Totally Liquid Planet
Name: Josh A.
Is it possible to have a 100% liquid planet?
Yes it is possible in principle, but the details depend on both the material and on
the mass of the planet. If the liquid was iron or nickel and if it is hot enough
then it is possible for a planet to be completely liquid from the surface to the
core. However, the planet would loose heat into space and progressively solidify.
The heat inside the Earth is maintained by the decay of radioactive elements that
are spread throughout the interior. Therefore, the heating can be maintained for a
period that depends on the amount of radioactive material present. If the planet
is too massive, then the iron/nickel will be in a solid phase in the core because
of the high pressures there.
What about a "water" planet?
When far from its star, the liquid will freeze (as with comets) and when too close
it will boil off into space. In the narrow range of temperature where the liquid
phase could exist, the liquid would immediately boil because it is surrounded by
the vacuum of space (there is no liquid phase possible in a vacuum). The
atoms/molecules would then form a gaseous atmosphere provided
there was enough mass to keep the atmosphere from drifting off into space. But if
it formed an atmosphere it would no longer be a "liquid" planet.
Grant Christie (by way of Howard Barnes)
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Update: June 2012