Mercury’s mysterious makeup may be the result of a grazing “hit and run” collision between two similar-sized protoplanets in the early solar system, a new study suggests.

Mercury, the smallest and innermost planet in our solar system, has a number of unusual characteristics that have long puzzled scientists. Despite being only slightly larger than Earth’s moon, Mercury is extraordinarily dense. It boasts a disproportionately large, iron-rich core that makes up about 60% of its mass — twice that of other rocky planets such as Earth, Venus or Mars — challenging commonly accepted theories of planetary formation.

Adding to the mystery, data from NASA’s MESSENGER probe, which orbited Mercury between 2011 and 2015, revealed that the planet’s surface is surprisingly rich in volatile elements, including potassium, sulfur and sodium. These elements should have been stripped away had the planet endured a rare, single, massive impact in its youth, as researchers previously theorized. One possible explanation is that young Mercury violently collided with a much larger protoplanet. However, simulations tracing terrestrial planet formation indicate that collisions among protoplanets of very different sizes and masses are rare, prompting scientists to seek alternate explanations of how Mercury could have lost so much outer material while preserving these volatiles.

The new simulations suggest that Mercury’s anomalous makeup may stem from a more frequent cosmic event: a grazing collision with a protoplanet of comparable size. “This kind of apparently ‘lucky shot’ would not have been unusual — and it might be exactly what created Mercury,” study lead author Patrick Franco, a postdoctoral researcher in astrophysics at the Paris Institute of Planetary Physics, told Live Science.

“Our work reinforces the idea that giant impacts are not just a part of planet formation — they may actually be the primary drivers shaping the final structure of planets in the solar system,” Franco said. The findings also raise questions about whether similar collisions could have shaped other planets, he added. The researchers reported their findings in a paper posted to the preprint server arXiv, which is yet to be peer-reviewed.