s because I think it’s a stupid definition. It was voted in by the International Astronomical Union in 2006, on the last day of a conference when just over 400 delegates out of 10,000 in the Union remained to vote. Some 237 voted for a resolution defining ‘planet’ in terms that meant Pluto and a lot of other new Kuiper belt objects, and Ceres, were ‘dwarf planets’. The nays totalled 157, so the fact is that Pluto was demoted on a majority of 80, in a motion where 95 percent of members did not vote at all. To me, that’s not particularly valid – and I’m far from the only one to think that. However, despite a meme circulating Facebook to the contrary, it hasn’t been rescinded.
Part of the public howl of protest was driven by the fact that Pluto – from its discovery by Clyde Tombaugh in 1930, right up until 2006 – was always the ninth planet. Walt Disney renamed Goofy’s dog Rover after it. Pluto became iconic – to the people of the mid-twentieth century, the last, lonely world out on the edge of our solar system (probably). It was a social definition. And then suddenly 237 scientists out of 10,000 killed a popular idea that been integral with society for 76 years.
But in any case, the definition of ‘planet’ on which the IAU voted is a rubbish one. Among other things, it requires the planet to have ‘cleared’ its vicinity of debris. Even Jupiter doesn’t match that, thanks to its Trojan asteroids. And to me, it has a philosophical problem: it’s trapped by the requirement in western thought to compartmentalise – to divide a complex and often smoothly gradiated universe into sharply defined categories.
Frequently it’s an ill-fit, and the IAU definition of ‘planet’ is no exception. The problem is that the reality of our solar system, particularly as it unfolded for us from 1992 (specifically), clearly defies such classification. Trying to jam its different contents into pre-defined ‘scientific’ categories misleads, because the bits of rock, dust, ice and gases that orbit the Sun in various ways are more complex than this.
More next week.
Copyright © Matthew Wright 2015