Iron meteorites are believed to represent the metallic cores of ancient, fragmented planetesimals, asteroid-sized rocky bodies which formed during the early stages of planet formation. Intriguingly, 182Hf-182W model ages of iron meteorites suggest that they formed within a few million years following the onset of accretion (solar system formation). This implies that at a significant proportion of the material which formed rocky planets such as Earth had already been modified by high temperature processes, including melting, segregation of iron-rich liquids, and loss of volatile elements to space. Understanding what these processes were provides insight into the early stages of solar system formation, but is also a necessary step in understanding the composition and subsequent evolution of rocky planets such as Earth. We know surprisingly little about core formation in this first generation of planetary bodies. One additional complication is that under the relatively low pressures of planetesimal interiors (up to 5 GPa), mixing of light elements (S, C, O, P) in core-forming, iron-rich liquids is highly non-ideal. Here, I present results of recent experiments which constrain the extent of liquid immiscibility, i.e. the unmixing of core-forming liquids due to non-ideal behaviour. Comparison of results with data from various groups of meteorites suggests that immiscibility affected a significant proportion of planetesimal cores. The extent of core immiscibility was controlled by thermal histories and volatile loss in planetary bodies. In fact, evidence for core immiscibility could be used to investigate planet-forming processes across the solar system, from rapid formation of planetesimals in the inner solar system, to slow differentiation of icy-rocky bodies in the outer solar system over billion year timescales.