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Sunday 26 July 2015

It's a planet, DEAL WITH IT.

A zebra is a type of horse. A lion is a sort of cat. A PC is a type of computer, and a platypus is a particularly weird mammal. A dwarf, without any other qualifying description, is a person.

But apparently Pluto is not a planet, because dwarf planets somehow aren't planets.

NGT and I got off on the wrong foot. For American readers, NGT is not much of a thing in the UK (neither was Carl Sagan actually). I've never got on with his I'm-delusional-with-excitement-about-everything attitude, and the first time I ever saw him on TV he was ranting with a pointless, surprisingly humourless fiery passion about why Pluto isn't a planet. I instantly failed to take him seriously.

The trouble is - amongst other things - is that such an uncompromising attitude is not conducive to rational debate. In fact it encourages the exact opposite, hence this post.

There are lots of different sorts of stars : yellow stars, red stars, blue stars... all are stars. No-one disputes this. There are grey areas like brown dwarfs, which don't shine by nuclear fusion, but, in most cases, one can look at a big bright shiny thing and say, "yep, that's a star". One can then go on to specify exactly what sort of star it is. And of course the same can be said for horses, cats, computers, and so on.

Image credit : me.
But not planets, apparently. That's because the IAU's definition of planet is - linguistically if nothing else - dreadful. Instead of defining different types of planets, it says that some things are planets and others are merely "dwarf planets". How can you have a dwarf thing that isn't a type of that thing ? That's like saying that dwarfs aren't people.

All dwarf planets are bastards in the IAU's eyes.
Then there's this bizarre idea that what a thing is depends on its environment - the famously ambiguous part of the definition that says a planet "must have cleared its orbit". Which Earth and Jupiter haven't, for instance. Now "moon", well, OK, but we all know what a moon is - something orbiting a planet. Being a moon isn't somehow demoting that object just because it happens to be orbiting something else. "Moon" is really only a specification of what the object is doing, not what it fundamentally is. The moons of Mars are likely captured asteroids; no-one thinks they've become magically different just by orbiting a planet.

But "planet" is different. We use it to distinguish it from other, very different objects like comets, nebulae, stars and galaxies. Saying that "planet" depends on its environment is like saying that someone could be counted as a dwarf if all their friends are very tall. It's nuts.

He's no Tyrion Lannister, that's for sure.
I propose a much simpler system. Sure, there will be grey areas, just like everything else, but here's a stab at something that should work well enough most of the time.

Let's use "planet" to mean anything that is large enough to be round but not so large that it shines by nuclear fusion. We'll have to agree on a precise definition of "round", but that's something of a detail. Anything smaller than this would be a comet or asteroid or possibly a stellar remnant.

The key is not to stop there. Instead of "planet" and "dwarf planet" we should have "giant planet" and "dwarf planet". Thus, both are very explicitly on equal footing. They are both types of planets. No inferiority is implied, they're just different, just as a whale isn't any better than a mongoose but both are mammals*. We could also have "terrestrial planets" and "gaseous planets" or even "ice planets". Yes, this would mean the Solar System would have a lot more planets, but it wouldn't have any more major planets. We can make as many sub-categories as we like, and make them as complicated as we like. We could even allow some moons to be types of planets.

* One may of course argue that the same is true of the current definition of planet/dwarf planet. Perhaps this is true, but I think it's a lot clearer if you label the giant planets as giant planets, not just planets.

Some people like to point out that Pluto would have a tail if it was closer to the Sun, and therefore it's not a planet but a giant comet. Uh, yeah, well, Earth would have a tail if it was close enough to the Sun. Anything would, because the Sun is jolly hot. We've known about Jupiter-mass evaporating planets for over ten years. No-one's claiming those are giant comets, because that's just plain silly.

Of course, there are very important fundamental differences in both composition and formation mechanism of the various "planets" in this system. Pluto orbits above the plane of the other planets, sure, but so what ? This taxonomic system in no way prevents you from classifying things in a very specific, detailed way : Pluto might become a "giant non-ecliptic minor ice planet", or something. But you retain the word planet to mean something that's already widely-accepted without confusing the heck out of the general public. You instantly convey to people that you're talking about a big round thing that's within a certain size range and not a star*. Which is surely the important thing.

* And of course we've been using terms like "gaseous planet" and "rocky planet" for decades, without confusing anyone. Again, the only reason people object to "dwarf planet" is because it's somehow not supposed to be a sub-category of planet but a distinct object.

Having a funny orbit shouldn't matter to what we define as a planet. Planets can be tidally ejected from their star systems altogether to become rogue planets - they don't stop being planets, they are now just, like moons, doing something a bit different (it's possible that some rogue planets might even form without a star at all - we don't really know as yet).

And no, being in the asteroid belt does not automatically make something an asteroid, any more than going into Church makes one a Christian. Ceres is, maybe, a small planet that happens to be in the asteroid belt. Sure, it may just be the largest asteroid - and here's where we reach a grey area. We don't really know what the difference between a planet and asteroid is (things are even worse when we include comets). But, while the proposed definition of planet may be unambiguous, being able to define-sub categories gives a lot of flexibility. As we learn more, perhaps we'll define the largest asteroids to be a special class of planets. So we haven't restricted ourselves in any way with this wide-ranging definition.

The spectacular results from New Horizons don't really make any difference to whether Pluto is a planet or not, but they do emphasise that this is a world. "Dwarf planet", given that "planet" is a separate term, sounds somehow condescending and just plain weird.
The point is, stop trying to say things aren't planets when they are quite clearly just different varieties of planets. Maybe they are radically - perhaps even fundamentally - different varieties of planets, but this broad definition doesn't prevent us from labelling them as such when we know enough about them (just as a whale is really quite a radically different animal from a mongoose). Right now, we don't have that information.

People also ask why people are vitriolically concerned about Pluto but don't give a monkeys about Ceres or other large Solar System bodies. Well, OK - but isn't this attacking the motivation rather than the argument ? Sure, Pluto is far more of a poster child than Ceres because History, but that doesn't mean we shouldn't re-evaluate our definitions.

Does any of this matter ? Yes and no. It doesn't really matter as far as the science goes because what you label something as doesn't change what it actually is. However, "stars and planets" are the two things most people think of when they think of astronomy. If we can't agree on the definition of "planet", then we look a bit silly. And it seems to me that a bit of common-sense understanding of the English language offers a simple way out of this unnecessarily complicated mess.


  1. Well, if Callisto (undifferentiated) and Hyperion (big and fluffy) were orbiting the sun, how would they be classified?

    1. If we take the definition I've proposed, Callisto would be considered a dwarf planet (or possibly a dwarf ice planet) since it's big and round. Hyperion isn't big or round so it would be considered an asteroid.


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