Calling all worldbuilders: Planet densities?

[Evil Dr Ganymede]

Ok, I'm trying to figure out a system for determining the bulk density of worlds generated for a scifi game (Traveller, if you must know ).

Right now, I'm working on the logic that worlds come in four main types depending on their bulk densities - Icy (1000-3000 kg/m3), Rocky (3000-4500 kg/m3), Earthlike (4500-6000 kg/m3), and Heavy Core (6000-8000 kg/m3). Unless specified exactly, Icy worlds are assumed to have a density of 1500 kg/m3, Rocky worlds are assumed to be 3500 kg/m3, Earthlike worlds are 5500 kg/m3, and Heavy Core worlds are 7000 kg/m3.

I'm assuming that densities are largely determined by the size of the planet and where it is in the system, though there can be some variation (there is a small chance that a given planet can be shifted up or down by one density type (ie Icy to Rocky, Earthlike to Rocky, etc)). But now I'm looking at this and wondering - is this actually true?

My basic assuption was that larger worlds would tend to be denser (cos you get Mars and the Moon and Europa and Io which would be classed as "Rocky" here, but the bigger planets - Venus and Earth - are "Earthlike" in density. Mercury is odd because it's closer to the sun, so has a better chance of having "Earthlike" density.

I had figured that bigger planets - up to about 9,600 km radius - could exist and they'd be more likely to be rather dense due to the increased compression in their cores by their great mass (hence the "Heavy Core"), but now I'm not so sure. Wouldn't "Panthalassic worlds" be low density since they're supposed to be mostly ice/water and rock? And they'd tend to be big too...?

This makes me wonder if my assumption is correct. Could you realistically get large worlds up to about 10,000 km radius that were of any density type? I could imagine a big Icy world (basically a Panthalassic), or a big Earthlike or Heavy Core World (basically a bigger version of Earth), but I can't picture a big Rocky world because it would have to have a really tiny iron core, and if the planet is that big then it surely would have differentiated. Or could you get small worlds that were in the "Heavy Core" density range (I guess they'd have really big iron cores that were poor in volatiles)?

I'm "umming and ahing" about which approach to take here... anyone got any suggestions here?

[Eburacum]

Well, you are better qualified to speculate about this than I, but I don't suppose that any world will have a density much greater than 8000 kg/m^3;
iron is the most likely core of such a world, and if a world is about 10 x Earth's mass, and almost pure iron, you could expect the core to be compressed by a respectable amount;
iron has a density of about 7600kg/m^3, so with 10% compression (WAG) that would increase it to 8360kg/m^3-
such a world would have a high gravity, so would hang on to any hydrogen it formed with and so retain a gas-giant like atmosphere...

so how could we get rid of that atmosphere?
What if another, similar but probably smaller world formed in one of the trojan points, and eventually broke free to smash into our newly formed world?

For a period of time the debris of this collision would remain in a chaotic state, and much of the volatile component might get lost; when the world settles down, it might be very dense and largely volatie free, and might have a lighter moon like our own.

That seem to be one way to form dense, low volatile wqorls; but the fact remains that once the dense world settles down, it is unlikely to lose any more atmosphere, as it will have a comparatively high gravity.

The diameter of a partially collapsed iron world of ten Earth masses would probably be about 24000 km and the gravity about 2.8 gee, I think. So such a world would hang onto any atmosphere it has after formation or after the last major collision;
it would also attract comets, I believe.