Why does Mercury have such a big iron core?

A new study disputes the prevailing hypothesis on why Mercury has a big core relative to its mantle. For decades, scientists argued that hit-and-run collisions blew away much of Mercury's rocky mantle and left the big, dense, metal core inside. But new research reveals that collisions are not to blame -- instead, the density, mass and iron content of a rocky planet's core is influenced by its distance from the sun's magnetic field.