An exoplanet's structure and composition are first-order controls of the planet's habitability. We explore which aspects of bulk terrestrial planet composition and interior structure affect the chief observables of an exoplanet: its mass and radius. We apply these perturbations to the Earth, the planet we know best. Using the mineral physics toolkit BurnMan to self-consistently calculate mass-radius models, we find that core radius, presence of light elements in the core and an upper-mantle consisting of low-pressure silicates have the largest effect on the final calculated mass at a given radius, none of which are included in current mass-radius models. We expand these results provide a self-consistent grid of compositionally as well as structurally constrained terrestrial mass-radius models for quantifying the likelihood of exoplanets being "Earth-like." We further apply this grid to Kepler-36b, finding that it is only ∼20% likely to be structurally similar to the Earth with Si/Fe = 0.9 compared to Earth's Si/Fe = 1 and Sun's Si/Fe = 1.19.

The interior composition of exoplanets is not observable, limiting our direct knowledge of their structure, composition, and dynamics. Recently described observational trends suggest that rocky exoplanets, that is, planets without significant volatile envelopes, are likely limited to <1.5 Earth radii. We show that given this likely upper limit in the radii of purely rocky super‐Earth exoplanets, the maximum expected core‐mantle boundary pressure and adiabatic temperature are relatively moderate, 630 GPa and 5000 K, while the maximum central core pressure varies between 1.5 and 2.5 TPa. We further find that for planets with radii less than 1.5 Earth radii, core‐mantle boundary pressure and adiabatic temperature are mostly a function of planet radius and insensitive to planet structure. The pressures and temperatures of rocky exoplanet interiors, then, are less than those explored in recent shock‐compression experiments, ab initio calculations, and planetary dynamical studies. We further show that the extrapolation of relevant equations of state does not introduce significant uncertainties in the structural models of these planets. Mass‐radius models are more sensitive to bulk composition than any uncertainty in the equation of state, even when extrapolated to terapascal pressures.

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