The IAU have decided (subject to ratification) than Pluto is a planet.

The issue is that there is not a really satisfactory lower definition on what makes a planet (though there is a reasonable upper limit, add too much mass and nuclear fusion happens - or happened).

Reasonable definitions either exclude pluto, or include pluto and others.

The options were, in essence:

1. Set the limit above Pluto, and have eight planets.
2. Set the limit below Pluto, and include Pluto, and others.
3. Set the limit above Pluto, but include Pluto as an anachronism.

They went for option 2, choosing to make an object a planet if it’s big enough that it’s own gravitational pull makes it into a spheroid. There are secondary rules about orbital period and inclination.

‘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.

One issue here is ‘how close does it have to be to the ideal hydrostatic equilibrium given the mass and spin of the object?’ Another is that a planet is not a planet if it orbits a planet? If two objects could seperately be considered planets, why should one of them lose status when the objects are close?

Of course, one thing’s certain. This decision will make examinations fun for the next few years. GCSE examiners thinking that recalling the planets is good evidence of understanding, and the bright kids realising that the expected answer of ‘Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto’ should, in the light of this decision) have Ceres between Mars and Jupiter, and other ‘Pluto-like objects’ (called ‘Plutons’ - yuk). These inlude UB313 and Charon (Pluto’s moon, which is similar in size to Pluto - so it’s really more a double-planet)

This is all a matter of semantics though, any reasonable definition has problems, this is why the definition has come so late in the history of astronomy.

Fortunately, the practical implications of this for physicists and astronomers are nil. It changes the face of the solar system not one jot.

Just yesterday, before I heard the decision, I was thinking that the ‘own gravity’ definition would make a good definition, on reflection I think it’s a bit too broad. I’d rather set a threshold which excluded Pluto (but one might still include it as an anachronism for historical reasons).

Update: This is all actually just a proposal, the vote is next week.

Update: Further reading on Cosmic Variance