How Rocky Exoplanets Are (Left): Mg, Fe and Si stellar compositions for Sun-like stars taken from the Hypatia catalog. Overlaid are contours of the predicted core mass fraction of any planets should a planet form with a bulk refractory composition the same as the stellar abundance which provide a baseline to test the hypothesis that host-star refractory composition is approximately that of the orbiting rocky exoplanet. It also outlines the diversity of compositions relative to the Solar System rocky planets. From Unterborn & Panero, 2019 JGR-Planets
How Rocky Exoplanets Were (Center): The TRAPPIST-1 planets have densities consistent with having ∼7 wt% surface water oceans and lacking a substantial extended atmosphere. They also likely migrated inward to there current orbital positions. Here, I show the orbital radius of a modeled water snow line (blue, red lines) in the protoplanetary disk of the TRAPPIST-1 M-dwarf system as a function of time of planet formation. As the disk cools, the ice-line moved inwards. The black dashed lines represent the minimum pre-migration orbital radius of TRAPPIST-1d if it formed outside the ice line in order to gain its water-rich compositions. This plot shows that if TRAPPIST-1d formed quickly (<3 Myr) it would have had to migrate farther to its current orbit. This represents a case where exoplanet composition can be used to analyze how and when rocky exoplanets formed. From Unterborn et al., 2018 Nature Astronomy
How Rocky Exoplanets Will Be (Right): Active degassing from a rocky exoplanet’s interior is a first-order constraint on whether it can sustain a temperate atmosphere over geologic timescales. Here are calculated times when degassing rates fall below 10% of Earth value as function of the total amount of the planet’s bulk K abundance relative to the Earth across observed stellar K abundances. This shows that the more potassium a stagnant-lid exoplanet contains directly affects how long it is able to degas. The degree of which K volatilizes during planet formation is a key parameter then on a planet’s potential to be habitable and for how long it can do so. As TRAPPIST-1 is ∼8 Gyr old, it may in fact be too old to sustain this critical process. From Unterborn et al., in review
The Earth is a habitable, dynamic planet. The above figure shows the results of a recent survey of planetary host and non-host stars which found variations of between 10 and 400% of Solar in the abundance of the major (Mg, Fe, Si) and minor (Ca, Al, Na) terrestrial planet-building elements. On the right is the terrestrial exoplanet dataset, showing planets ranging in size, mass and density from sub-Venus to super-Earth. These stars and planets represent compositions and structures unlike anything in our Solar System. This diversity raises the questions: How different are these planets and planetary systems from the Earth and our Solar System, and what, then is the likelihood of dynamic, habitable Earth-like planets in our Galaxy? My research combines observations and models from geodynamics, cosmochemistry, astronomy and astrophysics, geochemistry and mineral physics to answer these questions. I also look to our Solar System in my work, particularly Venus, to understand which compositional and evolutionary paths lead to lifeless, inhospitable planets, thus constraining from the outside-in, exactly what makes the Earth unique.