Consider a large spherical planet made of pure iron. Think of something similar to Mercury or Earth without its mantle, only much bigger, though those planets have elements other than iron mixed in their core.
What is the highest possible mass such a planet could have while still being stable? Can you tell the approximate radius it would have (if this is meaningful), and the pressure at the center?
If you tried to make a planet with mass larger than that, what would stop you?
I can think of two possible obstacles that would stop a heavier iron planet to form.
- The gravity of the planet cannot keep all its mass, so some of the iron will leak away to open space.
- The planet (or at least its center) will collapse under its own pressure, and undergo some sort of phase change so it is no longer made of liquid iron, but of something else: plasma or some other exotic phase of matter made of mostly iron nucleuses, matter made of fused or split atoms other than iron, neutron degenerate matter, other degenerate matter not containing nucleuses, black hole.
I am specifically not asking how heavy a planet could form in nature, but rather, how heavy a planet would be stable if it somehow got formed.
I did not specify a temperature for the planet. Imagine the planet to orbit around a star or as a free planet, whichever is convenient for you, and choose a realistic equilibrium temperature it could attain from radiation in that case. The iron ball shall not be a core inside a star or gas planet.
The planet shall rotate slowly enough that the rotation cannot significantly reduce the gravitational acceleration on its surface.