Does Mercury Have a big Iron Core Because it’s so Close to the Sun’s Magnetic Field?

Magnetic fields are great for lots of things – directing explorers, levitating trains, and containing nuclear fusion reactions are just an example of what these invisible forces can do.  Now we can ascribe another feature to magnetic fields – they can give planets a rocky core.

That is the result from research done by Dr. William McDonough at the University of Maryland and Dr. Takashi Yoshizaki from Tohoku University.  The pair developed a model that was published in Progress in Earth and Planetary Sciences that show how the sun’s magnetic field controlled the gradient of raw materials that the planets were formed out of.


We are still studying the sun’s magnetic field – and the Solar Orbiter spacecraft is a key part of the effort.

One of the outcomes of their research was a correlation between a newly formed planet’s “density and proportion of iron” and the strength of the star’s magnetic field during that planet’s formation.  Though without experimental controls the research is unable to show causation, it makes logical sense that iron, which is magnetic, would be affected by the massive magnetic fields emitted by a young star.

Our own solar system is a reasonable example of this – Mercury, despite being the smallest planet, has an iron core that makes up ¾ of its mass.  As planets get farther and farther away, their metallic cores make up less and less of their overall weight, with Venus and Earth coming in at about ? of their weight in their cores while Mars clocks in at ¼.


UT Video discussing the planetary formation process.

The cores themselves aren’t created by magnetic fields though.  Magnetism’s impact is more subtle, drawing chunks of iron together into newly formed protoplanetary balls.  Gravitational forces then take over in driving the dense iron into the core of the planet, where it either melted or cooled, depending on a variety of other planetary formation factors. U

Those planetary formation factors apply not only in our own solar system, but around the myriad stars that house extrasolar planets.  Unfortunately there is currently no way to detect the magnetic field of remote stars, so it would be impossible to include that data to try to understand exoplanet formation in existing systems.  But it is possible to infer what planets are made out of based on their emitted spectra and their estimated density.

Even Mars had a stronger magnetic field at one point, caused by it’s iron core, which might itself have been caused by the Sun’s magnetic field. Credit: NASA/JPL/GSFC

Density estimates will play a key factor in future exploration of this topic.  Dr. McDonough and his colleagues are looking to extrasolar planetary systems to confirm their theory.  They are interested in whether the density of planets decreases and they get farther away from the sun.  If they do, it’s a strong hint that magnetic forces might be causing heavier elements (i.e. iron) to move in towards the star.

For now the biggest impact this work will have is on modeling and future models of planetary formation.  With luck they will be able to confirm their theory in other solar systems, and solidify the importance of magnetic fields in planetary formation.

Lead Image –
Cutaway image of Mercury showing the size of its core.
Credit – NASA Goddard Space Flight Center

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