Well, comets are dirty ice through and through. So that's a start, no rocky core.
Pluto is though to have a rocky core.
But Uranus, if it's got a rocky core, it's tiny. I think the jury is still out there, and it could have an icy core. What is Uranus Made Of?
But - your question is actually about two different things.
For phases of various forms of ice, some at high pressures, and some liquid at high pressure, see Water phase diagram
Stack exchange discussion here may be useful Would a planet made completely of water be possible?
At the pressures at the center of, say, an Earth sized planet, you don't need to worry that it will fuse, or form degenerate matter. It will still be H2O, only question is if it is solid, or perhaps a supercritical liquid.
Now - if you say instead a hydrogen / helium planet - that is possible. In the early universe before any or much oxygen formed, there must have been many planets consisting entirely or almost entirely of hydrogen and helium. It's the addition of oxygen that makes it tricky because as soon as you have oxygen you also have abundant carbon too.
But you don't have to have silicon or iron or any of the heavier elements at that stage. So around stars with no or very little by way of silicates or iron, whether or not it happens in our solar system, must be possible.
Just a few thoughts, maybe others will expand on this in their answers :).
Pluto is though to have a rocky core.
But Uranus, if it's got a rocky core, it's tiny. I think the jury is still out there, and it could have an icy core. What is Uranus Made Of?
But - your question is actually about two different things.
- Is it entirely water, i.e. H2O? This seems quite hard to achieve (hesitate to say impossible) because any star that forms oxygen, atomic mass 16, surely also forms carbon, atomic mass 12, at an earlier stage in the fusion processes. So you would expect some carbon monoxide, carbon dioxide and methane as well. In our solar system at least, then all those are present in comets. You would need some process to separate out the water from the compounds of carbon before the planet forms, and I don't know if that is something that happens. I mean they do separate out on planets, when the oceans form - but how then can you get the oceans, and just the oceans, up into space again to form new water-only planets? But maybe there is some other process that does this??
- Does it lack a rocky core? This is much more plausible, because rocky elements condense out of a nebula closer to the sun as it collapses. And the "ices" methane, water, carbon dioxide and monoxide - condense out further from the newly forming star. In that case, also, it might have a core made of carbon (some form of diamond, say). A diamond core is not water, but not rock either, well would you call diamond a rock?
For phases of various forms of ice, some at high pressures, and some liquid at high pressure, see Water phase diagram
Stack exchange discussion here may be useful Would a planet made completely of water be possible?
At the pressures at the center of, say, an Earth sized planet, you don't need to worry that it will fuse, or form degenerate matter. It will still be H2O, only question is if it is solid, or perhaps a supercritical liquid.
Now - if you say instead a hydrogen / helium planet - that is possible. In the early universe before any or much oxygen formed, there must have been many planets consisting entirely or almost entirely of hydrogen and helium. It's the addition of oxygen that makes it tricky because as soon as you have oxygen you also have abundant carbon too.
But you don't have to have silicon or iron or any of the heavier elements at that stage. So around stars with no or very little by way of silicates or iron, whether or not it happens in our solar system, must be possible.
Just a few thoughts, maybe others will expand on this in their answers :).
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Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
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