Plausible explanation
I have heard about this, but I haven't seen the data yet. There are three plausible explanations for this:
1. It's an outer planet that migrated inward. When the star shed its outer layers to become a planetary nebula, an outer planet started plowing through the material and lost energy thereby spiraling in on the white dwarf. When the planet got closer, it disintegrated leaving just part of the core. A variation of this is that it is an iron (or metallic) asteroid that migrated inward.
2. The planet survived the death of the star, although this is highly unlikely. Mercury and Venus will be engulfed by the sun at the end of its life. As the planets plow through the solar material that the sun will puff off, the planets will lose energy and spiral inward. The high-energy plasma that makes up the star will evaporate the planets until there is nothing left. Even an iron core will not survive. Additionally, wildly shifting gravitational forces will cause uneven tidal pressures on the planets and they will be ripped apart. Earth may or may not be consumed in this way.
3. The planetoid formed (or reformed) after the star died and became a white dwarf. There is a fair amount of heavier elements such as silicon and oxygen (mostly hydrogen and helium though) which enabled a planetoid to form there.
Given these three possibilities, I think #1 is the most likely scenario. And yes, I am an astro-boffin.
Biting the hand that feeds IT
