Most planets are so large that their characteristic pressure exceeds by orders of magnitude current experimental capability. The behavior of materials in this regime is poorly understood, but likely to be rich, with important implications for our understanding of planetary formation and evolution. The discovery of exo-planets and the development of high energy density experiments motivate a closer look. We have been using density functional theory, combined with molecular dynamics and lattice dynamics to study materials in this extraordinary regime. I will explore two cosmically abundant planetary constituents at pressures up to 1 Gbar and temperatures up to 5 eV: helium and iron, focusing on changes in the electronic structure with compression and heating, including gap closure and changes in Fermi surface topology, and the connection between electronic structure and physical properties such as fluid or crystalline structure and electrical conductivity that may have important implications for the thermal evolution of planets and the generation of magnetic fields.