IMS_Blog

What a planet is now defined to be bares some repeating here:

(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 objects3 orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

IAU Resolution 6A

Oh dear. They’ve sort of got it right but they’ve made a complete hash of the wording. Written in a hurry in a desperate compromise. Although I mentioned that they probably couldn’t back out but maybe that’s exactly what the IAU should have done.

Firstly this now makes it abundantly clear we are dealing only with the little bit of the universe which is our solar system. No mention of stars now, just the Sun. That means we have all this coming again to decide when an extra-solar planet is a planet and when it is a small stellar system body. Surely the sensible thing would have been to find a general definition applicable to any old system

The wording of clause (c) is also problematic. Some people have objected that this means Earth, along with some of the rest, shouldn’t be counted because they’re accompanied by various asteroids. The people detracting on this point are obvious ignoring the spirit of the rule but given that the whole debate is one of semantics it should have been drafted more clearly. Some sort of mention of “objects a significant percentage of its size” tacked on the end, or something along those lines.

Interestingly the definition of dwarf planet (a poor choice of term in itself in my opinion—the whole point is they’re not planets) has mention of satellites tacked on the end. This is probably un-necessary for the planet definition (clear its path…) but maybe should have still been there for clarity. It’s also noticeable that satellite is no longer defined, so what Charon’s status now is I have no idea

My own personal view is that there are just too few categories to describe our solar syatem, never mind others. Asteroids aren’t planets, aren’t KBOs, aren’t… that’s why the different names to start with!

The arrived at “solution” is obviously some sort of botch job, and one can’t help feeling that the entire thing is bound to be looked at again. Perhaps the secret hope is that the exclusion of extra-solar considerations means that the entire question can be brought back up again in the future when people have had more time to think. It is to be hoped that what ever committee is set up to consider that question is more open and doesn’t wait until the last moment to publish its thoughts.

Update: Oh dear. I just knew it would drag on