A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
Inner core The only major layer of the earth that's liquid is the outter core, which is basically metal with the same composition of the inner core (mainly nickel and iron) but molten instead of solid. All the other major layers above and below it are though to be solid.
