Mercury's large iron core is thanks to the sun's magnetic field

One of the biggest arguments about the inner solar system among astronomers and scientists is why exactly Mercury has such a massive iron core. The prevailing theory is that collisions with other bodies destroyed much of the rocky mantle around Mercury during the formation of the planet, leaving the large and dense metal core inside covered by a relatively thin crust. However, new research has offered an alternative to that prevailing theory.

The new research has found that collisions aren't to blame for Mercury's large iron core; rather, the sun's magnetism is responsible. Researchers William McDonough and Takashi Yoshizaki have developed a model that shows the density, mass, and iron content of the rocky planet's core is influenced by the distance from the sun's magnetic field.

The researchers showed that the four inner planets of the solar system, including Mercury, Venus, Earth, and Mars, are all made up of different portions of metal and rock. Their model shows there is a gradient where metal content in the core drops off as the planets get further from the sun. Their paper outlines how that happens by showing the distribution of raw materials in the early forming solar system was controlled by the sun's magnetic field.

The researchers found that the density and proportion of iron in the core of a rocky planet correlates directly with the strength of the magnetic field around the sun during the planet's formation. The study suggests magnetism has to be factored into future attempts to describe the composition of rocky planets, including exoplanets outside our solar system.

The study could impact the search for life on other planets because the composition of the planet's core is important for its potential to support life. On Earth, the iron core creates a magnetosphere protecting life from cosmic rays. Researchers also note that the core has the majority of the phosphorus on the planet, which is an important nutrient for carbon-based life.

With the new model, researchers determined the speed at which gas and dust were sucked into the center of the solar system during its formation. When factoring in the magnetic field that would have been generated by the sun as it was born and calculating how that field would draw iron through the dust and gas cloud. The next step in the research will be to find another planetary system similar to ours with rocky planets spread over wide distances from the central star and determine if the density of the planets declines with their distance from the star.