Evolution of Mercury, illustration. The innermost planet, Mercury, has a very substantial iron core, occupying much of the planet's interior. It is far larger compared to the radius of the planet than the iron core of Earth, for example. Astronomers think this is because, shortly after the planet formed, more than four billion years ago, it was hit by another planet, as depicted in this illustration. (1) The impacting planet approaches the proto-Mercury. (2) The objects collide, an impact which removes Proto-Mercury's outer, rocky mantle, but leaves its iron core largely intact. (3) The rocky ejecta from the impact is blown into space, away from Mercury. It does not reaccrete. (4) After impact, Mercury cools and reforms. It is now smaller than before and, having lost much of its mantle, is left with an oversized iron core.