It's the part that is the same distance from all points on the surface.
The center of the planet is called the "core."
The center of the planet is called the "core."
The center of a planet star and or moon is the core
The distance from surface to surface through the center of a planet would be that planet's "diameter".
A big ball of gas and helium. Not if your planet is the Earth. Earth has iron at its center.
core
Egypt
Core
An imaginary line that runs through the center of a planet and its poles is called its axis.Or possibly a diameter, in regards to the planet's dimensions.
None. There is no known center to the universe.
No. There is not another planet inside Earth.
The center of a wheel is generally called a hub.