It's time to update our text books and websites etc. as we now have eight planets rather than nine. No, aliens didn't torpedo one out of the skies, the International Astronomical Union (IAU) have finally decided on a definition for a planet within our solar system and Pluto doesn't meet the criteria so it's been demoted to being a Dwarf Planet. Since the first time I ever gave it any real thought I've always felt that it was a mistake to call Pluto a planet. In fact I've blogged about this before: What exactly is a planet? At first I wasn't sure about the wording of this new definition but on reflection I think it was the best we could have hopped for and I'm now going to try convince you of that too!

[tags]Pluto, Planet, IAU[/tags]

The New Definition

So what is this new definition? Well, I'll give you the official wording first and then translate into English for you!

The IAU therefore resolves that planets and other bodies in our Solar
System be defined into three distinct categories in the following way:
(1) A planet is a celestial body that (a) is in orbit around the
Sun, (b) has sufficient mass for its self-gravity to overcome rigid
body forces so that it assumes a hydrostatic equilibrium (nearly round)
shape, and (c) has cleared the neighbourhood around its orbit.

(2) A dwarf planet is a celestial body that (a) is in orbit around
the Sun, (b) has sufficient mass for its self-gravity to overcome rigid
body forces so that it assumes a hydrostatic equilibrium (nearly round)
shape, (c) has not cleared the neighbourhood around its orbit, and (d)
is not a satellite.

(3) All other objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". 

So what does that really mean? Well, it means that to be a planet you need to satisfy the following:

• You must be round,
• Go round the Sun but not another planet,
• And you must dominate your neck of the woods (i.e. you have to be 'the daddy' in your neighbourhood).

The first two criteria are pretty self-explanatory but the last one is worth looking into a little more. Firstly, why is it there? Well, if you're made of mostly ice like most of the objects out beyond Pluto you really don't have to be that big to become round. Certainly no where near as big as objects made of rock need to be. Without this final criteria we could end up with hundreds of planets.

But what does it really mean? What it boils down to is that that you have to be very big to be a planet. Large objects have such a large gravitational influence that they clear themselves a space within the solar system. Objects that were in that space either get pushed aside or pulled into the large object. This means that the planets all orbit alone without other large objects near by. This is where Pluto fails. There are lots of objects with orbits very close to Pluto's.

Not a Line in the Sand

Something that I think almost everyone agreed was that it would be a bad idea to just draw a line in the sand and pick an arbitrary size and say "if you're bigger than this you're a planet if you're not you're not". This would have a been a stupid way to do this because we would have ended up picking a size like 1,000km which is only a nice number because of the units we happen to use for length. This would have no actually physical meaning and would lead to some interesting debates on objects that were approximately 1000km across but with an error in our best measurement of a few tens or even hundreds of km. What the IAU set out to do was to pick a definition that was based on physics and they have done so, there are no numbers mentioned anywhere in the definition.

Not a Siler Bullet

Anyone looking at the definition will of course be able to see that it is no silver bullet. You won't just be able to enter the details of any new planet candidate into a formula and get out an answer of "Planet" or "non-Planet". There will still have to be a debate in the IAU for each new planet candidate and a vote will still have to be taken on it. It will still be a judgement call. However, it will now be a judgement call based on three criteria rather than a judgement call based on no criteria. I don't think it is possible to come up with a silver bullet in this case so I'm happy that we now have a focus for all future debates on the planetary status of an object in our solar system.

But What About Pluto?

Pluto will now be relegated to the status of Dwarf Planet but it has been honoured as the prototype object for this whole new class. This makes sense. Making Pluto a planet in the first place was a mistake as I explained in my previous posting on defining the word 'planet'. I'm not going to go over those same arguments again here because it's ground that has been covered many times before. This mistake has now been rectified. It was also not the first such mistake and Pluto is not the first object to be demoted. Ceres is the biggest and brightest of the asteroids in the asteroid belt between Mars and Jupiter and hence it was the first of it's kind discovered. It was regarded as a planet for a number of years but as time went on more and more similar objects were discovered and it was realised that Ceres was not a planet but the biggest of a huge population of smaller bodies. When Ceres was relegated a new term was coined as a fudge to lighten the blow; "Minor Planet". This is all very similar to what has happened here with Pluto. It was the first of a whole new class of object to be discovered and as more of it's kind were found people realised it was a mistake to call it a planet and it has now been demoted and made the first of a new kind of object. It was also given a consolation prize like Ceres, only this time the term was Dwarf Planet rather than Minor Planet. An interesting twist is that this resolution further demotes Ceres to a Small Solar System Body and retires the term Minor Planet all together.

Conclusions

This new definition is no silver bullet. It still leaves a lot of room for debate and doesn't even begin to tackle what to call objects orbiting other stars. However, are we better off than we were before? I would say we are. Before we had no definition beyond "it's a planet if the IAU say it is", now we have a set of criteria that define a planet. True, each planet candidate will still have to be debated but now there is a focus for that debate which is something we've never had before. Finally, I think this finally un-does a 70 year old mistake and re-instates our eight real planets.

[tags]Pluto, Planet, IAU[/tags]