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At the moment it seems we have two different types of planet

  1. We have Rocky planets with a solid core that occupies most of the mass of the planet

  2. We have Gas Giants that contain a solid core but are mostly atmosphere.

We also have the possibility of a liquid planet as discussed here:

  1. Could a planet made completely of water exist?

Is it possible that other types of planets could exist (preferably naturally formed but alien intervention would be accepted).

For example could a Gas Giant exist with no solid core at all? Or would the pressure always create a solid at the center?

In other words could we have a true "gas world" where as you go down the pressure increases but you never reach a solid or liquid even if you pass through the core and start to rise out of the other side again? If that isn't possible then would a gas giant with a liquid but not a solid core be possible?

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  • $\begingroup$ That answer only answers half of the question. It doesn't address gas-giants. I'd be curious to know if a small enough gas-giant would have enough heavier gases to still maintain self-gravity, while losing the lighter ones, so as to avoid the solid-gas core. $\endgroup$ – user3082 Feb 9 '15 at 17:25
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    $\begingroup$ @user3082 I've modified the question to avoid the dupe, should be good to go now hopefully. $\endgroup$ – Tim B Feb 9 '15 at 18:00
  • $\begingroup$ The other question doesn't describe a planet with a liquid core. It describes a planet with a water core which is compressed into an exotic form of ice by the high pressure. $\endgroup$ – ckersch Feb 9 '15 at 18:17
  • $\begingroup$ @ckersch The other question doesn't specify that, although the answers might. $\endgroup$ – Tim B Feb 9 '15 at 18:20
  • $\begingroup$ You need to modify the question, a gas-giant will always have enough mass to solidify something, it's going to be the edge cases, of really small pockets of gas, or giants that get chunks blown out of them (but they'd have to be near enough something else that draws off that gaseous mass). Might lose some to very high solar wind and magnetosphere/photodisassociation? $\endgroup$ – user3082 Feb 9 '15 at 18:39
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Gas, no. Liquid, yeah, technically.

For the water planet, you can look at a very high temperature and pressure phase diagram to intuitively see that, although this may be true, it's not going to be very satisfying. At any reasonable internal planet temperature, the water planet will have a solid ice core.

enter image description here

The neat thing to take away from this is that certain types of water ice can burn you if you touch them. Of course, they'd be burning your horribly crushed body, because they only exist at very high pressure.

But back to other materials than water. The diagram is from this publication. It's a proposed phase diagram for water at very high temperatures and pressures. Specifically those inside Uranus and Neptune. It turns out that water stops being water under those conditions. This occurs with all substances at the extremes of temperature and pressure.

For instance, as ckersch pointed out, hydrogen will turn into a metallic liquid under high pressure and sufficient temperature. Like those experienced inside a planet. Metallic liquid hydrogen can be 5-40 times denser than liquid hydrogen. What this means is that a planet made purely of hydrogen would smoothly transition from a gas to a liquid and the densest liquid would be several times denser than liquid water. There would be no surface. As the density increases the atoms eventually can't stay away from each other and the planet becomes a star.

It's easy to imagine that in the very early universe, all planets were made entirely of hydrogen and helium. That is to say, at one point there may have been more planets without a solid core than planets that had a solid core.

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    $\begingroup$ This only applies for large planets. If you go for IAU's definition, apply it to a "clean" solar system, a water globule that is in a solar orbit is a planet. Quibbles like that aside, the question is, can you get a large enough ball that it will hold its own gas (or liquid) via gravity? $\endgroup$ – user3082 Feb 9 '15 at 20:03
  • $\begingroup$ @user3082 Ok, so using your semantics argument, a liquid water planet can exist as a single drop of water orbiting a star. That's fine, if true, but it feels disingenuous to the question. Besides, I find it hard to believe a water globule, or small gas accumulation, could satisfy the third criteria from the IAU. $\endgroup$ – Samuel Feb 9 '15 at 20:10
  • $\begingroup$ If it's a clean system, then it has cleared its orbital path (there is nothing in its orbital path). But, see my answer. Does Mercury count as a planet for you? $\endgroup$ – user3082 Feb 9 '15 at 20:22
  • $\begingroup$ @user3082 Yes, obviously Mercury is a planet. $\endgroup$ – Samuel Feb 9 '15 at 20:26
  • $\begingroup$ Then, as I said in my answer, we can have a thoroughly liquid planet, of that mass. $\endgroup$ – user3082 Feb 9 '15 at 20:31
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Liquid Core

It's definitely possible to have a core of liquid hydrogen. It may even be possible in a gas giant, but the properties of materials at those sorts of temperatures and pressures are mostly theoretical.

Unlike water, which is always a solid at high enough pressures, sufficiently hot pressurized hydrogen turns first into a liquid, and then (theoretically) into a liquid metal. This may be what happens in gas giants in our solar system, though their cores have enough other elements in them that they are solid.

The phase diagram for high pressure hydrogen looks like this:

Phase diagram for high pressure hydrogen

So yes, you could have a planet with a core of liquid hydrogen. Make it about as big as Jupiter, but remove the impurities.

Gas cores

I need to do a bit more research for gas cores, but I don't think they'll work, at least for a stable planet. To be as big as a planet and hot enough to maintain a gaseous core, my guess is that either you'll either ignite nuclear fusion in the core and form a star or else the outer layers of the atmosphere will have too much energy to be held in by gravity and will be lost to space.

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  • $\begingroup$ How small can a gas planet get before it starts losing gases at a fearful rate, because of its lack of gravity? $\endgroup$ – user3082 Feb 9 '15 at 21:45
  • $\begingroup$ Where is the diagram from? $\endgroup$ – Samuel Feb 9 '15 at 22:04
  • $\begingroup$ NAU's meteorite study's program. $\endgroup$ – user3082 Feb 9 '15 at 22:07
  • $\begingroup$ @HDE226868 I've replaced it with what I think was the original image. At the very least, with a relevant image... $\endgroup$ – ckersch Feb 22 '17 at 17:41
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Short answer: yes, planets must have a liquid/rocky core

For almost all substances, at extremely high pressure/temperatures, one of two things happens: either you are past the critical point, and the distinction between gas and liquid becomes meaningless, or you have compressed your substance into a solid.

I do not know the way to calculate it, but I suppose there may be a possibility that there exists a gas that would resist liquefaction sufficiently that a planet made of the gas would both be have enough gravity to prevent that gas from escaping and low enough mass to prevent the gas from going supercritical/liquifying.

However, solar systems are not usually uniform. They instead contain lots of different matter, including matter that would be liquid/solid at the temperatures and pressure of a planet core. So, while it may be possible for a planet to theoretically exist (and I'm hesitant to believe that is actually possible), it frankly couldn't happen that way in practice.

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  • $\begingroup$ You forgot the "made by aliens" stipulation. So, this isn't really an answer. $\endgroup$ – user3082 Feb 9 '15 at 19:54
  • $\begingroup$ @user3082 How is this not really an answer? Because I didn't specifically use the word alien? Unless we give our aliens the leeway to operate outside the laws of physics (which would make this whole exercise pointless) they would still have the same challenges that a natural solar system would. $\endgroup$ – Nick2253 Feb 9 '15 at 20:10
  • $\begingroup$ Aliens can compose a planet of specific things; ie: pure water, and clear the orbital path, and keep things from impacting it. ie: They are not limited by natural solar system conditions. $\endgroup$ – user3082 Feb 9 '15 at 20:21
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Edit:
Yes such a planet could exist (even made of water):

Basically we're looking for how much mass will put 100kBar or less on the center of the planet at the distance from the star which provides 650K of heat to the planet?

Mercury seems a good fit for the heat, let's run with it:

enter image description here

Given that phase diagram, you could have a planet that doesn't experience more than 100kBar of pressure at the center of its mass, that orbits at the distance of Mercury. Mercury has an internal pressure of 400,000 atmospheres, but is 5.4x as dense as water. Reduce the internal pressure of our waterworld by 1/5th because our hypothetical planet is 1/5th as dense. We only need to stay below ~98,000 atm (we're at 74,074 atm) to be under 100kBar. Which means we could have a liquid planet bigger than the size of Mercury, at slightly greater than Mercury's distance from the sun or less. Assuming even heating throughout the planet.

According to this link, however, that's not a problem, we've placed our planet at (or nearer) the 650K heat-distance from the sun.

The top of the ocean is boiling but the atmosphere at the surface is also at 100% humidity. Any rain (due to troposphere cooling) will boil before it hits, but you'll probably have cloud cover somewhere inbetween. Rain might potentially be able to fall on the night side, or if cloud cover provides enough shade/heat blocking. However cloud cover doesn't make Venus cold. Cloud cover also blocks heat-loss to space, which means the night side of the planet will be warmer than Mercury's night side.

No magnetosphere means we'd constantly be losing this water vapor atmosphere (well, and pure oxygen, since the hydrogen is stripped via photodisassociation). I'm not sure how long the planet would last after it's creation and heating.


So many thoughts on this concept/question, and I don't want to clutter up the comments even more.

Some thoughts:

You'd need exceptionally pure liquids/gases to form a non-solid planet. Anything that could precipitate out would cause a solid core. That's a probably a lot less than one ppm (or even 1 ppt). I'm unsure of how precisely pure we'd need to make the above liquid-water planet. I'm pretty sure it's beyond our current capabilities. OTOH, maybe you let the core form, and then lift it out using massive engines.

You also (most likely) need an exceptionally clean solar-system, as one of the definitions of a planet is something which cleans up its orbital path. You can't have asteroids (and probably not even space dust, for age values over a billion years), or anything else which can get sucked into your planet, or they will form a solid core. Of course, our aliens could keep pulling out cores, tossing them at the current moon, and letting that moon reform into a oblate spheroid.

edit:
In case it wasn't clear by other comments, and other parts of this answer - the planet would have to be small. I'm unsure of how small, but it could not be a gas-giant, as that's big enough for its gravity to solidify gases (and most likely its liquids).

You may end up with a gas-planet with a liquid core - especially if your planet picks up comets, or has spare free hydrogen/oxygen that can get hit by lightning, or anything else. I'm assuming the whole point of this exercise is that you want to pass through your planet - hitting the liquid surface (that exists at vast pressure) at sufficient speed (falling speed alone, much less powered flight) might be a problem.

I'd be curious to know if a small enough gas-planet (not a giant, obviously) would have enough heavier gases to still maintain self-gravity, while losing the lighter ones, so as to avoid the solid-gas core. Or would a small enough planet even form out of gases? IIRC, they can't form at the interior of solar systems (get blown away), but form at the outer edge, and get thrown inwards and melted...

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    $\begingroup$ As it is, -1, it appears you're just discussing the question further, rather than offering an answer. $\endgroup$ – Samuel Feb 9 '15 at 19:04
  • $\begingroup$ Some parts of an answer are in there, if you bother reading: must be composed of clean materials, can't form in inner system, etc. But yes, there's a lot of discussion of the question. And it's definitely not a complete answer, nor even an attempt at one. $\endgroup$ – user3082 Feb 9 '15 at 19:06
  • $\begingroup$ What's the matter with solid material turning to liquid under the extreme pressure in the center of a massive planet? It doesn't matter if it picks up an asteroid or two - or three, or four. $\endgroup$ – HDE 226868 Feb 9 '15 at 19:32
  • $\begingroup$ I'm sorry, I don't know of solid materials which turn liquid under greater pressure. Please cite. As for liquids/gases turning solid, that's why I was careful to say it would only apply to non-massive planets. $\endgroup$ – user3082 Feb 9 '15 at 19:55
  • $\begingroup$ The phase diagram I gave is for water. You're trying to apply it to iron. $\endgroup$ – Samuel Feb 9 '15 at 20:31

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