Hi LJK

Marvin Herndon is a geophysical radical otherwise known for his nuclear-reactor model for the cores of planets, including Earth. There's something to that basic idea, though he misrepresents some data, but I am not so sure about the decompressed Earth concept. Mantle convection is pretty well established as a process so it seems odd he'd want to do away with it. Also he is yet to explain how the Earth disposes of ~ 300 Earth masses of Hydro/Helium to become an exposed core.

Interestingly Micheal Woolfson - another cosmogonic radical - provides just such a scenario. He has modelled a collision between two Jovians and generally their high density cores survive the impact - this is the origin of Earth & Venus. However the masses of the planets involved are somewhat lower than Herndon's Jupiter scale protoplanets, so I am unsure how much the decompression would change their radii.

Adam