aesmael

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.

Originally published at a denizen's entertainment. You can comment here or there.

Increasingly less recently I had been discussing with Pazi a game project. I wanted to do something useful so I offered to craft a little program module which could generate 'realistic enough' star systems. Something that would not simulate stellar formation and evolution directly, but which could use information derived from those simulations. I suspect such a tool could find a few uses, all of roughly the same sort, but apart from being useful I mainly think it a pretty nifty idea. Of course I would need to first finish my current project, but that is badly in need of finishing anyway.

Trying to determine what I would consider a 'first finished' version, not the first steps I would take in writing it but what it would need to do for me to consider it getting to completed. That, and some sorting out in my mind of how to approach the workings of this thing.

My thinking is to start with the star itself and build up the system around it in a sort of abbreviated history. Presumably, get it working for single stars before worrying about multiple systems, unless those turn out not really harder to put in. We get a few basic figures for the star, stuff like mass, spectral type, luminosity, then use those to proceed. Stellar mass typically correlates with disk mass available immediately after its formation, determining how much material is available to form planets and potential early migration activity for them. Plus, the distances at which various sorts of planets can form. I think we can relate those figures to get some nice functions for populating planets from. Start by placing giant planets, unless consensus has changed on the order of formation, then the little ones, abstract some pseudo-orbital-evolution and there we have our own impromptu planetary system.

Each planet's numbers - semi-major axis, eccentricity, mass, composition - get to interact with the host star's numbers, and we get a rough guide to the world's likely surface conditions, atmosphere and chemistry of note. There might be more than one class a given world could fall into.

I find the prospect of this fairly daunting. I imagine for an experienced programmer it would fall into the class of 'relatively simple' but for me, it's not. I'm only just barely learning, still. But I look forward to accomplishing this. I suspect the first thing needing doing is collecting papers with relevant modelling statistics to use; been wondering if it might be much harder too to get information for moons and small bodies.

I worry I have somehow done badly by describing my goals in the terms I have, rather than some technical description of coding intentions, but suspect this to be a silly worry. Am expecting to be learning a lot of how in the doing, and believe it is advantageous to have clear ideas on what I want the outcome to be.

Once that 'first finished' version is working I would want to add some fancier abilities, like aging. This would be needed anyway to incorporate giant stars since those again modify their planets as they age and expand and brighten, but also would be useful for incorporating very young stars still in the process of forming planets and details like at what point terrestrial worlds are likely to become tectonically dead. I suppose an approach that might work is to generate values for the whole system's duration (at least, mark points when things change and the values before and after). I suppose it is unlikely this program would get used for anything that depicts time-spans long enough for stars to visibly age, but at least for some sort of manual editing or inspection it would not end up generating a new history every time the age is tweaked. Perhaps too much, but I cannot help but feel it would be a useful approach somehow.

As for populating a larger 3- or 2-dimensional space, I suspect that would be harder to do, maybe want for another tool that talks to this one. Currently I'm thinking finding a good way to place stars in a field would be a more difficult thing to do than getting population kind distributions for various stellar environments. Hopefully by the eventual time I get to attempting that I will know much better.

That's main thoughts and plans on this for now.

[At Systemic can be found an excellent example of why this will likely be difficult to do. At the moment our models aren't reliably predicting the sizes of giant planets in some important situations. And we're still finding situations where our models are in error, which is why I suspect I will have to aim for 'realistic enough' and post refinements as astronomers make refinements. Or as soon after as I am able to.]