Early-Forming Solids May Have Changed the Composition of the Protoplanetary Disk

ALMA image of the planet-forming disk around the young, Sun-like star TW Hydrae. The inset image (upper right) zooms in on the gap nearest to the star, which is at the same distance as the Earth is from the Sun, suggesting an infant version of our home planet could be emerging from the dust and gas. The additional concentric light and dark features represent other planet-forming regions farther out in the disk. Credit: S. Andrews (Harvard-Smithsonian CfA), ALMA (ESO/NAOJ/NRAO).

In the earliest periods of solar system formation, the protoplanetary disk was a hot environment full of gas and dust. As the disk cooled, various elemental species condensed out of the gas based on their condensation temperatures, starting with the most refractory (e.g., aluminum, calcium, magnesium), followed by the more volatile. This process is responsible for the growth and elemental compositions of solids in the solar system. The major element compositions of planetary materials are often compared to “solar” values in order to understand how they differ from the Sun itself. The “solar” composition is best understood using a pristine type of meteorite called carbonaceous chondrites, and in particular, CI chondrites. The use of CI chondrites to define solar composition is, in part, the result of scientists’ ability to directly study these unaltered meteorites in laboratories rather than having to rely on remote observations of the Sun. Relative to CI chondrites, Earth (along with the non-CI carbonaceous chondrites) is enriched in refractory elements, while other inner solar system and non-carbonaceous chondrite materials are depleted in these elements. Many previous attempts at explaining the starting composition of Earth have struggled with this conundrum.

A study led by Alessandro Morbidelli from the Observatoire de la Côte d’Azur proposes that a hypothetical early generation of planetesimals, which are not represented in our meteorite collections, may have changed the composition of the protoplanetary disk. Morbidelli and colleagues demonstrated that formation of such a generation of planetesimals could have depleted the protoplanetary disk in refractory elements. This could account for the current population of planetary materials, some of which are enriched and some depleted in refractory elements relative to the CI chondrites. Based on their findings, the authors suggest that the Earth formed from a mixture of approximately 60% solar-composition material and approximately 40% refractory element-rich, first-formed planetesimals. This model by Morbidelli and colleagues has the potential to answer long-standing questions about the refractory element abundances of planetary materials and the starting composition of Earth. These findings, while thought provoking, will need to be tested through a comprehensive examination of the refractory element compositions of planetary materials. READ MORE