Stellar remnants are completely different from planets.
The Earth was never a star and fusion has never occurred in the Earth's core at any time in its history. When a small to medium sized star dies, and the outer layers are lost, what remains is a white dwarf. It is still much more massive than the Earth, and is very hot. It is crushed by its own gravity, causing the matter to become "degenerate".
On Earth, atoms are held together by chemical bonds. Lots of interesting patterns can be formed by the atoms, producing minerals, rocks, seas and life. Nothing like this can happen in degenerate matter.
In degenerate matter, the atoms are pushed together by gravity. Degenerate matter is unlike regular matter. It is much much more dense, and chemical bonds are not a significant force between atoms.
White dwarfs are formed of carbon, oxygen, hydrogen and helium (with the lighter elements on the surface). Even after a white dwarf has cooled, it would still be degenerate, and quite unlike the Earth, in composition, properties and in density.