IMS_Blog

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.