Extreme Measures: The Pressure and Temperature Phase Space of Rocky Super-Earths

With the launch of TESS, the rate of exoplanet discovery is due to increase. The compositions and thermal states of these planets are unknown and unlikely to be directly observed. Geoscience, however, can help us infer and constrain these planetary parameters and outline the diversity of rocky worlds. The current dataset of planetary properties is biased to Earth-like compositions. Stellar compositions, however, suggest a range of potential rocky planet compositions that is quite different from Earth's. The lack of thermal conductivity, water storage, and phase equilibria datasets across this range of compositions limits our ability to model these planets. Such data is vital for characterizing a planet's potential habitability.

Recent astronomical observational surveys have statistically established a maximum radius of rocky exoplanets of 1.5 Earth radii. Planets larger than these super-Earths are likely to be mini-Neptunes with large gaseous envelopes. In response to these new classes of planets, ab initio and shock-wave measurement efforts are exploring the properties of silicates at pressures >1 TPa.

We present here the open-source mass-radius-composition calculator, ExoPlex. For a given planetary bulk composition, ExoPlex self-consistently calculates mantle phase equilibria as well as density, adiabatic temperature and gravity profiles. We outline the pressure and temperature regime of rocky planet mantles and cores across a wide variety of interior compositions as outlined by stellar abundance data. We empirically demonstrate that for rocky planets smaller than 1.5 Earth-radii, core mantle boundary (CMB) pressure and adiabatic temperature are independent core mass fraction, reaching a maximum of 600 GPa and 4000 K assuming a 1600 K mantle potential temperature. This is despite central core pressures varying from 1.5-2.5 TPa. Inclusion of a volatile layer, atmosphere or light elements in the core only lowers CMB pressure for a given radius.

Experimental studies relevant to our understanding rocky planet mantles may then be safely limited to pressures <600 GPa as those planets where silicates reach pressures >1 TPa are likely to be very rare. We argue that for the characterization of rocky exoplanets, it would be more fruitful for geoscience to explore a wider compositional space at pressures ≤600 GPa.