The Microscopic Foundations of Solid Compositions in Planetary Systems

Abstract Dust grains in the interstellar media and protoplanetary disks have roughly equal abundances in carbon and silicates. When coagulating and growing into larger astronomical objects, theories bridging grains to the growth products struggle to quantitatively explain the <10−2 carbon to silicon ratio in rocky planets. Here we propose a new paradigm that self-consistently establishes the microscopic foundations for dust evolution theories by integrating them with ab initio computational chemistry. CO and H2O, the most substantial astrophysical molecules that can be “glues” in grain growths, exhibit drastically weaker adsorption on carbonaceous grains than silicates. This effect inhibits the inclusion of carbon in planetesimals by the desorption of these “glue molecules” from warm carbonaceous grains close to the host star (≲ 8 AU for a solar-system-equivalent), giving rise to the high Si-C abundance ratios in warm regions. The same mechanism semi-quantitatively yields the trend of increasing carbon abundances in the outer parts of our Solar system and likely generic planetary systems.