  Recent advance in understanding the supercritical state posits the existence
of a new line above the critical point separating two physically distinct
states of matter: rigid liquid and non-rigid gas-like fluid. The location of
this line, the Frenkel line, remains unknown for important real systems. Here,
we map the Frenkel line on the phase diagram of supercritical iron using
molecular dynamics simulations. On the basis of our data, we propose a general
recipe to locate the Frenkel line for any system, the recipe that importantly
does not involve system-specific detailed calculations and relies on the
knowledge of the melting line only. We further discuss the relationship between
the Frenkel line and the metal-insulator transition in supercritical liquid
metals. Our results enable predicting the state of supercritical iron in
several conditions of interest. In particular, we predict that liquid iron in
the Jupiter core is in the "rigid liquid" state and is highly conducting. We
finally analyse the evolution of iron conductivity in the core of smaller
planets such as Earth and Venus as well as exoplanets: as planets cool off, the
supercritical core undergoes the transition to the rigid-liquid conducting
state at the Frenkel line.
