Back in the early twentieth century the Solar System was a simple place. Four small planets (three of them possibly inhabited) huddled close to the Sun, girdled by an asteroid belt widely supposed to be an exploded planet. Beyond lay four gas giants with their own moons (also likely inhabited). In and around them orbited various comets.
Into this uncomplicated system swept astronomer Percival Lowell, who as early as 1894 believed – from unexplained movements of Uranus – that another world lay beyond Neptune. Planet X. The problem was finding it; but twentieth century technology offered methods unavailable to earlier astronomers. By taking repeated photographs of the night sky and comparing them, manually, with an apparatus known as a ‘blink comparator’, it was possible to find a faint object moving against background stars.
The work began under astronomer Vesto Slipher at the Lowell Observatory, in 1929. The math of planetary pertubations gave a general start-point for the search. For those working the blink comparator it was arduous, strain-filled work. But then in 1930, young astronomer Clyde Tombaugh hit the jackpot after many hours staring into the machine.
The new planet was an instant sensation. It was the first planet discovered since 1848 – and it leaped into pop-culture almost overnight. Naming rights fell to the Lowell Observatory, and after a slight flurry the name Pluto was suggested by 11-year old English schoolgirl Venetia Birney – after the Greek God of the Underworld. This also commemorated Percival Lowell’s initials. Pluto became a household name – even Walt Disney got in on the act, renaming Goofy’s dog Rover after the new world the following year.
Pluto was so distant that virtually nothing could be discovered other than the fact that it was in a strange orbit – one that ran inside the orbit of Neptune, and which was tilted against the plane of the ecliptic. The mystery deepened when Pluto was found to be too small to account for Uranus’ wanderings, a point confirmed in 1978, when Pluto’s mass was nailed down by the discovery of a moon – Charon – orbiting it every six days. They are, in effect, a double planet system, orbiting a common centre of gravity (barycentre). Everything orbits that way, actually – but in the case of Pluto the barycentre is in space.
But then the simple picture of the solar system began to unravel. In 1992, David Jewitt and Jane Luu discovered the first of many objects outside the orbit of Neptune; asteroids made not of rock, but of ices. Pluto, it turned out, was part of a spread of icy debris now known as the Kuiper Belt. This unfolded as more instruments were available, finally emerging as a vast and complex cloud orbiting beyond the major planets – apparently left over from the formation of the Solar System. Pluto, it seemed, was one of the objects, orbiting quite close to the inner edge.
For a while it seemed Pluto was the largest. But then other objects were discovered, among them Haumea, which is football-shaped and in an eccentric orbit: Sedna – which has an insanely elongated orbit taking it to over 930 times the distance of Earth from the Sun at apehelion: and others such as Makemake, Varuna, Quaoar and Eris.
All were big enough to be called planets, and some had moons. Suddenly the old ‘nine and maybe ten planet’ model began to unravel. How many planets did the Sun have? Twenty? More? The trigger that got the International Astronomical Union (IAU) into action was Eris. Discovered by Mike Brown of CalTech and originally nicknamed Xena, Eris turned out to be about the same size as Pluto, with a radius of 1163 km, +/- 6. It had a moon, Dysnomia (originally dubbed ‘Gabrielle’). It could have been the tenth planet – but its discovery in 2005 triggered a debate.
Suddenly, it seemed, the solar system had a class of icy bodies that didn’t fit prior classification, and this prompted the IAU to come up with a new definition for ‘planet’. One voted into reality by just 237 members in 2006, with a majority of only 80 against the 157 nay-voters, out of a total membership of 10,000. It demoted Pluto to ‘dwarf’ status.
The problem to my mind – apart from the fact that 95 percent of IAU members didn’t vote at all – was that the new definition was still prisoner to old prejudices – merely adding a category. Whereas the new discoveries, realistically, should have triggered a deeper re-think. More on that soon.
What these new discoveries did make clear was that Pluto was merely one of many similar bodies orbiting beyond Neptune and wasn’t cause of the odd movements of Uranus. In short, the fact that Pluto had been found was a testament to Clyde Tombaugh’s personal dedication and his incredibly hard work.
The fact that the IAU then tripped over their definition of planet – both because of the definition itself, and of the fact that it was adopted without the participation of most members – raises another question. How far is our definition of astronomical reality driven by our relentless drive to classify and compartmentalise nature, always framing it with a mind-set that in many ways is still shrugging off the down-side legacies of the Age of Reason? And is there another way we can conceptually organise all the different stuff that’s slinging around the Sun, including us?
More in a little while. Meantime, let’s wait on the Pluto encounter and see what New Horizons reveals. I’m excited. Are you?
Copyright © Matthew Wright 2015