In the deepest region of the Earth’s mantle, just above the core, much of the material consists of crystals of magnesium oxide. In larger rocky exoplanets called super-Earths, the higher pressures in the lower mantle may drive magnesium oxide to transform into a new high-pressure crystal structure or a metallic, electrically conductive liquid that generates a magnetic field. And in gas giants, magnesium oxide could exist in the cores themselves.