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Belegaer
2006-Sep-02, 01:44 PM
Could we have a rocky body with mass like that of Earth (or even higher) but with significantly lower density? For example is it possible to have a planet with mass twice as high as that of Earth but with density 3000 kg per m3 (so in fact less dense than Mars)?

As this is highly speculative question maybe I should reformulate it a bit: do we know about anything prohibiting existence of such body?

I pressume such hypothetical planet should be almost completely free of iron, nickel or any other heavier element.

antoniseb
2006-Sep-02, 01:57 PM
The limits really depend on the compressability of the materials making up the planet. Diamond has a density of 3.52 grams per CC. This is almost light enough to meet your criterion. There are probably other fairly rigid materials that are even lighter. Aluminosilicate has a density of 2.6, and might be strong enough to hold up an entire planet.

Belegaer
2006-Sep-02, 02:12 PM
The limits really depend on the compressability of the materials making up the planet. Diamond has a density of 3.52 grams per CC. This is almost light enough to meet your criterion. There are probably other fairly rigid materials that are even lighter. Aluminosilicate has a density of 2.6, and might be strong enough to hold up an entire planet.

I understand - and compressability will surely depend on other factors such a temperature or/and phase. Could aluminosilicate exist even in molten state and in presence of oxygen in the planet's mantle or core? Or is it rather rare mineral/alloy existing only under very specific circumstances?

antoniseb
2006-Sep-02, 02:24 PM
I don't know that much about it, but I imagine Google could tell you some details. My educated (but often wrong) guesses say that it should not have a big problem with compression, even when molten. If this is for a plausible science fiction story, or game, you could just wave your hands and say it is 'good enough'. I think the chances of getting a planet with lots of Aluminum and no Iron is near zero, since my understanding of the element creation processes in the universe suggest that you won't get one without the other.

Ken G
2006-Sep-02, 02:38 PM
But don't forget the other way to get low densities-- use lots of ice and water. I don't know if it's terribly plausible for an Earth-sized planet to be composed of 50% water or some such thing, but it wouldn't disobey any laws of physics.

Belegaer
2006-Sep-02, 03:01 PM
Ken, yes I know but I was particularly interested in rocky planet resembling inner planets in solar system. :)

Ken G
2006-Sep-02, 03:02 PM
There is no particular mystery as to why the planets are prograde, virtually all plausible formation scenarios achieve this result easily. Furthermore, it has nothing at all to do with this thread.