Is a planet entirely made of only liquid possible?

Approaching this as a mathematician, interested in the planets

Assuming you mean a liquid ocean of water which beings sufficiently adapted could potentially swim all the way through, it would have to be small because water when compressed enough becomes ice - or else - to have a hot core, which it might have soon after formation, or be tidally heated. And I’m taking the liquid to be water rather than, say, liquid nitrogen, or liquid helium, or organic liquids such as methane, ethane etc.

So, first, the easiest case, if you don’t need it to have enough gravity to hold an atmosphere, I don’t see why not. Basically you want a large comet, in an orbit which keeps it permanently liquid. We could create such a world artificially in our solar system with mega engineering by diverting a comet into just the right orbit around the Sun.

However, unless we add something extra to the picture, it wouldn’t last long. The problem is that water evaporates rapidly in a vacuum. With surface temperature of 273.15 K and using the equation for mass loss of liquid water in a vacuum of

(pe/7.2) * squrt (M/T) kg / m2 / sec
(equation 3.26 - compare calculation results here: Modern Vacuum Physics)

where M is the molar mass, T is the temperature in kelvin, pe is the vapour pressure, which for water at 0 C (273.15 K) is 611.3Pa, (Vapour pressure of water at 0 C), M = 0.018 kg, gives

(611.3/7.2) * sqrt(0.018/295) = 0.663 kg / m2 / sec.

So you lose about 57 meters a day thickness of liquid water exposed to a vacuum, or about 20.9 kilometers of water per year. The rate of loss goes up if the temperature increases and is 2.495 kg / m2 /sec at 295 k, or 22 C. That’s 215.6 meters per day and 78.6 km per year.

So, a liquid water comet would not last for long. That is unless you get a constant influx of other comets bringing more water to it.

LARGER PLANET WITH SIGNIFICANT GRAVITY

What if the object is large enough to retain liquid water for long periods of time?

That’s only possible if it has at least enough gravity to retain a significant amount of atmosphere, even if the atmosphere is just water vapour, or oxygen (after dissociation of the water by radiation).

~But then - it will surely have a solid ice core. In that case, if the water is also salty, it might well have a “club sandwich” type pattern of alternating layers of ice and water as suggested for Ganymede, of various types of ice, with some of them “snowing upwards”

Ganymede׳s internal structure including thermodynamics of magnesium sulfate oceans in contact with ice

But even Ganymede is not large enough to retain an atmosphere to protect the surface layer of water. Its diameter is 5,268 km so if brought close enough to the Sun to have a permanently liquid surface layer, it would vanish completely in 67 years.

It could build up a temporary atmosphere though as the water evaporated. It’s gravity is similar to the Moon’s.

Can we terraform the Moon? If yes, how difficult is it and is it possible with the current technology?

So using a calculation from that answer, if you hit it with a comet 164 km in diameter you’d have enough material for an atmosphere which would last for 10,000 years. Since the volume goes up as the cube, that means with a similar pressure atmosphere, a moon the size of Ganymede could last for 10,000*(5,268/164)^3 = 331 million years before evaporating completely if it built up an Earth pressure atmosphere. And the atmosphere would consist of water vapour and oxygen, so might well be breathable too, especially if you can somehow introduce some nitrogen as a buffer gas.

But that’s still no good if you want the core to be liquid all the way through.

ANOTHER SOLUTION - “DIRTY OCEAN” WITH ORGANICS

There is another solution though. If you are willing to do it artificially, you could cover the entire surface of a small comet with a low density liquid which also has a low evaporation pressure.

Indeed, comets are rich in organics anyway, so if you could bring a comet to just the right distance from the Sun, not too far, not too close, then as it melted, it would develop a layer of scum like that. And that might well be habitable too, with organics and an oxygen rich ocean too, due to similar processes to the ones that make Europa’s ocean oxygen rich.

Organics with a high evaporation rate would disappear leaving only those with a low evaporation rate, and perhaps solid layers as well.

So if you are okay with your planet being a tiny comet sized object, and your water can be a bit “dirty” with organics, which means it can also support life, I’d say yes, it does seem possible. So how large can it be?

EXAMPLE OF EUROPA

Europa’s ocean may be as much as 100 km thick, with a surface layer 10 - 30 km thick.

Based on that, you could have a minor planet made of ice, 260 km in diameter, and consisting entirely of water, I think, with a surface layer of organic ionic fluids or a scum of organics in solid form floating on the surface. That could last for billions of years.

That makes it about the same size as 88 Thisbe

Vesta’s double that diameter

Vesta, Ceres and the Moon to scale at 20 km per px

I’m just using the figures for Europa and the depth of its subsurface ocean, which is kept liquid by tidal heating, and assuming the situation is similar - so this is just a rough estimate as it would depend on what you have by way of an energy source to keep your planet or moon warm. With just surface heating, surely the center would cool down eventually.

Tidal heating could be a way to keep your planet liquid just as for Europa, so if you make it so that it orbits a hot Jupiter - those are planets like Jupiter that end up in orbits close to their sun, and they may well have liquid water moons.

Another solution, without the layer of ionic liquids or similar, is to have a constant influx of comets to replenish the water. I can imagine some scenarios where that could work, e.g. soon after formation of a solar system. It also might work for a while later on in a white dwarf star with material brought into it through destruction of its Oort cloud and perturbing effects of an extra planet, see Our Solar System Could Lose One Or More Of Its Gas Giants Billions Of Years In The Future - and that would also help keep it hot. In a situation like that maybe even quite a large minor planet would stay hot enough to stay liquid all the way through. But the tidal heating + surface thin layer seems the easiest solution to me.

So, in short, I think this scenario could actually exist in nature, if you don’t mind having an ocean rich in organics, covered with a thin layer of organics, and make it a moon orbiting a gas giant rather than a planet on its own.

This is just a rough estimate. Would be interesting if someone was to do a paper on it - has anyone? Would a liquid water world the size of Vesta or even Ceres be possible, with tidal heating to keep it warm? Can a hot Jupiter have a moon of pure ice? (I don’t see why not if it formed far enough away from its host star originally, but would be interesting to know how likely that is).

STACK EXCHANGE DISCUSSION AND STAR TREK VOYAGER EPISODE

See also stack exchange discussion here, where I’ve just added this answer (a copy of the original text which I wrote here)

Could a planet made completely of water exist?

The original question there is motivated by a story in Star Trek Voyager about a planet made entirely of water with the water prevented from escaping by a “containment field”

Thirty Days (episode)