Forming exo-Mercuries: Making dense planets via impacts

The planet Mercury has an exceptional, massive iron core. Mercury's core accounts for about 70% of its mass, far more than the Earth's core which is only about 30% of its mass. It is unclear how such an extreme iron-enrichment arose.

One popular idea is that early in its life Mercury was hit by another large planetary body that stripped it of much of its original rocky mantle. The major argument against this model is that the rocks stripped off of early Mercury would just be spread around its orbit and eventually fall back onto the planet, reversing any iron-enrichment caused by the impact.

Recent successes in exoplanet detection have shown that Mercury is not alone. There is a small but significant population of exoplanets with similarly high iron-enrichment across the mass range of rocky planets (0.1-10 times the mass of the Earth). These super-dense, iron-rich planets are known as exo-Mercuries and/or super-Mercuries.

The fundamental questions we propose to answer are: how did these Mercury-like planets form? and what is their relation to our own Mercury?

We will carry out high resolution, detailed supercomputer simulations of potential Mercury-forming giant impacts to examine how much rocky mantle can be removed. We will then investigate how the rocky debris behaves over millions of years to determine how much returns to the planet and how much is lost. We will apply this same two-stage coupled simulation approach to investigate giant planetary impacts as a mechanism for forming Mercury-like planets outside our solar system.

Finally, we will explore an alternative planetary collision scenario and investigate how much the rocky mantle can be eroded by the cumulative effects of many smaller collisions amongst the building blocks of Mercury-like planets. Combined, the results of all our simulations will allow us to answer whether Mercury, and super-Mercury planets, can form via planetary collision processes.

These questions are a fundamental part of our understanding of planet formation across the Universe.