Volatile loss, Differentiation and Collisions: Key to the Composition of Rocky Exoplanets

Abstract

<p><strong>Volatile loss, Differentiation and Collisions: Key to the Composition of Rocky Exoplanets</strong></p> <p>Many of the key characteristics and geology of our planet Earth today were determined during the planet’s formation. What about rocky exoplanets? How does rocky planet formation determine the properties, composition, geology and ultimately, presence of life on rocky exoplanets?<span class="Apple-converted-space"> </span></p> <p>In this talk I will discuss projects that investigate the link between rocky planet formation and the composition of rocky exoplanets. This work utilises unique observations that provide us with the bulk composition of rocky exoplanetary material. These observations come from the old, faint remnants of stars like our Sun, known as white dwarfs.<span class="Apple-converted-space"> </span></p> <p>White dwarfs should have clean hydrogen or helium atmospheres. This means that planetary bodies as small as asteroids can show up in the white dwarf’s atmosphere. Metallic species such as Fe, Mg or Ca provide the bulk composition of the accreted body. Several thousand polluted white dwarfs are now known.</p> <p>Models indicate that outer planetary systems, like our Solar System beyond Mars, should survive the star’s evolution to the white dwarf phase. Scattering is a common process, and any bodies that are scattered inwards, a bit like sun-grazing comets in our Solar System, would show up in the white dwarf atmosphere.</p> <p><strong>What determines the composition of the rocky exoplanetary bodies accreted by white dwarfs?<span class="Apple-converted-space"> </span></strong></p> <p>Models presented in Harrison et al, 2018, 2020 (submitted) find that the abundances observed in the atmospheres of white dwarfs can be explained by three key processes, notably galactic chemical evolution, loss of volatiles (thermal processing) and large scale melting<span class="Apple-converted-space">  </span>which leads to the segregation of material between the core, mantle and crust. Galactic chemical evolution determines the initial composition of the planet forming material. Thermal processing determines the loss of volatiles, be that CO and other gases, water, or moderate volatile species such as Na. Collisions between planetary bodies that have differentiated to form a core can lead to fragments dominated by core-rich or mantle-rich material.<span class="Apple-converted-space"> </span></p> <p><strong>Core-Mantle differentiation is a common process in exoplanetary systems</strong></p> <p>High abundances of siderophile (iron-loving) compared to lithophile (silicate loving) speeches in some polluted white dwarfs indicate that accretion of a planetary body composed primarily of material from a planetary core (or alternatively mantle). Harrison et al, 2020, based on data from Hollands et al, 2017, 2018, present several examples of systems with extreme abundances, core-rich, mantle-rich or crust-rich.<span class="Apple-converted-space"> </span></p> <p>Bonsor et al, 2020 concludes that most polluted white dwarfs (>60%) have accreted the fragment of a differentiated exoplanetesimal.<span class="Apple-converted-space"> </span></p> <p><strong>Post-Nebula volatilisation in exoplanetary bodies</strong></p> <p>Mn and Na trace the loss of volatiles in planetary bodies. The difference in behaviour of Mn and Na under oxidising/reducing conditions makes them a strong indicator of the conditions prevalent when volatile loss occurred. Mn/Na for the Moon/Mars indicate post-Nebula volatile loss<span class="Apple-converted-space">  </span>(Siebert et al, 2018). Harrison et al, 2020, in prep, provides the first evidence of post-nebula volatilisation in exoplanetary bodies utilising the Mn/Na abundances of polluted white dwarfs.<span class="Apple-converted-space"> </span></p>