Metallic Hydrogen apparently just means "hydrogen in a state that conducts electricity". No idea what kind of pressure and temperate would form actually solid hydrogen, but from my understanding the gas giants are quite hot internally.

And by that diagram the core would be massive.

From what I can see it's a diffuse core, meaning it's a gas until it isn't, but with no real line between the two.

[–] gandalf_der_12te@discuss.tchncs.de 3 points 1 week ago* (last edited 1 week ago)

yeah i read up on it and all four giants (jupiter, saturn, uranus, neptune) have no "clear" surface: they all have a gaseous atmosphere on the surface, but when you go down, it goes above the critical point and therefore continuously changes into a liquid phase with no clear line in between. very deep inside, they all have cores made from rocks, but it's rather small compared to the total size of the planet.