This theory was shown to not be possible by someone called Harvey Turner.
In order for a planet to remain intact, it needs to be ‘self gravitating’ which means that it has to be able to keep its own shape under gravity.
This process requires a large mass to keep all the forces in balance. Moreover, in order to prevent the planet from collapsing in on itself, a high pressure and temperature are needed in the centre and inner regions to create outward forces to counteract gravity.
Obviously, a single state cannot be held by the whole planet if its interior and exterior are different, and there will certainly be regions of the planet which is made vapour and solid due to the forces acting on the planet.
Interestingly, it is possible for a planet to be covered completely by a liquid over a solid core or there to be a mixture between solid, liquid and gas in the outer regions (much like Earth) and for the planet to still be stable.
Keep your eyes open for my next post,