A super-Earth is an extrasolar planet with a mass between 1 to 10 times the mass of the Earth and our Solar System does not have any planets that are within this mass regime. A number of super-Earths have already been discovered around other stars. The four distinct types of materials that could make up a super-Earth with different proportions are iron alloys, silicates, volatiles/ices and hydrogen-helium gas. For a given mass, a less dense super-Earth will have a larger diameter while a denser super-Earth will have a smaller diameter. Thus, a pure hydrogen-helium gas planet will have the largest possible diameter while a pure iron planet with have the smallest possible diameter. However, the upper and lower limiting diameters for a super-Earth of a given mass are highly unlikely with regard to the physical processes involved in planet formation.
A paper by Robert A. Marcus, et al. (2010) entitled “Minimum Radii of Super-Earths: Constraints from Giant Impacts” examines the smallest possible diameter a super-Earth of a given mass can have. Therefore, volatiles/ices and hydrogen-helium gas are not considered and only rocky planets with an iron core and a silicate mantle are considered here. The only way to significantly increase the density of a planet requires the removal of the silicate mantle while preserving the iron core. An effective way to do that is by the stripping of the planet’s silicate mantle by giant impacts.
An example of mantle stripping via collision in our own Solar System is the planet Mercury. By mass, Mercury is 70 percent iron and 30 percent silicate, while the Earth is one-third iron and two-thirds silicates and other materials. Proportional to its mass, Mercury has a higher iron content than any other planet in the Solar System. It is currently theorized that Mercury was initially over twice its current mass with an iron core and a substantial silicate mantle. A large object, roughly one-third Mercury’s current mass, struck the planet and stripped away much of the planet’s original crust and silicate mantle, leaving behind the iron core together with a thin layer of the original crust and silicate mantle.
The conclusions derived from this paper show that the collision stripping of mantle material is an effective mechanism in producing a super-Earth with a higher mean density by increasing the iron mass fraction. It is easier for the collision stripping of mantle material for a lower mass super-Earth to produce a large iron mass fraction as compared to a higher mass super-Earth.
However, even with the most extreme impact conditions, the collision stripping of mantle material from a super-Earth is still unable to produce anything close to a pure iron planet. The maximum mass of a super-Earth with over 70 percent iron by mass is most probably 5 Earth masses since its formation via the stripping of its silicate mantle by a giant impact requires an initial object of 10 Earth masses. The maximum mass of a super-Earth is expected to be around 10 times the mass of the Earth since a more massive planet will probably undergo runaway growth via accretion of hydrogen-helium gas and become an even more massive gas giant planet.
NASA’s Kepler space telescope is expected to find a few hundred planets in the super-Earth mass regime and a sample of them will probably have masses too large for their observed diameters based on standard planet formation. The formation of such dense “cannonball” super-Earths can then be explained by the collision stripping of mantle material to produce a larger iron mass fraction.