The role of high-pressure experiments on determining super-Earth properties

Abstract

Super-Earths are the newest class of extra-solar planets with a mass range between about 1–10M ⊕ . With their large masses, they experience very large internal pressures. The central pressure scales proportionately with mass, reaching values that require us to extend our understanding of rock and H2O behavior to such extreme conditions. Pressure also constrains the power law relationship between mass and radius of solid planets R∼M β . The value for the exponent is 0.262≤β≤0.274 as constrained by the different internal structure models for super-Earths, while it is 0.3 for planets between 5–50% the mass of Earth. Despite uncertainties in planetary composition, temperature structure and equation of state, the mass-radius relationship is robust, and thus, useful for inferring the expected signal in transit searches. In the next few years many super-Earths will be discovered and their masses and radii will be known with some uncertainty. Even without errors in both the data and structure models, a large number of compositions can fit the same average density. However, the follow-up observations with space telescopes will yield very precise radius measurements and even probe the atmospheres of super-Earths. This radius uncertainty will then be comparable to the current error derived from the equation of state used by the structure models. Thus, there is a need for accurate equations of state of solid planetary materials. Furthermore, information on the structure, such as the size and state of the core, crucially depends on the exact behavior of super-Earth materials (i.e. silicates, iron, iron alloy and ices) at high pressures and temperatures. In addition, information about the atmospheric composition of these planets may prove useful in constraining their interiors. Ultimately any inference on the structure of super-Earths, including information from atmospheres, depends on the precision of interior models, which in turn require accurate equations of state.