Enriching inner discs and giant planets with heavy elements

Abstract: Giant exoplanets seem to have on average a much larger heavy element content than the solar system giants. Past attempts to explain these heavy element contents include collisions between planets, accretion of volatile rich gas and accretion of gas enriched in micro-metre sized solids. However, these different theories individually could not explain the heavy element content of giants and the volatile to refractory ratios in atmospheres of giant planets at the same time. Here we combine the approaches of gas accretion enhanced with vapor and small micro-meter sized dust grains. As pebbles drift inwards, the volatile component evaporates and enriches the disc, while the smaller silicate core of the pebble continues to move inwards. The smaller silicate pebbles drift slower, leading to a pile-up of material interior to the water ice line, increasing the dust-to-gas ratio interior to the ice line. Under the assumption that these small dust grains follow the motion of the gas, gas accreting giants accrete large fractions of small solids in addition to the volatile vapor. The effectiveness of the solid enrichment requires a large disc radius to maintain the pebble flux for a long time and a large viscosity that reduces the size and inward drift of the small dust grains. However, this process depends crucially on the debated size difference of the pebbles interior and exterior of the water ice line. On the other hand, the volatile component released by the inward drifting pebbles can lead to a large enrichment with heavy element vapor, independently of a size difference of pebbles interior and exterior to the water ice line. Our model stresses the importance of the disc's radius and viscosity on the enrichment of dust and vapor. Consequently we show how our model could explain the heavy element content of the majority of giant planets by using combined estimates of dust and vapor enrichment.

• The paper demonstrates how inward-drifting pebbles enrich inner discs via volatile evaporation at the water-ice line.
• It models the combined accretion of dust and vapour, linking disc viscosity and radius to heavy-element uptake in forming giants.
• The study implies that heavy element gradients in exoplanets arise from dual enrichment processes in evolving protoplanetary discs.

Enriching Inner Discs and Giant Planets with Heavy Elements

The formation and compositional diversity of giant exoplanets continue to intrigue the astronomical community. The focal point of the study by Bitsch and Mah lies in the elucidation of the processes that enrich inner discs and subsequently, giant planets with heavy elements. The paper postulates a dual mechanism involving the accretion of volatiles, alongside micrometre-sized dust grains, to account for the heavy-element enrichment observed in giant exoplanets.

Key Findings

The research integrates models of inward-drifting and evaporating pebbles into a cohesive framework that highlights how such processes can enrich protoplanetary discs with heavy elements. The model reveals that as pebbles migrate towards the star, their volatile components evaporate, enriching the inner disc. Notably, the research emphasizes the significance of the water-ice line, where silicate grains drift at varied speeds, causing a material pile-up, thus augmenting the dust-to-gas ratio.

• Disc Parameters: The efficacy of this enrichment process is contingent on large disc radii, which ensure a sustained pebble flux, and high viscosities, which slow the drift of dust grains. The paper acknowledges this mechanism’s sensitivity to the debated size differences of pebbles across the water-ice line.
• Heavy Element Enrichment: It suggests that the heavy-element-rich environments of many giant planets could result from combined dust and vapour accretion. The authors propose that such environments are achievable when both dust grains and gaseous elements are abundantly accreted by forming planets, supported by low-viscosity conditions and large disc radii.

Implications

The theoretical model posited by Bitsch and Mah holds significant implications for our understanding of planetary formation, particularly regarding the compositional diversity of giant planets. It provides a plausible explanation for the higher metallicity observed in exoplanets compared to the giants of our Solar System.

• Planetary Composition: This research might illuminate the varied volatile-to-refractory ratios detected in planetary atmospheres, offering insights into the pathways of planetary formation.
• Formation Pathways: The model underscores the complexity of planet formation environments, advocating for further investigation into the roles of disc viscosity and radius in dictating planetary composition.

Future Directions

The study paves the way for more nuanced investigations into disc evolution, focusing on how conditions within the disc influence the heavy-element enrichment of nascent planets. Future research could explore the dynamical effects on disc material distribution and employ advanced simulations to dive deeper into 3D disc and pebble dynamics.

This model also opens avenues for comparative studies involving different stellar metallicities, disc chemistries, and environmental conditions that might produce diverse planet formation outcomes. Such research is paramount to refining our understanding of not only our own Solar System but also the plethora of planetary systems within our galaxy.