The melting curve of iron at extreme pressures: Implications for planetary cores

Exoplanets with masses similar to that of Earth have recently been discovered in extrasolar systems. A first order question for understanding their dynamics is to know whether they possess a liquid metallic core like the Earth. However, the iron melting curve is unknown at conditions corresponding to planets several times the Earth’s mass (over 1500 GPa for planets with 10 times the Earths mass (ME)). In the density-temperature region corresponding to the core of those super-Earths, we calculate the high pressure melting curve using a first principles approach based on density functional theory. We perform direct molecular dynamics simulations using the computer intensive two phases approach. The resulting melting exhibits a slope steeper than any calculated planetary adiabatic temperature profile. This suggests that the presence of molten metallic cores is less likely as the size of terrestrial planets increases.