It’s no reflection on the awesomeness of Pluto to say it’s a different thing, honest.
As New Horizons proves once again that humans are freakin’ amazing and brilliant when we work together and use our mighty brains to solve interesting problems (and as we all pore over the photos that have been released and get excited about the ones yet to come – what are those impact crater-looking things? Is that a volcanic mare, or a mighty series of dunes? Why is it a different colour to its main moon Charon? EVERYTHING IS THE MOST INTERESTING THING!), the debate over whether Pluto should or shouldn’t be a “planet” has threatened to flare up again.
You might recall that in 2006 it was determined that Pluto was no longer considered to be a planet on par with the now-limited-to-eight main bodies in the solar system.
The reason for this was that bodies had been discovered further out in the Kuiper Belt (the region of icy rubble beyond the orbit of Neptune) and it was thought likely by the International Astronomical Union that as our technology improved we’d be discovering more and more of these things – which turned out to be correct.
So we had a choice: either redefine planet to mean the eight things that we currently define them as – thereby eliminating Pluto – or adding dozens of new planets to the list every year. Thanks to technological advances like the Hubble telescope and better computer algorithms for automatically recognising odd orbits in data, there are almost 400 candidate planet-like objects out in the Kuiper Belt. That’s a lot of things for eight year olds to memorise.
Thus in 2006 the IAU settled on a definition for planet that had three main tenets: if you’re big enough to be round, you’re going around the sun and there’s no other significantly-sized body in your orbit, then guess what? You’re a planet.
Pluto ticks most of the boxes, but the definitions are a little bit arbitrary.
The first two seem OK. A planet has to be orbiting the Sun so moons and satellites, which orbit planets that orbit the Sun, therefore don’t count.
That might seem obvious, but it’s worth remembering that almost all of the moons around the planets are a) basically spheres and b) in two cases, larger than one of the planets (the Jupiter moon Ganymede and the Saturnian moon Titan are both larger than Mercury – although they’re not as dense and therefore don’t have as much mass. See? It’s complicated).
Second, the thing has to be big enough to undergo “hydrostatic equilibrium” under it’s own mass. Thus lumpy little asteroids don’t count (and technically the planets aren’t actually spherical – they’re slightly flattened at the poles and wider at the equator because of their rotation, making them “oblate spheroids”, but that’s nit-picking).
Thirdly – and this where Pluto fell down – they have to have “cleared their orbit” of other bodies. This is the most arbitrary distinction of all, but the argument goes like this: you need to be big enough that your gravity has either thrown everything else out of your orbit or it’s been attracted to you and collected it.
Pluto doesn’t count for two reasons. One, that there are other things in its wide orbit (“plutininos”) that its gravity is not powerful enough to disturb. It also, technically, crosses the orbit of Neptune although the two are in what’s called an orbital resonance, so they’re never in the same place at the same time – otherwise the utterly enormous Neptune would easily slingshot Pluto out of the Solar System entirely.
It’s been pointed out that most of the planets absolutely have significant things in their orbits – aside from Neptune, which hasn’t cleared Pluto from its orbit, the Earth, Mars and Jupiter all have bodies of asteroids scattered in our orbits and no-one appears to be arguing we’re not planets as a result. But this was a definition invented mainly to find a way to exclude Pluto and get things to a manageable system.
But to be honest, “planets” is a problematic term in any case.
It’s derived from the the Greek term for “wanderer” and were so-called because the the five close enough to be visible to humans before the invention of the telescope (Mercury, Venus, Mars, Jupiter and Saturn) were points of light that moved through the skies when viewed from Earth, unlike the apparently fixed stars.
The fact we invented the word to describe little points of light in the sky is why we have the same name for “little solid lumps of rock”, like Venus, as we do for “massively giant seething balls of gas”, like Jupiter.
You could make a very solid argument to call those types of things by different, less Earth-centric names, the way we do for other non-planet things in our solar system like asteroids and comets, but we’re kinda stuck with it.
So I rather like that we have a different name for the amazing objects out beyond the orbit of the last of the gas (well ice) giants.
Out past Neptune is Pluto and the other “trans-Neptunian” objects, including a load of amazing dwarf planets: in orbits wider than Pluto’s, there are Eris, Makemake, Haumea, Sedna, Orcus, Salacia and others that haven’t officially been given names yet.
They’re all smaller than our Moon and they’re – probably, we think – made of ice, mainly water and methane. In fact, they appear to be more like giant comets than they are like the four inner planets.
And that’s why, dumb as the term might be, I like that there’s a different word for them. “Dwarf planet” is a stupid name (and it’s not what astronomers use, incidentally: they call them “planetoids’) but it at least acknowledges that they’re different things*, an exciting group of different bodies about which we barely know anything and which we’re the first generation to be able to discover.
And if the fact that you’ve heard of Pluto means that you might be interested to learn more about Eris or Makemake, that’s a great thing in itself (their orbits are nuts, by the way). And hell, it makes more sense to celebrate Pluto as Amazing Gateway to the Glories of the Kuiper Belt rather than The Shitty Last Planet After The Proper Ones.
And in any case, New Horizons has made the distinction kinda moot. Look at that photo at the top of the page again: Pluto is now a place.
It’s a real, tangible world ten years and billions of kilometres away, with its own characteristics and mysteries and beauty. However we define it, we’re the first humans in history to see its face – and that’s the thing that fills me with hope and joy for our species.
Dammit, we’re an amazing species when we want to be.
* Of course, then you have the question of why tumbling balls of frozen methane are labelled the same as a solid sphere of rock in a diffuse belt of other rocks, since the major asteroid Ceres has also been deemed a dwarf planet, but that’s a different rant. Dammit, what’s wrong with just declaring Ceres The Mighty King Of The Asteroid Belt rather than confusing the issue by making people think it’s like Pluto, only closer?