How drifting and evaporating pebbles shape giant planets I: Heavy element content and atmospheric C/O (2105.13267v3)

Abstract: Recent observations of extrasolar gas giants suggest super-stellar C/O ratios in planetary atmospheres, while interior models of observed extrasolar giant planets additionally suggest high heavy element contents. Furthermore, recent observations of protoplanetary disks revealed super-solar C/H ratios, which are explained by inward drifting and evaporating pebbles, enhancing the volatile content of the disk. We investigate how the inward drift and evaporation of volatile rich pebbles influences the atmospheric C/O ratio and heavy element content of giant planets growing by pebble and gas accretion. To achieve this goal, we perform semi analytical 1D models of protoplanetary disks including the treatment of viscous evolution and heating, pebble drift and simple chemistry to simulate the growth of planets from planetary embryos to Jupiter mass objects by accretion of pebbles and gas while they migrate through the disk. Our simulations show that the composition of the planetary gas atmosphere is dominated by the accretion of vapour, originating from inward drifting evaporating pebbles. This process allows the giant planets to harbour large heavy element contents. In addition, our model reveals that giant planets originating further away from the central star have a higher C/O ratio on average due to the evaporation of methane rich pebbles in the outer disk. These planets can then also harbour super-solar C/O ratios, in line with exoplanet observations. However, planets formed in the outer disk harbour a smaller heavy element content, due to a smaller vapour enrichment of the outer disk. Our model predicts that giant planets with low/large atmospheric C/O should harbour a large/low total heavy element content. We further conclude that the inclusion of pebble evaporation at evaporation lines is a key ingredient to determine the heavy element content and composition of giant planets.

Drifting and Evaporating Pebbles in Giant Planet Formation

The investigation of the composition and formation processes of giant planets is central to our understanding of planetary systems. This research examines the role of drifting and evaporating pebbles in shaping the atmospheric and heavy element content of giant planets. Building upon recent observations of protoplanetary disks, which indicate super-solar carbon-to-hydrogen (C/H) ratios, and atmospheric studies showing super-stellar carbon-to-oxygen (C/O) ratios in some gas giants, this paper explores how these volatile abundances are influenced by pebble dynamics.

Methodology

The research employs semi-analytical, one-dimensional (1D) models to simulate the evolution of protoplanetary disks. These models incorporate viscous evolution, pebble drift, and basic chemical processes to simulate the growth of planetary cores into Jupiter-sized objects through pebble and gas accretion. Key processes such as viscous heating, evaporation, and drift of pebbles are central to this model, providing insights into how pebble dynamics influence the heavy element inventory and volatile ratios in developing planets.

Key Findings

1. Heavy Element Content:
   • The simulations reveal that the composition of a planet's gaseous atmosphere is significantly affected by the accretion of vapor from pebbles that evaporate upon crossing evaporation fronts. This process allows a substantial accumulation of heavy elements in gas giants, especially in regions where traditional models, lacking pebble evaporation, predict lower abundances.
2. C/O Ratios:
   • The research shows that giant planets that originate further from their host stars tend to exhibit higher C/O ratios. This is attributed to the evaporation of carbon-rich pebbles, particularly methane, in the outer regions of the disk. Consequently, these planets frequently record super-stellar C/O ratios, aligning with observational data of exoplanets.
3. Dependence on Viscosity:
   • The paper underscores the influence of disk viscosity on pebble dynamics. Lower viscosities enhance radial pebble drift and result in greater volatile enrichment, whereas higher viscosities allow for more rapid inward vapor diffusion and subsequently lower overall heavy element enrichment in inner disk regions.

Implications

The findings have significant implications for the theoretical understanding of giant planet compositions and their formation environments. Notably, the correlation between high C/O ratios and planet formation at greater orbital distances suggests that observed atmospheric compositions can inform constraints on planet formation pathways. Moreover, the results advocate for the inclusion of pebble evaporation processes in planetary formation models to more accurately capture the heavy element content of gas giants.

Future Directions

Looking forward, further refinements to the models, such as incorporating planetesimal accretion alongside pebble dynamics, will be critical in capturing the complete picture of giant planet formation. Additionally, integrating chemical reactions on growing pebble surfaces and evaluating different chemical compositions of protoplanetary disks could provide deeper insights into varying planetary compositions observed across different systems.

This paper provides a meaningful contribution to the field of planet formation, primarily by enhancing the understanding of how compositional gradient arises during the growth of giant planets and subsequently informs observable properties of exoplanets. The findings open new avenues for speculation about the variation in planetary compositions and offer predictive power concerning the relationship between formation location and resulting atmospheric characteristics. Future astronomical observations, potentially by missions such as JWST and ARIEL, will be essential in testing these theoretical predictions and informing further refinement of these models.