How does the chemical composition of solids influence the formation of planetesimals?

Abstract

The formation of planetesimals is a necessary step in the formation of planets. While several mechanisms have been proposed, a local dust-to-gas ratio above unity is a strong requirement to trigger the collapse of pebble clouds into planetesimals. A prime location for this is the water-ice line, where large water-rich pebbles evaporate and release their smaller silicate cores. This enhances the local dust-to-gas ratio due to the different inward drift speeds of large and small pebbles. Previous work suggested that planetesimal formation becomes difficult at overall dust-to-gas ratios below 0.6\%, consistent with the occurrence of close-in super Earths. However, the influence of disc composition on planetesimal formation remains unclear. Observations of stellar abundances show both a decrease and a wide spread in C/O ratios for low-metallicity stars. Using the C/O ratio as a proxy to determine water ice abundance in discs, we use the 1D disc evolution code chemcomp to simulate protoplanetary discs with varying C/O and dust-to-gas ratios over 3 Myr. Planetesimal formation is modeled using conditions based on dust-gas dynamics and pebble fragmentation. Our results confirm that planetesimal formation strongly depends on disc metallicity, with lower metallicity discs forming significantly fewer planetesimals. A lower carbon fraction generally promotes planetesimal formation by increasing water ice, while higher carbon fractions suppress it. The opposite is seen for oxygen: higher oxygen content leads to more efficient planetesimal formation at the same dust-to-gas ratio. We thus predict that planets around low-metallicity stars should be more common when their C/O ratio is low and oxygen is enhanced, a trend that can be tested observationally. Our simulations thus open a pathway to understand if the composition of the planet forming material influences the growth of planets.