A mineral from the inside of planets reveals a surprising lesson: How you push matters

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.