In new research shared out by Tohoku University, researchers ask, “Why does Mercury have a big iron core?” It turns out that this tiny planet has a metallic core making up about three-quarters of its mass. If Earth were built the same way, gravity would be much stronger and in general, we’d enjoy our world a lot less. In trying to understand how our world and Mercury ended up so different, researchers looked at both the building blocks of planets and the conditions in the early solar system.

Planets are largely a mix of the same stuff that makes up meteorites: iron, silicon, carbon, oxygen, and magnesium all mixed together. In the early solar system, the Sun had a strong magnetic field that interacted with all the grains and small objects in the protoplanetary disk, and the iron in that disk did what iron does in a magnetic field: it migrated toward the magnet. This made the volume near the Sun where Mercury formed extra iron-rich, allowing Mercury to be all that much denser than Earth.

The moral of the story: if you want to shine in all the colors of the rainbow, form in shadow, but if you want to be a dense lump, the spotlight might be the place to be, at least for planets. Your results may vary.

More Information

Tohoku University press release

University of Maryland press release

“Terrestrial planet compositions controlled by accretion disk magnetic field,” William F. McDonough and Takashi Yoshizaki, 2021 July 2, Progress in Earth and Planetary Science