Must all planets have a solid/liquid core?

Question

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:

3. 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?

Answer 1

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.

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.

Answer 2

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:

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.

Answer 3

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.

Answer 4

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:

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.

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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...

Answer 5

Nobody seems to be mentioning the one planet we know of that definitely does have a liquid core:

The Earth's inner core is solid iron, but it's surrounded by an outer core of liquid iron.

It's almost certain that earlier in the planet's history the entire core was liquid. The inner core only formed once the inside of the planet cooled down enough for the iron to start solidifying. The inner core will grow more and more over time. It's basically like an ice cube floating in water that's slowly being cooled down, the only difference being that solid iron is heavier than liquid iron, so it sinks to the bottom. Eventually the whole core will freeze, but that will take billions of years.

There is also a planet that might answer this question

If that isn't possible then would a gas giant with a liquid but not a solid core be possible?

Jupiter is thought to have a smallish rocky core underneath the (liquid) metallic hydrogen layer, but it's not known for sure if that exists. It's possible that Jupiter formed without one, or that it dissolved afterwards. In that case the core of the planet would be entirely liquid.

Even if it's not the case for Jupiter, it's certainly possible to have a Jupiter-like planet without a rocky core. This would be especially easy if it formed much earlier in the Universe's history, because there didn't used to be as many heavy elements as there are now. Assuming planets could form around the first generation of stars (I see no reason why they couldn't), they must have been made entirely of hydrogen and helium and would have lacked any solid cores.

(Presumably some of these will have been flung away from their stars and will still exist today in interstellar space. However, they will cool down over time, and hydrogen can freeze if it gets cold enough - I don't know whether or not they would cool slowly enough to still be liquid all the way through.)