Institute for Advanced Study / Princeton University Joint Astrophysics Colloquium

Planets are not layered in general. The strong evidence of layering on Earth arises because the main components (mantle and core) are immiscible. This is a thermodynamic property and nothing to do with gravity. However, immiscible phases can macrosegregate, allowing gravity to place the more dense component at the bottom. Diffusion of the higher molecular weight species is slow and frustrated by convection. However, central concentration of heavy elements can arise in giant planets (including ice giants) because of the way planets are assembled: The densest material is also the material that aggregates first, and hydrogen is accreted on top. In this case, the various constituent parts are miscible (unlike the case of mantle and core on Earth) but separated at birth. In sufficiently massive "SuperEarths", there should be no core and mantle because entropy wins. I will discuss our current understanding of the imperfect separation of core from mantle on Earth and the extent to which a compositional gradient in giant planets may be legacy of formation.