Selective Aggregation Experiments on Planetesimal Formation and Mercury-Like Planets

TL;DR

This paper reviews experimental and theoretical mechanisms that could explain the formation of Mercury-like planets, emphasizing selective processing such as iron enrichment and magnetic aggregation in protoplanetary disks.

Contribution

It introduces potential processes like photophoresis and magnetic aggregation that could lead to Mercury's high density during planet formation.

Findings

Photophoresis could separate iron and silicate in early planet formation.

Magnetic properties of iron may facilitate aggregation of Mercury-like planets.

Selective processing mechanisms are plausible in explaining Mercury's composition.

Abstract

Much of a planet's composition could be determined right at the onset of formation. Laboratory experiments can constrain these early steps. This includes static tensile strength measurements or collisions carried out under Earth's gravity and on various microgravity platforms. Among the variety of extrasolar planets which eventually form are (Exo)-Mercury, terrestrial planets with high density. If they form in inner protoplanetary disks, high temperature experiments are mandatory but they are still rare. Beyond the initial process of hit-and-stick collisions, some additional selective processing might be needed to explain Mercury. In analogy to icy worlds, such planets might, e.g., form in environments which are enriched in iron. This requires methods to separate iron and silicate at early stages. Photophoresis might be one viable way. Mercury and Mercury-like planets might also form due to the ferromagnetic properties of iron and mechanisms like magnetic aggregation in disk magnetic fields might become important. This review highlights some of the mechanisms with the potential to trigger Mercury formation.