How disc initial conditions sculpt the atmospheric composition of giant planets

TL;DR

The paper investigates how initial protoplanetary disc conditions sculpt the atmospheric compositions of giant planets in a pebble-drift and evaporation framework. Using a semi-analytic 1D model with a simple chemical partitioning scheme, it shows that most disc parameters have limited impact on gas-giant atmospheres, except for the dust-to-gas ratio which scales the enrichment. Crucially, atmospheric abundances are primarily set by where and how planets form and migrate, crossing evaporation fronts that alter the volatile inventory accreted in gas and vapour-rich gas. Consequently, C/O alone is not a reliable tracer of formation location; instead, a combination of atmospheric abundances (e.g., C/H, O/H, S/H) and stellar abundances is required, with volatile-rich giants likely forming in the inner disc and migrating inward. The study thus links observed exoplanet atmospheric diversity to formation location and migration history, guiding interpretation of atmospheric data in the context of disc evolution.

Abstract

Past studies have revealed the dependency of the disc parameters (mass, radius, viscosity, grain fragmentation velocity, dust-to-gas ratio) on the formation of giant planets, where more massive discs seem beneficial for giant planet formation. It is unclear how the different disc properties influence the composition of forming giant planets. The idea that the atmospheric abundances can trace directly the formation location of planets is put into question, due to the chemical evolution of the disc, caused by inward drifting and evaporating pebbles. This complicates the idea of a relation between atmospheric abundances and planet formation locations. We use planet formation simulations that include the effects of pebble drift and evaporation and investigate how the different disc parameters influence the atmospheric composition of giant planets. We focus on the atmospheric C/O, C/H, O/H and S/H ratios allowing us to probe tracers for volatiles and refractories and thus different accretion pathways of giant planets. We find that most of the disc parameters have only a limited influence on the atmospheric abundances of gas giants, except for the dust-to-gas ratio, where a larger value results in higher atmospheric abundances. However the atmospheric abundances are determined by the planetary formation location, even in the pebble drift and evaporation scenario. Our study suggests that volatile-rich giant exoplanets predominantly form in the inner disc regions, where they can accrete large fractions of vapour-enhanced gas. Our study shows that simulations that try to trace the origin of giant planets via their atmospheric abundances do not have to probe all disc parameters, as long as the disc parameters allow the formation of giant planets. Our study thus suggests that the diversity of observed planetary compositions is a direct consequence of their formation location and migration history.

Figures (10)

• Figure 1: Evolution of planetary mass and the planetary heavy element mass fraction against position (top row) and [C/H], [O/H], [S/H], [C/O], [C/S] and [O/S] ratios (middle and bottom rows), using the standard parameters (in green) and changing to $u_{frag}$=1.0 m/s (in purple) with initial positions of 1, 3, 10 and 30 AU. The dots mark time steps of 0.5 Myr after the embryos start growing (unless the planet has reached the inner edge). The solid lines represent pebble accretion, while the dotted lines represent gas accretion, separated by a black dot.
• Figure 2: Same as Fig. \ref{['fig:growth_ratios_ufrag']}, but now the purple colour represent planets growing in discs with $\alpha=10^{-3}$ rather than with $\alpha=10^{-4}$ (green colour).
• Figure 3: Final planetary mass against final distance from the host star. The colour codes depict the C/O (top left), C/S (top right), O/S (bottom left) ratios, all normalised to the stellar value, as well as the total heavy element mass of the planets (bottom right).
• Figure 4: Violin plots of the C/O ratio ( normalised to stellar value) of the planets above 100 Earth masses within our sample. The different panels show the C/O ratio split by the different parameters that are varied in our simulations (initial orbital distance, disc mass, disc radius, fragmentation velocity, $\alpha$-viscosity, starting time and dust-to-gas ratio). The green and purple vertical lines correspond to the median and mean of the distributions.
• Figure 5: Same as Fig. \ref{['fig:growth_ratios_ufrag']}, but now the purple colour represents planets growing in discs with $M_{\rm disc}=0.01M_\odot$ rather than $M_{\rm disc}=0.1M_\odot$ (green colour).