# Why does half of a planet's radius end up being the radius of the core?

This answer makes the assumption that the ratio between the radius of a planet and the radius of its core is roughly constant shortly after planetary formation. Why should this be the case? Do we know why, when a planet forms, it ends up with half of its radius being core?

• Isn't this just based on assuming the same mix of heavy core-forming elements and lighter mantle/crust forming elements? Take any mass of the mix, turn it into a sphere and have a core form, and the ratio should be the same. – Anders Sandberg Mar 24 at 9:52
• I don't think your linked question/answer makes that assumption for any other than the smaller , inner planets in our system. – Carl Witthoft Mar 24 at 17:11