planetary structural layer planetary core core "Not to be confused with planetary core in the core accretion theory, referring to a central accretionary body surrounded by a halo of dust and gas which serves to trap debris and increase the rate of accretion.." WIkipedia:Planetary_core. This class is meant to be applicable accross planets. Earth's core would be an instance. The "layer" aspect of cores may be problematic. A planetary core is the innermost structural layer or layers of a planet. The core may be entirely liquid, entirely solid, or have both liquid and solid parts.