Can Planets Be Entirely Liquid and Still Viable in Space?

[EnumaElish]

Is it physically possible to have planets consisting entirely of liquids? I don't mean an ocean world, but a planet with nothing but liquid?

 

[chroot]

I don't know of any substances which can be liquid when exposed to the vacuum of space, so the planet would have to have at least some atmosphere, the vapor phase of whatever liquid makes up the rest of the planet.

I also don't really know if any substances can remain in the liquid phase under the extremely high pressure near the core of a planet; even hydrogen solidifies.

The planet would have to be massive to have strong enough gravity to keep its atmosphere; if it were too small, the atmosphere would constantly be escaping, and thus the planet would be basically boiling away all the time. If the planet is too big, though, I'd imagine that its core would end up solidifying.

So, I think the answer really depends on whether or not a suitable substance exists. Anyone else have any ideas?

- Warren

 

[EnumaElish]

What property of gases makes them a viable interface with the vacuum?

 

[mgb_phys]

What property of gases makes them a viable interface with the vacuum?

It's more an inevitable interface with vacuum. At low pressure liquids boil to give vapour so if you had a liquid planet exposed to vacuum you would boil some of the surface off creating an atmosphere.

 

[cesiumfrog]

I don't see why there's a problem in principle. Technically a rock in space should be slowly sublimating, but for common solids that vapour pressure is negligible. I imagine a blob of pure liquid mercury would remain so for a long time. In practice though, the obvious problem is that liquid is an intermediate phase between solid and gas: you need to position your planet so that the average temperature is in that precise range, ensuring also that it hasn't time to freeze at night nor boil on the sun-side. A large volume to surface ratio probably helps, but then you have the pressure gradient to deal with as well.. Ultimately, the common substances of the galaxy (hydrogen and rock) don't lend themselves to this balancing act.

 

[mgb_phys]

In practice you must always have somefinite vapour pressure and so will have an atmosphere however tenuous.
It's less clear (to me - I'm not a solid state person) why you can't have a liquid core as chroot says, the Earth has a liquid core but that may only be due to it not having cooled down yet.

 

[Astronuc]

What property of gases makes them a viable interface with the vacuum?

What does one mean by viable.

Gas molecules have mass, momentum and kinetic energy/temperature.

The maximum velocity must be less than the escape velocity, which is a function of the molecular mass and local gravity field strength.

Planets could be liquid, but in order to have sufficient mass to have enough gravity to retain liquid or vapor molecules (if the temperature range is wide enough), the core or somewhere between core and surface might be solid (function of pressure and temperature).