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Originally posted August 18 2006 at 21:08 under Physics. 2 Comments. 2 Trackbacks (now closed). Last modified: 18 August 2006 at 22:04

OMG! Planets!

Slightly Bemused
Earth (still a planet)

It’s all over the news. We’ve got some new planets. And we didn’t even have to tender contracts to get them built. Well I might as well add my own thoughts to get lost in the noise.

First it’s worth pointing out that the International Astronomical Union have found themselves in an impossible position, so much so that they’ve spent the last two years trying to find a way out of it. They’ve been in a classic catch 22, damned if they do, damned if they don’t. It just happens that I think they’ve damned themselves the wrong way if they ratify the proposed planet definition. Appropriately enough they’ve been damned by a god of the underworld, Pluto

According to the IAU:

(1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.
Draft Resolution for GA-XXVI: Definition of a Planet

This is on the surface fairly sensible—a planet is a big round thing (not a moon though) which orbits a star. The accompanying notes to this section are also quite sensible; they basically say that normally this means a big, heavy thing and that if there are two objects with a centre of mass outside both then they’re a binary planet. The real hints that something isn’t quite right come with point number two:

(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet”
Draft Resolution for GA-XXVI: Definition of a Planet

Ceres is a planet?! The problem with point 1 is that it makes lots of things planets that, really, nobody in general would refer to as planets. Ceres obviously has a lot more in common with the rest of the asteroid belt (most of which won’t be planets) than Mercury, Venus, Earth et al. This is all Pluto’s fault. Since Pluto’s discovery people have gone around calling it a planet, mainly because back when it was discovered this was perfectly reasonable…it was seen as a relatively large lump of rock going around the Sun in a fairly non-crazy orbit. We’ve been teaching people there are nine planets for seventy-odd years; it’s a bit difficult to just dump. Yet the fact is that’s exactly what they should have done. Let’s face it, if they’d dropped Pluto from the planet list nobody would have taken much notice anyway. The fact Pluto wasn’t really a planet would be the sort of thing only brought up by annoying pedants and those compiling good pub quiz questions. Points 3 and 4 more or less show the difficulties the IAU is getting into

(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.

(4) All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”

Draft Resolution for GA-XXVI: Definition of a Planet

Anybody who just happened to read point 3 might well think it was getting along to some sort of definition of Kuiper Belt Objects, and for good reason. Had Pluto been discovered with the knowledge we now have of the outer solar system then that’s exactly what it would have been classified as a particularly large KBO. This has been increasingly clear, especially since Charon and “Sedna” were found, let alone “Xena”. The IAU should have just bitten the bullet, given a definition for KBOs and declared Pluto and Charon to be large examples there of, instead of going down this silly route of having every Tom, Dick and Harry of the solar system a planet (and at this rate we’ll need so many names we’ll be using Tom, Dick and Harry).

Of course this isn’t to say that having decided this then defining a planet would become much easier, it would just become more sensible and you wouldn’t have to worry about having Pluto fit into whatever you came up with. Some sort of minimum mass/size added to the planet definition might be all that’s needed (and maybe you could still squeeze Pluto in even if it doesn’t make sense). Or perhaps the “nearly circular orbit close to the ecliptic plane” could shine some light. It’s this sort of thing which has been debated. The disappointment is that after two years of trying the power of compromise has forced us to this.

Update: This just in…maybe pluto won’t make it after all. The important new bit “by far the largest body in its local population” might be a way out, but then again…what happens if we discover a binary where the components are roughly equal in mass but way bigger than Pluto/Charon? By this definition neither would be a planet (and neither a satellite presumably, as that would require a planet to orbit). Two years and still nothing which really works. Perhaps we should just say planets are Mercury to Pluto and come up with a completely new term to use as well.

Comments (2):


There is a split between people who feel that the type of object is the most important factor, irregardless of how unique the object is, and others who feel that ‘planets’ should be as unique as possible. Also, almost eveyone feels that objects around the size of Jupiter in obit around a star shoukd be planets.

Those who feel the type of object is most important disagree with ‘Ceres obviously has a lot more in common with the rest of the asteroid belt’, as I do. Ceres is spherical, layered, and might possibly have a subsurface ocean (some have suggested adding a Magnetometer to Dawn for this reason).

In a similar fasion, Pluto, Quaoar, and the like have properties that in my firm opinion, make them more than mere comets. Quaoar may have (or had) ice volcanoes. Pluto has a tenuous atmophere, and seasons.

In short, I think spherical objects are more than just ‘useless lumps of ice/rock’, as I have heard some people label Pluto, and are well deserving of at least some sort of special category, if not outright planethood.

Also, the ‘nearly circular orbit close to the ecliptic plane’ tends to cause problems when look at known extrasolar systems, where objects of Jupiter size or more orbit in very eccentic orbits (inclinations or ecliptic planes unknown).

Made by JamesFox on Aug 20, 2006 at 15:02


To take the last point first, I agree that trying to use something along the lines of orbital eccentricity and inclination to the ecliptic plane, whilst it might work for our own system, is probably asking for trouble when trying to deal with “planetary” systems in general. Part of the whole debate is that it is actually so difficult to find a good definition.

I don’t see how people can argue that type is so important and then still want to lump everything together as a planet based on something that has not much to do with type but more to do with size and density. On the other hand if we start giving too much thought to uniqueness then we end up demanding a new category for every different object. The demands of good taxonomy are a balance between the differences between groups and the differences within a group.

No where do I claim Pluto is just a lump of ice and rock (and I don’t think I’ve heard anyone claim it’s akin to a comet). It’s an interesting object. However, pluto and it’s companion distant “plutons” are KBOs. There’s no way Pluto would have been designated as a planet in the first place had the it been discovered after much study of the Kuiper belt and I fail to see why people think it needs a further category; its planetary designation is a historical accident, which isn’t a very good reason for struggling to continue to include it. Indeed, had it been properly categorised to start with we probably wouldn’t be having the debate at all!

Ceres may be a particularly large example of an asteroid, and because of this it may have some fairly unique and again interesting features. But (as far as I know—I confess I haven’t fully researched the point) its formation and history is awfully close to its companions. If it’s a planet then so are all the asteroids, which is patently silly.

Personally I think we have about four or five clear categories of objects (at least in our system). The rocky planets (of which Earth is the best known). The gas giants (Jupiter et al), KBOs, cometary objects, and asteroids. If we’re going to reclassify in a scientific manner then perhaps we should just abandon the word planet to popular culture and find a different term without the connotations to describe what we mean (though that probably wouldn’t reduce the debate!)

One final thought. One must be a little careful with statements along the lines of “almost everyone feels that objects around the size of Jupiter in obit around a star should be planets” because there’s no cut off size. Currently any “brown dwarf” which is orbiting a star would be a planet, which I think is very open to debate.

Made by Ian Scott on Aug 20, 2006 at 15:35

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Trackbacks (2):

OMG! Still Planets!

There's still argument about what a planet is. Things aren't really progressing either. Rather than one proposal there are now three separate, but not alternate, proposals which, as I'll go into, haven't really changed anything. Anyone interested might... [Read More]

Tracked from IMS_Blog on Aug 22, 2006 at 22:25

OMG! Still Planets!

There's still argument about what a planet is. Things aren't really progressing either. Rather than one proposal there are now three separate, but not alternate, proposals which, as I'll go into, haven't really changed anything. Anyone interested might... [Read More]

Tracked from IMS_Blog on Aug 22, 2006 at 22:36

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Dr Ian Scott
Croydon (and Gateshead), United Kingdom
Bullding Services Engineer (EngDesign), PhD in Physics (University of York), football fanatic (Newcastle United), open source enthusiast (mainly Mozilla)

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