Tsk
Originally published at a denizen's entertainment. You can comment here or there.
I disagree with the IAU's decision on defining planets, sure. But articles like Should The Cluttered Skies Demote Earth? (and comments of a similar style) make me want to ask people to please stop helping.
Pluto and other 'dwarf planets' have crossed a threshold. Their mass is sufficient that gravity[1] dominates their shape. They are probably all differentiated bodies. So I consider them interesting in ways different to other small bodies, the smaller asteroids, comets, kuiper belt objects, and irregular moons. Besides, in astronomical terms our sun is a dwarf star, but it is still a star.
However. If one looks at the dynamical properties of these bodies, differences between these 'dwarf planets' and the others become sharply apparent. They just don't have the same heft. Looking at the table in that article it is clear that one of the most common arguments against the dwarf planet categorisation - that if you put Earth or some other planet out in the kuiper belt, it would be called a dwarf planet (and that location should not determine what an object is) and thus the whole dwarf planet thing must be nonsense. But according to those figures Earth would have to be moved nearly 3,000 times further from the sun before it would even approach the dynamical insignificance of Pluto or Eris, and that doesn't account for the effect Earth would have had on that region had it been orbiting out there these several billion years - I am pretty sure those figures are derived from the solar system as it is now, not as it might have been.
I do think for planetary bodies a combination of physical and dynamical properties ought to be considered when deciding what to call them. If Ganymede or Titan orbited the sun we'd probably call them planets. If Mercury orbited one of the other planets, we'd probably call it a moon (actually we might call it a double planet if it orbited Earth or anything smaller, but never mind). So I think location matters. Context matters. And arguments like "If we moved X it would / wouldn't be a planet" aren't convincing because they're trivial or false. Nor, to say Earth should be a dwarf planet on account of the stuff around our orbit because the total mass of it is insignificant - in any interaction between them Earth will do the scattering, not be knocked around or much affected. As arguments go, they're pathetic, and I'm embarrassed to see people somewhat on my side using them.
There is an article by S. Alan Stern and Harold F. Levison proposing a planetary classification scheme which meets all of my requirements, and adds some new dimensions I had been hoping for but had not thought of. Although I am partial to major and minor planets rather than über and unter planets the latter set is probably a better choice for not conflicting with our existing use of minor planets. I recommend reading the linked pdf for anyone interested in this topic. It is clear and straightforward, and I doubt I could express it better here. If I were Tyrant of Astronomy I might make that document law and then abdicate. Maybe.
I keep trying to write a description of the proposed scheme here despite what I said immediately above. Probably because I worry people won't follow the link. So, short of copy-pasting large chunks, we simplify: a planet is defined as an object massive enough that its shape is dominated by gravity, but which does not ever sustain fusion. A mathematical test is proposed to decide whether a particular planet is über (dynamically dominant) or unter (not dynamically dominant) in its orbit. Some planetary satellites are considered planetary bodies themselves. Further classification subdivisions are proposed according to composition (rock, ice, or hydrogen) and mass (subdwarf, dwarf, subgiant, giant and supergiant[2]). In this scheme, Earth would for example be a rocky dwarf über planet.
Much of what pleases me about this scheme is its scope for recognising both the variety and the similarities of planets. Pluto is different from Mercury is different from Earth is different from Uranus is different from Jupiter. They're all different from Titan, Triton, Mars and Ceres. But they're all interesting, and have qualities in common which non-planets do not have. Plus, it feeds my sense of wonder and excitement about the universe.
[1] Along with other details such as their material properties and angular momentum, e.g. 20000 Varuna. Also happens with stars, for example Achernar.
[2] This is fairly directly taken from how we classify stars according to size.
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