The Pressure and Temperature Limits of Likely Rocky Exoplanets

Abstract

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.

Super‐Earth Internal Structures and Initial Thermal States

“…Density and pressure profiles within the interior of super‐Earths with 1 to 10 Earth masses ( M E ) and CMFs of 0.2 (Mars like), 0.32 (Earth like), and 0.68 (Mercury like) are shown in Figure 2. Statistics of observed exoplanet populations suggest that exoplanets with radii larger than 1.5 Earth radii have relatively small bulk density indicating the presence of a substantial gaseous envelope (Rogers, 2015; Unterborn & Panero, 2019; Weiss & Marcy, 2014). Even though super‐Earths larger than 1.5 R E may be rare, we argue they are still worth investigating.…”

Transits of Known Planets Orbiting a Naked-eye Star

“…Given the mass and radius for both planets, there are significant constraints for inferences to their interior structures in the context of composition. Namely, the stellar elemental abundances (see Section 2) would need to be measured to a higher precision, 0.02-0.04dex (Hinkel & Unterborn 2018), necessary to meaningfully constrain the mineralogy (Unterborn & Panero 2019). In addition, fundamental to all of the mass-radius models are critical assumptions regarding the composition of rocky exoplanets and the underlying mineral physics.…”

“…In addition, fundamental to all of the mass-radius models are critical assumptions regarding the composition of rocky exoplanets and the underlying mineral physics. These assumptions typically cause over-or underpredictions in empirical models (e.g., Zeng et al 2016) when characterizing ultrahigh pressures present in the cores of super-Earths and mini-Neptunes (Unterborn & Panero 2019).…”

Machine-learning Inference of the Interior Structure of Low-mass Exoplanets

“…Planets identified with both techniques are characterized by their mass and radius, which, combined, provide a first indication of the bulk composition through comparison with theoretical mass-radius curves (e.g., Valencia et al 2006;Sotin et al 2007;Zeng & Sasselov 2013). A common approach to the interior characterization of exoplanets is the use of numerical models to compute interior structures which comply with the measured mass and radius of the planet (e.g., Sotin et al 2007;Valencia et al 2007;Fortney et al 2007;Wagner et al 2011;Zeng & Sasselov 2013;Unterborn & Panero 2019). As this is an inverse problem, it requires the calculation of a large number of interior models to obtain an overview over possible interior structures (Rogers & Seager 2010a;Dorn et al 2017;Brugger et al 2017).…”