Mercury, the planet in our solar system that is nearest to the Sun, has been likened to an orange. It has an enormous core, which mirrors an orange's juicy part, while its mantle and crust corresponds to the fruit's thin rind.

This is an unusual phenomenon that has baffled astronomers for many decades because the conventional means whereby planets are formed do not produce relatively large cores. Terrestrial planets such as the Earth usually have smaller cores and are likened to peaches, making their overall density lower.

Why is Mercury's core so big for its size?

This led astronomers to speculate that the planet Mercury may have experienced massive impacts that have stripped any silicate mantle it may have had before. It may also be possible that the planet's outer layers may have evaporated from the intense heat of our sun.

However, in the past few years, the Messenger probe from NASA discovered volatile elements such as potassium within the crust of the planet. According to many models, evaporation or some kind of impact would have removed such elements.

Additionally, recent exoplanet observations by scientists seem to suggest that the structure of Mercury may be common, after all. Corot-Exo-7b and Kepler-10b are known to be the smallest exoplanets with known densities, and they are denser than scientists have expected.

This suggests that they also have an orange-like structure, just like Mercury. These planets are also situated near their own sun as well. With this, a new scientific theory is set to possibly explain the phenomena with the heat of starlight.

Gas molecules colliding with hot dust grains pick up heat and bounce off, which shoves those grains. University of Duisburg-Essen's Gerhard Wurm of Germany, together with colleagues, made calculations on how such a photophoretic force affects dust grains that swirl around stars.

The research team discovered that since metallic grains are conductors of heat, they become heated evenly throughout. This results in them being shoved on all sides so that they do not move very far from their star. However, insulating grains like silicates that are less-dense have hot sides--the sides that face their sun--where the gas molecules that are departing give a stronger shove than gases from the other colder side.

Wurm and his team said that this results in the "sorting" of the grains over time in a young solar system, where metals remain closer to their sun while less dense particles are pushed farther out. Eventually, planets form using these grains. This process may explain why Mercury is very dense.

Wurm further adds that objects that are rich in metal stay closer to their star, as they couldn't be pushed; meanwhile, the farther out from the system one goes, the fewer metals are available for building planets. The team's research will soon be published in The Astrophysical Journal.

Carnegie Institution of Washington's Larry Nittler cautions, however, that the theory is not yet conclusive, stressing that the mantle-stripping theory is still not ruled out. He suggests that computer simulations may be done on our own system, which will account for photophoresis, and then compare them with the measurements taken by NASA's Messenger probe.

Wurm, on the other hand, plans to run a real-world simulation. He will be dropping a capsule that contains dust and metals from the Bremen Drop Tower in Germany, where space's weightlessness is simulated. Wurm aims to zap infrared on the capsule to check if the contained materials will separate as predicted.